LTC3608 - Linear Technology
LTC3608
18V, 8A Monolithic
Synchronous Step-Down
DC/DC Converter
Description
Features
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8A Output Current
Wide VIN Range = 4V to 18V
Internal N-Channel MOSFETs
True Current Mode Control
Optimized for High Step-Down Ratios
tON(MIN) ≤ 100nsec
Extremely Fast Transient Response
Stable with Ceramic COUT
±1% 0.6V Voltage Reference
Power Good Output Voltage Monitor
Adjustable On-Time/Switching Frequency
Adjustable Current Limit
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Low Shutdown IQ: 15µA
Available in a 7mm × 8mm 52-Lead QFN Package
Applications
Point of Load Regulation
Distributed Power Systems
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L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 6100678, 6580258, 5847554, 6304066.
The LTC®3608 is a high efficiency, monolithic synchronous
step-down DC/DC converter that can deliver up to 8A output
current from a 4V to 18V (20V maximum) input supply. It
uses a valley current control architecture to deliver very
low duty cycle operation at high frequency with excellent
transient response. The operating frequency is selected
by an external resistor and is compensated for variations
in VIN and VOUT.
The LTC3608 can be configured for discontinuous or
forced continuous operation at light load. Forced continuous operation reduces noise and RF interference while
discontinuous mode provides high efficiency by reducing
switching losses at light loads.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and an optional
short-circuit shutdown timer. Soft-start capability for supply sequencing is accomplished using an external timing
capacitor. The regulator current limit is user programmable.
A power good output voltage monitor indicates when
the output is in regulation. The LTC3608 is available in a
compact 7mm × 8mm QFN package.
Typical Application
Efficiency and Power Loss
vs Load Current
High Efficiency Step-Down Converter
VON
ION
RUN/SS
VIN
100pF
187k
100
0.8µH
SW
1500pF
11.3k
0.22µF
ITH
BOOST
SGND
INTVCC
30.1k
FCB
VRNG
PGOOD
EXTVCC
100µF
×2
10000
EFFICIENCY
90
1000
85
80
75
70
100
POWER LOSS
65
10
60
VIN = 12V
VOUT = 2.5V
EXTVCC = 5V
55
4.7µF
PGND
50
0.01
VFB
3608 TA01a
VOUT
2.5V
8A
95
9.53k
POWER LOSS (mW)
10µF
×3
LTC3608
VIN
4V TO 18V
EFFICIENCY (%)
0.1µF VOUT
0.1
1
LOAD CURRENT (A)
1
10
3608 TA01b
3608fc
LTC3608
41 SW
42 SW
43 SW
44 SW
45 SW
46 SW
47 SW
48 PVIN
49 PVIN
TOP VIEW
50 PVIN
Input Supply Voltage (SVIN, PVIN, ION)........ 20V to –0.3V
Boosted Topside Driver Supply Voltage
(BOOST)................................................. 26V to –0.3V
SW Voltage............................................. 20V to –0.3V
INTVCC, EXTVCC, (BOOST – SW), RUN/SS,
PGOOD Voltages....................................... 7V to –0.3V
FCB, VON, VRNG Voltages............. INTVCC + 0.3V to –0.3V
ITH, VFB Voltages........................................ 2.7V to –0.3V
Operating Junction Temperature Range
(Notes 2, 4)......................................... –40°C to 125°C
Storage Temperature Range.................... –55°C to 125°C
pIN CONFIGURATION
51 PVIN
(Note 1)
52 PVIN
Absolute Maximum Ratings
PVIN 1
40 PGND
PVIN 2
39 PGND
PVIN 3
38 PGND
53
PVIN
PVIN 4
PVIN 5
37 PGND
55
SW
36 PGND
PVIN 6
35 PGND
PVIN 7
34 PGND
SW 8
33 SW
NC 9
32 INTVCC
SGND 10
31 INTVCC
BOOST 11
30 SVIN
54
SGND
RUN/SS 12
29 EXTVCC
VON 13
28 NC
SGND 26
NC 25
NC 24
VFB 23
ION 22
NC 21
SGND 20
FCB 19
ITH 18
VRNG 17
SGND 15
27 SGND
PGOOD 16
SGND 14
WKG PACKAGE
52-LEAD (7mm × 8mm) QFN MULTIPAD
TJMAX = 125°C, θJA = 29°C/W
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3608EWKG#PBF
LTC3608EWKG#TRPBF
LTC3608WKG
52-Lead (7mm × 8mm) Plastic QFN
–40°C to 125°C
LTC3608IWKG#PBF
LTC3608IWKG#TRPBF
LTC3608WKG
52-Lead (7mm × 8mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
3608fc
LTC3608
Electrical
Characteristics
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
SVIN
Operating Input Voltage Range
IQ
Input DC Supply Current
Normal
Shutdown Supply Current
VFB
Feedback Reference Voltage
4
ITH = 1.2V, –40°C to 85°C (Note 3)
ITH = 1.2V, –40°C to 125°C (Note 3)
l
0.594
0.590
18
V
900
15
2000
30
µA
µA
0.600
0.600
0.606
0.610
V
V
ΔVFB(LINEREG)
Feedback Voltage Line Regulation
VIN = 4V to 18V, ITH = 1.2V (Note 3)
0.002
ΔVFB(LOADREG)
Feedback Voltage Load Regulation
ITH = 0.5V to 1.9V (Note 3)
–0.05
–0.3
%/V
%
IFB
Feedback Input Current
VFB = 0.6V
–5
±50
nA
gm(EA)
Error Amplifier Transconductance
ITH = 1.2V (Note 3)
mS
VFCB
Forced Continuous Threshold
IFCB
Forced Continuous Pin Current
VFCB = 0.6V
tON
On-Time
ION = 60µA, VON = 1.5V
ION = 60µA, VON = 0V
tON(MIN)
Minimum On-Time
tOFF(MIN)
l
1.4
1.7
2
l
0.54
0.6
0.66
V
–1
–2
µA
220
280
110
340
ns
ns
ION = 180µA, VON = 0V
60
100
ns
Minimum Off-Time
ION = 30µA, VON = 1.5V
320
500
ns
IVALLEY(MAX)
Maximum Valley Current
VRNG = 0.5V, VFB = 0.56V, FCB = 0V
VRNG = 0V, VFB = 0.56V, FCB = 0V
IVALLEY(MIN)
Maximum Reverse Valley Current
VRNG = 0.5V, VFB = 0.64V, FCB = 0V
VRNG = 0V, VFB = 0.64V, FCB = 0V
ΔVFB(OV)
Output Overvoltage Fault Threshold
VRUN/SS(ON)
RUN Pin Start Threshold
VRUN/SS(LE)
RUN Pin Latchoff Enable Threshold
RUN/SS Pin Rising
4
4.5
V
VRUN/SS(LT)
RUN Pin Latchoff Threshold
RUN/SS Pin Falling
3.5
4.2
V
IRUN/SS(C)
Soft-Start Charge Current
VRUN/SS = 0V
–0.5
–1.2
–3
µA
IRUN/SS(D)
Soft-Start Discharge Current
VRUN/SS = 4.5V, VFB = 0V
0.8
1.8
3
µA
VIN(UVLO)
Undervoltage Lockout
INTVCC Falling
l
3.4
3.9
V
VIN(UVLOR)
Undervoltage Lockout Release
INTVCC Rising
l
3.5
4
V
RDS(ON)
Top Switch On-Resistance
Bottom Switch On-Resistance
10
8
19
14
mΩ
mΩ
l
l
l
5
8
11
16
A
A
5.5
7.5
A
A
7
10
13
%
0.8
1.5
2
V
3608fc
LTC3608
Electrical
Characteristics
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
5
5.5
V
–0.1
±2
%
Internal VCC Regulator
VINTVCC
Internal VCC Voltage
6V < VIN < 18V, VEXTVCC = 4V
ΔVLDO(LOADREG)
Internal VCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 4V
VEXTVCC
EXTVCC Switchover Voltage
ICC = 20mA, VEXTVCC Rising
ΔVEXTVCC
EXTVCC Switch Drop Voltage
ICC = 20mA, VEXTVCC = 5V
ΔVEXTVCC(HYS)
EXTVCC Switchover Hysteresis
l
l
4.7
4.5
4.7
150
V
300
mV
500
mV
PGOOD Output
ΔVFBH
PGOOD Upper Threshold
VFB Rising
7
10
13
%
ΔVFBL
PGOOD Lower Threshold
VFB Falling
–7
–10
–13
%
ΔVFB(HYS)
PGOOD Hysteresis
VFB Returning
1
2.5
%
VPGL
PGOOD Low Voltage
IPGOOD = 5mA
0.15
0.4
V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: TJ is calculated from the ambient temperature TA and power
dissipation PD as follows:
TJ = TA + (PD • 29°C/W)(θJA is simulated per JESD51-7 high effective thermal conductivity test board)
θJC = 1°C/W (θJC is simulated when heat sink is applied at the bottom of the package.)
Note 3: The LTC3608 is tested in a feedback loop that adjusts VFB to
achieve a specified error amplifier output voltage (ITH). The specification at
85°C is not tested in production. This specification is assured by design,
characterization, and correlation to testing at 125°C.
Note 4: The LTC3608 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC3608E is guaranteed to meet specifications from
0°C to 125°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3608I is guaranteed over the full –40°C to 125°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.
typical performance characteristics
Transient Response
Transient Response
VOUT
200mV/DIV
Start-Up
VOUT
200mV/DIV
RUN/SS
2V/DIV
IL
5A/DIV
IL
5A/DIV
VOUT
1V/DIV
ILOAD
5A/DIV
ILOAD
5A/DIV
20µs/DIV
LOAD STEP 0A TO 8A
VIN = 12V
VOUT = 2.5V
FCB = 0V
FIGURE 6 CIRCUIT
IL
5A/DIV
3608 G01
20µs/DIV
VIN = 12V
VOUT = 2.5V
FCB = INTVCC
FIGURE 6 CIRCUIT
3610 G02
40ms/DIV
VIN = 12V
VOUT = 2.5V
RLOAD = 0.5Ω
FIGURE 6 CIRCUIT
3608 G03
3608fc
LTC3608
Typical Performance Characteristics
Efficiency vs Load Current
Efficiency vs Input Voltage
100
100
40
30
20
10 VIN = 12V
FREQ = 550kHz
0
0.01
0.1
1
LOAD CURRENT (A)
VOUT = 5V
VOUT = 3.3V
VOUT = 2.5V
VOUT = 2.5V
VOUT = 1.8V
VOUT = 1.2V
VOUT = 1V
ILOAD = 1A
85
80
10
550
500
ILOAD = 1A
5
400
20
10
15
INPUT VOLTAGE (V)
0.80
5
10
15
INPUT VOLTAGE (V)
3608 G05
Load Regulation
2.5
FIGURE 6 CIRCUIT
20
3608 G06
ITH Voltage vs Load Current
0.60
600
CONTINUOUS MODE
DISCONTINUOUS MODE
ITH VOLTAGE (V)
400
300
2.0
0.40
ΔVOUT (%)
500
0.20
0
–0.20
200
–0.40
100
1.5
CONTINUOUS
MODE
1.0
DISCONTINUOUS
MODE
0.5
–0.60
0
2
4
6
LOAD CURRENT (A)
8
10
–0.80
25
VRNG =
20
0
3608 G07
Load Current vs ITH Voltage and VRNG
2
4
6
LOAD CURRENT (A)
8
0
10
On-Time vs ION Current
10000
5
LOAD CURRENT (A)
10
3608 G09
On-Time vs VON Voltage
1000
VVON = 0V
1V
0
3608 G08
ION = 30µA
800
0.7V
ON-TIME (ns)
0.5V
10
5
0
1000
ON-TIME (ns)
15
100
600
400
200
–5
–10
ILOAD = 10A
450
3608 G04
700
FREQUENCY (kHz)
ILOAD = 10A
90
Frequency vs Load Current
LOAD CURRENT (A)
FREQUENCY (kHz)
50
EFFICIENCY (%)
EFFICIENCY (%)
60
FCB = 0V
FIGURE 6 CIRCUIT
600
95
70
0
650
FCB = 5V
FIGURE 6 CIRCUIT
90
80
Frequency vs Input Voltage
0
0.5
1.0
1.5
2.0
ITH VOLTAGE (V)
2.5
3.0
3608 G10
10
1
10
ION CURRENT (µA)
100
3608 G11
0
0
1
2
VON VOLTAGE (V)
3
3608 G12
3608fc
LTC3608
Typical Performance Characteristics
200
150
100
50
0
–50 –25
25
75
0
50
TEMPERATURE (°C)
100
15
10
0.7
0.8
VRNG VOLTAGE (V)
0.9
MAXIMUM VALLEY CURRENT (A)
15
10
5
0
50
75
25
TEMPERATURE (°C)
100
3608 G14
3
3608 G15
Maximum Valley Current Limit in Foldback
125
16
14
12
10
8
6
4
4
8
3608 G16
20
12
16
INPUT VOLTAGE (V)
2.0
0.62
10
5
0
0
0.1
0.2
3608 G17
0.3
VFB (V)
0.4
0.5
0.6
3608 G18
Error Amplifier gm vs Temperature
1.8
0.61
gm (mS)
FEEDBACK REFERENCE VOLTAGE (V)
6
15
Feedback Reference Voltage vs Temperature
0.60
0.59
0.58
–50
9
0
1.65 1.90 2.15 2.40 2.65 2.90 3.15 3.40
RUN/SS VOLTAGE (V)
1.0
18
–25
12
Input Voltage vs Maximum Valley Current
20
MAXIMUM VALLEY CURRENT LIMIT (A)
0.6
0.5
3608 G13
Maximum Valley Current Limit vs Temperature
0
–50
15
20
5
125
Maximum Valley Current Limit vs RUN/SS Voltage
MAXIMUM VALLEY CURRENT LIMIT (A)
ON-TIME (ns)
250
25
IION = 30µA
VVON = 0V
MAXIMUM VALLEY CURRENT LIMIT (A)
300
Maximum Valley Current Limit vs VRNG Voltage
MAXIMUM VALLEY CURRENT LIMIT (A)
On-Time vs Temperature
1.6
1.4
1.2
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3608 G19
1.0
–50
–25
50
0
75
25
TEMPERATURE (°C)
100
125
3608 G20
3608fc
LTC3608
Typical Performance Characteristics
Input and Shutdown Currents vs Input Voltage
0.20
25
800
20
SHUTDOWN
600
15
400
10
EXTVCC = 5V
0
5
10
15
INPUT VOLTAGE (V)
20
14
0
–0.30
0
–0.40
FCB PIN CURRENT (µA)
4
2
2
0
40
10
20
30
INTVCC LOAD CURRENT (mA)
100
125
–0.25
2
–0.50
–0.75
–1.00
3608 G24
25
75
0
50
TEMPERATURE (°C)
100
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
RUN/SS PIN CURRENT (µA)
4.5
LATCHOFF ENABLE
4.0
LATCHOFF THRESHOLD
75
0
25
50
TEMPERATURE (°C)
900
1000
3608 G23
125
PULL-DOWN CURRENT
1
0
–1
–2
–50
PULL-UP CURRENT
–25
3608 G25
0
50
75
25
TEMPERATURE (°C)
100
125
3608 G26
Undervoltage Lockout Threshold
vs Temperature
5.0
–25
600
700
800
FREQUENCY (kHz)
RUN/SS Pin Current vs Temperature
3
–1.50
–50 –25
500
3608 G22
0
RUN/SS Pin Current vs Temperature
3.0
–50
0
400
50
–1.25
3.5
8
FCB Pin Current vs Temperature
6
0
50
75
25
TEMPERATURE (°C)
10
4
3608 G21
8
–25
12
6
–0.20
5
VIN = 20V
16
–0.10
10
EXTVCC SWITCH RESISTANCE (Ω)
18
0.10
EXTVCC Switch Resistance vs Temperature
0
–50
20
RUN/SS PIN CURRENT (µA)
200
SHUTDOWN CURRENT (µA)
INPUT CURRENT (µA)
35
30
1000
0
0.30
IEXTVCC (mA)
EXTVCC OPEN
1200
40
ΔINTVCC (%)
1400
IEXTVCC vs Frequency
INTVCC Load Regulation
100
125
3608 G27
4.0
3.5
3.0
2.5
2.0
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3608 G28
3608fc
LTC3608
Pin Functions
PVIN (Pins 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53):
Main Input Supply. Decouple this pin to power PGND with
the input capacitance, CIN
SW (Pins 8, 33, 41, 42, 43, 44, 45, 46, 47, 55): Switch
Node Connection to the Inductor. The (–) terminal of the
bootstrap capacitor, CB, also connects here. This pin swings
from a diode voltage drop below ground up to VIN.
SGND (Pins 10, 14, 15, 20, 26, 27, 54): Signal Ground. All
small-signal components and compensation components
should connect to this ground, which in turn connects to
PGND at one point.
BOOST (Pin 11): Boosted Floating Driver Supply. The (+)
terminal of the bootstrap capacitor, CB, connects here.
This pin swings from a diode voltage drop below INTVCC
up to VIN + INTVCC.
RUN/SS (Pin 12): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/µF) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.
VON (Pin 13): On-Time Voltage Input. Voltage trip point for
the on-time comparator. Tying this pin to the output voltage or an external resistive divider from the output makes
the on-time proportional to VOUT. The comparator input
defaults to 0.7V when the pin is grounded and defaults to
2.4V when the pin is tied to INTVCC. Tie this pin to INTVCC
in high VOUT applications to use a lower RON value.
PGOOD (Pin 16): Power Good Output. Open-drain logic
output that is pulled to ground when the output voltage
is not within ± 10% of the regulation point.
VRNG (Pin 17): Current Limit Range Input. The voltage at
this pin adjusts maximum valley current and can be set
from 0.5V to 0.7V by a resistive divider from INTVCC. It
defaults to 0.7V if the VRNG pin is tied to ground which
results in a typical 16A current limit.
ITH (Pin 18): Current Control Threshold and Error Amplifier
Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V with 0.8V corresponding to zero sense
voltage (zero current).
FCB (Pin 19): Forced Continuous Input. Tie this pin to
ground to force continuous synchronous operation at low
load, to INTVCC to enable discontinuous mode operation at
low load or to a resistive divider from a secondary output
when using a secondary winding.
NC (Pins 9, 21, 24, 25, 28): No Connection.
ION (Pin 22): On-Time Current Input. Tie a resistor from VIN
to this pin to set the one-shot timer current and thereby
set the switching frequency.
VFB (Pin 23): Error Amplifier Feedback Input. This pin
connects the error amplifier input to an external resistive
divider from VOUT.
EXTVCC (Pin 29): External VCC Input. When EXTVCC exceeds
4.7V, an internal switch connects this pin to INTVCC and
shuts down the internal regulator so that controller and
gate drive power is drawn from EXTVCC. Do not exceed
7V at this pin and ensure that EXTVCC < VIN.
SVIN (Pin 30): Supply Pin for Internal PWM Controller.
INTVCC (Pins 31, 32): Internal 5V Regulator Output. The
driver and control circuits are powered from this voltage.
Decouple this pin to power ground with a minimum of
4.7µF low ESR tantalum or ceramic capacitor.
PGND (Pins 34, 35, 36, 37, 38, 39, 40): Power Ground.
Connect this pin closely to the (–) terminal of CVCC and
the (–) terminal of CIN.
3608fc
LTC3608
Functional Diagram
RON
VON
ION
13
22
FCB
EXTVCC
19
29
SVIN
30
4.7V
0.7V
2.4V
+
1µA
PVIN
–
1, 2, 3, 4, 5, 6,
7, 48, 49, 50,
51, 52, 53
0.6V
REF
0.6V
CIN
5V
REG
INTVCC
+
–
31, 32
F
11
VVON
tON =
(10pF)
IION
R
S
Q
FCNT
SW
+
ICMP
–
L1
DB
VOUT
8, 33, 41, 42,
43, 44, 45,
46, 47, 55
SWITCH
LOGIC
IREV
–
SHDN
1.4V
COUT
OV
M2
+
CVCC
17
PGND
×
(0.5 TO 2)
0.7V
34, 35, 36, 37,
38, 39, 40
16 PGOOD
1
240k
+
1V
Q2 Q4
–
Q6
ITHB
R2
0.54V
UV
23
Q3 Q1
R1
SGND
OV
+
–
VFB
+
–
0.8V
–
SS
+
1.2µA
+
–
6V
+
0.6V
18
ITH
10, 14, 15,
20, 26, 27, 54
0.66V
RUN
SHDN
EA
×3.3
–
VRNG
CB
M1
ON
20k
+
BOOST
0.4V
12
27 NC
9, 21, 24,
25, 28
3608 FD
RUN/SS
CSS
3608fc
LTC3608
Operation
Main Control Loop
The LTC3608 is a high efficiency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input voltage range of 4V to 18V (20V maximum) and provides a
regulated output voltage at up to 8A of output current. The
internal synchronous power switch increases efficiency
and eliminates the need for an external Schottky diode. In
normal operation, the top MOSFET is turned on for a fixed
interval determined by a one-shot timer OST. When the
top MOSFET is turned off, the bottom MOSFET is turned
on until the current comparator ICMP trips, restarting the
one-shot timer and initiating the next cycle. Inductor current
is determined by sensing the voltage between the PGND
and SW pins using the bottom MOSFET on-resistance.
The voltage on the ITH pin sets the comparator threshold
corresponding to inductor valley current. The error amplifier, EA, adjusts this voltage by comparing the feedback
signal VFB from the output voltage with an internal 0.6V
reference. If the load current increases, it causes a drop
in the feedback voltage relative to the reference. The ITH
voltage then rises until the average inductor current again
matches the load current.
At light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
comparator IREV which then shuts off M2 (see Functional Diagram), resulting in discontinuous operation. Both
switches will remain off with the output capacitor supplying
the load current until the ITH voltage rises above the zero
current level (0.8V) to initiate another cycle. Discontinuous mode operation is disabled by comparator F when
the FCB pin is brought below 0.6V, forcing continuous
synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to maintain regulation. The one-shot timer generates an on-time
that is proportional to the ideal duty cycle, thus holding
frequency approximately constant with changes in VIN.
The nominal frequency can be adjusted with an external
resistor, RON.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage exits a ±10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned
off and M2 is turned on and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is
shorted to ground. As VFB drops, the buffered current
threshold voltage ITHB is pulled down by clamp Q3 to
a 1V level set by Q4 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
VFB approaches 0V.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor, CSS. When this voltage
reaches 1.5V, the controller turns on and begins switching,
but with the ITH voltage clamped at approximately 0.6V
below the RUN/SS voltage. As CSS continues to charge,
the soft-start current limit is removed.
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTVCC
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor, CB. This capacitor is recharged from
INTVCC through an external Schottky diode, DB, when
the top MOSFET is turned off. When the EXTVCC pin is
grounded, an internal 5V low dropout regulator supplies
the INTVCC power from VIN. If EXTVCC rises above 4.7V,
the internal regulator is turned off, and an internal switch
connects EXTVCC to INTVCC. This allows a high efficiency
source connected to EXTVCC, such as an external 5V supply or a secondary output from the converter, to provide
the INTVCC power. Voltages up to 7V can be applied to
EXTVCC for additional gate drive. If the input voltage is
low and INTVCC drops below 3.5V, undervoltage lockout
circuitry prevents the power switches from turning on.
3608fc
10
LTC3608
Applications Information
The basic LTC3608 application circuit is shown on the
front page of this data sheet. External component selection
is primarily determined by the maximum load current.
The LTC3608 uses the on-resistance of the synchronous
power MOSFET for determining the inductor current. The
desired amount of ripple current and operating frequency
also determines the inductor value. Finally, CIN is selected
for its ability to handle the large RMS current into the
converter and COUT is chosen with low enough ESR to meet
the output voltage ripple and transient specification.
VON and PGOOD
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3608 applications is determined implicitly by the one-shot timer that controls the
on-time, tON, of the top MOSFET switch. The on-time is
set by the current into the ION pin and the voltage at the
VON pin according to:
VVON
(10pF)
IION
The LTC3608 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3608 also has a VON pin that
allows the on-time to be adjusted. Tying the VON pin high
results in lower values for RON which is useful in high VOUT
applications. The VON pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where VOUT changes and to correct minor
frequency shifts with changes in load current.
Tying a resistor RON from VIN to the ION pin yields an
on-time inversely proportional to VIN. The current out of
the ION pin is
V
ION = IN
RON
VRNG Pin and ILIMIT Adjust
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
The VRNG pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3608 can deliver. The
maximum output current is given by:
IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL
The IVALLEY(MAX) is shown in the figure “Maximum Valley
Current Limit vs VRNG Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTVCC can be used to
set the voltage on the VRNG pin from 0.5V to 1V, or it can
be simply tied to ground force a default value equivalent
to 0.7V. When setting current limit ensure that the junction temperature does not exceed the maximum rating of
125°C. Do not float the VRNG pin.
tON =
f=
VVON
VOUT
[H ]
RON (10pF) Z
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. The VON pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high VOUT applications, tying VON to INTVCC so that the
comparator input is 2.4V results in a lower value for RON.
Figures 1a and 1b show how RON relates to switching
frequency for several common output voltages.
3608fc
11
LTC3608
Applications Information
Because the voltage at the ION pin is about 0.7V, the current into this pin is not exactly inversely proportional to
VIN, especially in applications with lower input voltages.
To correct for this error, an additional resistor, RON2,
connected from the ION pin to the 5V INTVCC supply will
further stabilize the frequency.
RON2 =
5V
R
0.7V ON
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITH pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITH pin to the VON pin
as shown in Figure 2a. Place capacitance on the VON pin
to filter out the ITH variations at the switching frequency.
The resistor load on ITH reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 2b.
SWITCHING FREQUENCY (kHz)
1000
VOUT = 3.3V
VOUT = 1.5V
100
100
VOUT = 2.5V
1000
RON (kΩ)
10000
3608 F01a
Figure 1a. Switching Frequency vs RON (VON = 0V)
SWITCHING FREQUENCY (kHz)
1000
VOUT = 12V
VOUT = 5V
VOUT = 3.3V
100
100
1000
RON (kΩ)
10000
3608 F01b
Figure 1b. Switching Frequency vs RON (VON = INTVCC)
3608fc
12
LTC3608
Applications Information
2.0
CVON
0.01µF
RVON2
100k
RC
VON
LTC3608
ITH
CC
(2a)
VOUT
INTVCC
RVON1
3k
10k
CVON
0.01µF
RVON2
10k
RC
Q1
2N5087
CC
VON
LTC3608
ITH
1.0
0.5
0
0.25
0.50
0.75
DUTY CYCLE (VOUT/VIN)
1.0
3608 F03
Figure 3. Maximum Switching Frequency vs Duty Cycle
Minimum Off-time and Dropout Operation
The minimum off-time, tOFF(MIN), is the smallest amount
of time that the LTC3608 is capable of turning on the bottom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 320ns.
The minimum off-time limit imposes a maximum duty
cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
tON + tOFF(MIN)
tON
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3608 develops a 0.6V reference voltage between
the feedback pin, VFB, and the signal ground as shown in
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
⎛ R2 ⎞
VOUT = 0.6V ⎜ 1+ ⎟
⎝ R1⎠
DROPOUT
REGION
3608 F02
Figure 2. Correcting Frequency Shift with Load Current Changes
1.5
0
(2b)
VIN(MIN) = VOUT
SWITCHING FREQUENCY (MHz)
VOUT
RVON1
30k
To improve the frequency response, a feed forward capacitor C1 may also be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
Inductor Selection
Given the desired input and output voltages, the inductor value and operating frequency determine the ripple
current:
⎛V ⎞⎛ V ⎞
ΔIL = ⎜ OUT ⎟ ⎜ 1− OUT ⎟
VIN ⎠
⎝ f L ⎠⎝
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of IOUT(MAX). The largest ripple current
occurs at the highest VIN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
⎛ V
⎞⎛
VOUT ⎞
OUT
1−
L=⎜
⎟⎜
⎟
⎝ f ΔIL(MAX) ⎠ ⎝ VIN(MAX) ⎠
3608fc
13
LTC3608
Applications Information
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores.
A variety of inductors designed for high current, low voltage applications are available from manufacturers such as
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
CIN and COUT Selection
The input capacitance, CIN, is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
IRMS ≅IOUT(MAX)
VOUT
VIN
VIN
–1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/2. This simple worst-case condition is
commonly used for design because even significant deviations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
the capacitor.
The selection of COUT is primarily determined by the
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔVOUT is approximately
bounded by:
⎛
1 ⎞
ΔVOUT ≤ ΔIL ⎜ ESR +
8fCOUT ⎟⎠
⎝
Since ΔIL increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but
have lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications providing that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible
piezoelectric effects. The high Q of ceramic capacitors with
trace inductance can also lead to significant ringing. When
used as input capacitors, care must be taken to ensure
that ringing from inrush currents and switching does not
pose an overvoltage hazard to the power switches and
controller. To dampen input voltage transients, add a small
5µF to 50µF aluminum electrolytic capacitor with an ESR in
the range of 0.5Ω to 2Ω. High performance through-hole
capacitors may also be used, but an additional ceramic
capacitor in parallel is recommended to reduce the effect
of their lead inductance.
Top MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor, CB, connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode DB from INTVCC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to VIN and the BOOST pin rises
to approximately VIN + INTVCC. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications an 0.1µF to 0.47µF, X5R
or X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when inductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
with changes in VIN. Tying the FCB pin below the 0.6V
3608fc
14
LTC3608
Applications Information
Fault Conditions: Current Limit and Foldback
threshold forces continuous synchronous operation, allowing current to reverse at light loads and maintaining
high frequency operation.
The LTC3608 has a current mode controller which inherently limits the cycle-by-cycle inductor current not only
in steady state operation but also in transient. To further
limit current in the event of a short circuit to ground, the
LTC3608 includes foldback current limiting. If the output
falls by more than 25%, then the maximum sense voltage is
progressively lowered to about one sixth of its full value.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating
in discontinuous mode. The secondary output VOUT2 is
normally set as shown in Figure 4 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then VOUT2 will droop. An external
resistor divider from VOUT2 to the FCB pin sets a minimum
voltage VOUT2(MIN) below which continuous operation is
forced until VOUT2 has risen above its minimum:
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3608. The INTVCC pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum of
4.7µF tantalum or ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers.
⎛ R4 ⎞
VOUT2(MIN) = 0.6V ⎜ 1+ ⎟
⎝ R3 ⎠
= SGND
52
NC
SGND
SVIN
EXTVCC
INTVCC
SW
PGND
PGND
PGND
PGND
INTVCC
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
1
2
3
4
5
6
7
8
9 10 11 12 13 14
SW
26
25
24
23
22
21
20
19
18
17
16
R4
OPTIONAL EXTVCC
CONNECTION
5V < VOUT2 < 7V
R3
15
SGND
= PGND
SW
VON
51
NC
LTC3608
RUN/SS
50
SW
BOOST
49
ION
SGND
48
VFB
SW
NC
VIN
SW
SW
CIN
+
NC
PVIN
47
SW
PVIN
46
NC
PVIN
45
SW
PVIN
44
SGND
PVIN
43
PGND
PGND
•
+
SW
PVIN
COUT
41
42
T1
1:N
PVIN
VOUT1
+
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
VOUT2
CSEC
1µF
SW
GND
IN4148
•
INTVCC Regulator and EXTVCC Connection
3608 F04
SGND
Figure 4. Secondary Output Loop and EXTVCC Connection
3608fc
15
LTC3608
Applications Information
The EXTVCC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTVCC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTVCC
pin to INTVCC. INTVCC power is supplied from EXTVCC
until this pin drops below 4.5V. Do not apply more than
7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN. The
following list summarizes the possible connections for
EXTVCC:
1. EXTVCC grounded. INTVCC is always powered from the
internal 5V regulator.
2. EXTVCC connected to an external supply. A high efficiency
supply compatible with the MOSFET gate drive requirements (typically 5V) can improve overall efficiency.
3. EXTVCC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The
system will start-up using the internal linear regulator
until the boosted output supply is available.
additional 1.3s/µF, during which the load current is folded
back until the output reaches 75% of its final value.
After the controller has been started and given adequate
time to charge up the output capacitor, CSS is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging CSS. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
The overcurrent protection timer requires that the soft‑start
timing capacitor, CSS, be made large enough to guarantee
that the output is in regulation by the time CSS has reached
the 4V threshold. In general, this will depend upon the
size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor
can be estimated from:
CSS > COUT VOUT RSENSE (10 –4 [F/V s])
Soft-Start and Latchoff with the RUN/SS Pin
Generally 0.1µF is more than sufficient.
The RUN/SS pin provides a means to shut down the
LTC3608 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3608 into a low quiescent current shutdown (IQ <
30µA). Releasing the pin allows an internal 1.2µA current
source to charge up the external timing capacitor CSS. If
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short
circuit by the current foldback circuitry and latchoff operation can prove annoying during troubleshooting. The
feature can be overridden by adding a pull-up current
greater than 5µA to the RUN/SS pin. The additional current prevents the discharge of CSS during a fault and also
shortens the soft-start period. Using a resistor to VIN as
shown in Figure 5a is simple, but slightly increases shutdown current. Connecting a resistor to INTVCC as shown
in Figure 5b eliminates the additional shutdown current,
but requires a diode to isolate CSS. Any pull-up network
must be able to pull RUN/SS above the 4.2V maximum
threshold of the latchoff circuit and overcome the 4µA
maximum discharge current.
tDELAY =
1.5V
C = (1.3s/µF ) CSS
1.2µA SS
When the voltage on RUN/SS reaches 1.5V, the LTC3608
begins operating with a clamp on ITH of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH
is raised until its full 2.4V range is available. This takes an
3608fc
16
LTC3608
Applications Information
INTVCC
RSS*
VIN
3.3V OR 5V
D1
RUN/SS
RSS*
CSS
D2*
RUN/SS
2N7002
CSS
3608 F05
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
(5a)
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3608 circuits:
1. DC I2R losses. These arise from the resistance of the
internal resistance of the MOSFETs, inductor and PC
board traces and cause the efficiency to drop at high
output currents. In continuous mode the average output
current flows through L, but is chopped between the top
and bottom MOSFETs. The DC I2R loss for one MOSFET
can simply be determined by [RDS(ON) + RL] • IO.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region during switch node transitions. It depends upon
the input voltage, load current, driver strength and
MOSFET capacitance, among other factors. The loss
is significant at input voltages above 20V and can be
estimated from:
Transition Loss ≅ (1.7A–1) VIN2 IOUT CRSS f
3. INTVCC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by supplying INTVCC current through the EXTVCC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
4. CIN loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I2R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in fuses or
batteries.
Other losses, including COUT ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR), where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or discharge COUT generating a feedback error signal used by the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot
or ringing that would indicate a stability problem. The ITH
pin external components shown in Figure 6 will provide
adequate compensation for most applications. For a
detailed explanation of switching control loop theory see
Application Note 76.
3608fc
17
LTC3608
Applications Information
Design Example
Next, set up VRNG voltage and check the ILIMIT. Tying VRNG
to 0.5V will set the typical current limit to 11A, and tying
VRNG to GND will result in a typical current around 16A.
CIN is chosen for an RMS current rating of about 5A at
85°C. The output capacitors are chosen for a low ESR
of 0.002Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
As a design example, take a supply with the following
specifications: VIN = 5V to 20V (12V nominal), VOUT =
2.5V ± 5%, IOUT = 8A, f = 550kHz. First, calculate the timing resistor with VON = VOUT:
2.5V
RON =
≈ 187k
550kHz ) (10pF )(2.4V)
(
and choose the inductor for about 40% ripple current at
the maximum VIN:
2.5V
⎛ 2.5V ⎞
L=
1−
= 1.24µH
550kHz ) ( 0.4) ( 8A ) ⎜⎝
20V ⎟⎠
(
Selecting a standard value of 1.2µH results in a maximum
ripple current of:
2.5V
⎛ 2.5V ⎞
ΔIL =
1–
= 3A
550kHz ) (1.2µH) ⎜⎝ 12V ⎟⎠
(
ΔVOUT(RIPPLE)= ΔIL(MAX) (ESR)
= (3A) (0.002Ω) = 6mV
However, a 0A to 8A load step will cause an output change
of up to:
ΔVOUT(STEP) = ΔILOAD (ESR) = (8A) (0.002Ω) = 16mV
An optional 22µF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 6.
INTVCC
CVCC
4.7µF
6.3V
PGND
CF
RF1
0.47µF 1Ω
25V
SW
EXTVCC
C4
0.01µF
VIN
SGND
51
52
NC
SGND
SVIN
EXTVCC
INTVCC
SW
INTVCC
PGND
PGND
PGND
PGND
PGND
SW
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
26
25
24
R1
9.5k
1%
C1
1
2
3
4
5
6
7
8
9 10 11 12 13 14
RON
187k
1%
(OPTIONAL)
23
22
21
CON
0.01µF
20
INTVCC
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
DB
CMDSH-3
C2
VOUT
CC1
1500pF
17
16
R3
0Ω
15
RPG1
100k
INTVCC
C3
CC2
100pF
(OPTIONAL)
SGND
RVON
0Ω
RSS1
510k
CB1
0.22µF
SW
(OPTIONAL)
VIN
R5
11.3k
18
SW
CIN: TAIYO YUDEN GMK325BJ106MM-B
COUT: TDKC2012X5ROJ226M
L1: CDEP85NP-R80MC-50
C5: MURATA GRM31CR60J226KE19
R2
30.1k
1%
(OPTIONAL)
19
SGND
(OPTIONAL)
50
LTC3608
VON
+
NC
RUN/SS
C6
10µF
35V
49
ION
SW
BOOST
CIN
10µF
35V
3×
GND
48
SW
SGND
47
VIN
VIN
5V TO 18V
VFB
NC
46
SW
SW
45
NC
PVIN
44
(OPTIONAL)
GND
SW
PVIN
43
NC
PVIN
+
SGND
SW
PVIN
COUT1
100µF
×2
SW
PVIN
C5
22µF
6.3V
42
PVIN
41
L1
0.8µH
PVIN
VOUT
2.5V AT
8A
PGND
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
CSS
0.1µF
VIN
(OPTIONAL)
VOUT
0.1µF
3608 F06
= PGND
= SGND
Figure 6. Design Example: 5V to 18V Input to 2.5V/8A at 550kHz
18
3608fc
LTC3608
Applications Information
How to Reduce SW Ringing
As with any switching regulator, there will be voltage ringing on the SW node, especially for high input voltages.
The ringing amplitude and duration is dependent on the
switching speed (gate drive), layout (parasitic inductance)
and MOSFET output capacitance. This ringing contributes
to the overall EMI, noise and high frequency ripple. One
way to reduce ringing is to optimize layout. A good layout
minimizes parasitic inductance. Adding RC snubbers from
SW to GND is also an effective way to reduce ringing. Finally,
adding a resistor in series with the BOOST pin will slow
down the MOSFET turn-on slew rate to dampen ringing,
but at the cost of reduced efficiency. Note that since the
IC is buffered from the high frequency transients by PCB
and bondwire inductances, the ringing by itself is normally
not a concern for controller reliability.
PC Board Layout Checklist
When laying out a PC board follow one of the two suggested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents, a
multilayer board is recommended to help with heat sinking
of power components.
• The ground plane layer should not have any traces and
it should be as close as possible to the layer with the
LTC3608.
• Place CIN and COUT all in one compact area, close to
the LTC3608. It may help to have some components
on the bottom side of the board.
• Use a compact plane for the switch node (SW) to improve
cooling of the MOSFETs and to keep EMI down.
• Use planes for VIN and VOUT to maintain good voltage
filtering and to keep power losses low.
• Flood all unused areas on all layers with copper. Flooding with copper reduces the temperature rise of power
components. Connect these copper areas to any DC
net (VIN, VOUT, GND or to any other DC rail in your
system).
When laying out a printed circuit board without a ground
plane, use the following checklist to ensure proper operation of the controller. These items are also illustrated in
Figure 7.
• Segregate the signal and power grounds. All smallsignal components should return to the SGND pin at
one point, which is then tied to the PGND pin.
• Connect the input capacitor(s), CIN, close to the IC.
This capacitor carries the MOSFET AC current.
• Keep the high dV/dT SW, BOOST and TG nodes away
from sensitive small-signal nodes.
• Connect the INTVCC decoupling capacitor, CVCC, closely
to the INTVCC and PGND pins.
• Connect the top driver boost capacitor, CB, closely to
the BOOST and SW pins.
• Connect the VIN pin decoupling capacitor, CF , closely
to the VIN and PGND pins.
• Keep small-signal components close to the LTC3608.
• Ground connections (including LTC3608 SGND and
PGND) should be made through immediate vias to
the ground plane. Use several larger vias for power
components.
3608fc
19
LTC3608
Applications Information
CVCC
SW
VIN
NC
SGND
SVIN
EXTVCC
INTVCC
SW
INTVCC
PGND
PGND
PGND
PGND
PGND
SW
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
SGND
52
NC
LTC3608
1
2
3
4
5
6
7
8
9 10 11 12 13 14
VON
51
SW
RUN/SS
50
CIN
ION
BOOST
49
SW
SGND
48
VFB
NC
47
SW
SW
46
NC
PVIN
VOUT
SW
PVIN
45
NC
PVIN
44
SW
PVIN
43
SGND
PVIN
COUT
SW
PVIN
42
PVIN
41
PGND
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
26
25
24
23
R1
R2
RON
22
21
20
19
18
RC
CC1
17
16
15
CC2
CB
DB
CSS
3608 F07
Figure 7. LTC3608 Layout Diagram
3608fc
20
LTC3608
Typical Applications
3.6V Input to 1.5V/8A at 750kHz
VIN2 = 5V
INTVCC
CVCC
4.7µF
6.3V
PGND
EXTVCC
C4
0.01µF
CF
0.47µF
25V
SW
SGND
50
51
52
(OPTIONAL)
NC
SGND
SVIN
EXTVCC
INTVCC
SW
PGND
PGND
PGND
PGND
PGND
PGND
INTVCC
NC
LTC3608
SW
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
1
2
3
4
5
6
7
8
9 10 11 12 13 14
CIN: TAIYO YUDEN TMK432BJ106MM
COUT1: TDKC4532X5ROJ107M
L1: CDEP85NP-R20MC-50
C5: TAIYO YUDEN JMK316BJ226ML-T
INTVCC
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
26
25
24
R1
20.5k
1%
C1
RON
113k
1%
(OPTIONAL)
23
22
21
CON
0.01µF
20
17
39.2k
16
(OPTIONAL)
C2
VOUT
VIN
R5
6.19k
18
15
R2
30.1k
1%
(OPTIONAL)
19
CC1
3300pF
INTVCC
11k
RPG1
100k
SGND
49
SW
VON
GND
ION
RUN/SS
+
SW
BOOST
C6
10µF
10V
VFB
SGND
CIN
10µF
3×
48
SW
NC
47
VIN
VIN
3.6V
NC
SW
46
SW
PVIN
45
NC
PVIN
44
(OPTIONAL)
GND
SGND
SW
PVIN
43
SW
PVIN
COUT1
100µF
×2
42
PVIN
C5
22µF
6.3V
L1
0.2µH
PVIN
41
+
PVIN
VOUT
1.5V AT
8A
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
CC2
100pF
INTVCC
SGND
VOUT
CB1
0.22µF
VOUT
RSS1
510k
CSS
0.1µF
0.1µF
VIN
(OPTIONAL)
3608 TA02
= PGND
= SGND
Transient Response
Efficiency Curve
100
95
90
EFFICIENCY (%)
IL
5A/DIV
VOUT
200mV/DIV
20µs/DIV
LOAD STEP 1A-8A
VIN = 3.6V
VOUT = 1.5V
FCB = 0V
3608 TA02a
85
DCM
CCM
80
75
70
65
60
55
50
100
VIN = 3.6V
FREQ = 750kHz
1000
1000
LOAD CURRENT (A)
10000
3608 TA02b
3608fc
21
LTC3608
Typical Applications
5V to 18V Input to 1.2V/8A at 550kHz
RF1
1Ω
INTVCC
CVCC
4.7µF
6.3V
PGND
VIN2
EXTVCC
C4
0.01µF
CF
0.47µF
25V
SW
SGND
50
51
52
(OPTIONAL)
NC
SGND
SVIN
EXTVCC
INTVCC
SW
INTVCC
PGND
PGND
PGND
PGND
PGND
PGND
NC
LTC3608
SW
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
1
2
3
4
5
6
7
8
9 10 11 12 13 14
C5: TAIYO YUDEN JMK316BJ226ML-T
CIN: TAIYO YUDEN TMK432BJ106MM
COUT1: TDKC4532X5R107M
L1: CDEP85NP-R50MC-125
R1
30k
1%
25
24
23
DB
CMDSH-3
C1
RON
187k
1%
(OPTIONAL)
22
21
CON
0.01µF
20
R2
30.1k
1%
(OPTIONAL)
C2
VOUT
VIN
(OPTIONAL)
19
R5
7.68k
18
CC1
1500pF
17
16
15
RPG1
100k
CC2
100pF
INTVCC
SGND
VOUT
CB1
0.22µF
INTVCC
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
26
SGND
+
SW
VON
C6
10µF
35V
49
ION
RUN/SS
CIN
10µF
25V
3×
GND
SW
BOOST
VIN
5V TO 18V
48
VFB
SGND
47
VIN
SW
NC
46
NC
SW
45
NC
SW
PVIN
44
(OPTIONAL)
GND
SW
PVIN
43
PVIN
+
SGND
PVIN
COUT1
100µF
×2
SW
PVIN
C5
22µF
6.3V
42
PVIN
41
L1
0.5µH
PVIN
VOUT
1.2V AT
8A
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
RSS1
510k
CSS
0.1µF
RVON
0.1µF
VOUT
(OPTIONAL)
CVON
VIN
(OPTIONAL)
3608 TA03
= PGND
= SGND
Transient Response
Efficiency vs Load Current
90
VIN = 12V
85 FREQ = 550kHz
80
EFFICIENCY (%)
IL
5A/DIV
VOUT
200mV/DIV
75
70
65
DCM
OCM
60
20µs/DIV
LOAD STEP 1A-8A
VIN = 12V
VOUT = 1.2V
FCB = 0V
3608 TA03a
55
50
100
1000
1000
LOAD CURRENT (A)
10000
3608 TA03b
3608fc
22
LTC3608
Typical Applications
5V to 18V Input to 1.8V/8A All Ceramic 1MHz
RF1
1Ω
INTVCC
EXTVCC
C4
0.01µF
CF
0.1µF
25V
CVCC
4.7µF
6.3V
PGND
VIN
SW
SGND
52
NC
SGND
SVIN
EXTVCC
INTVCC
SW
INTVCC
PGND
PGND
PGND
PGND
PGND
PGND
SW
SGND
PVIN
FCB
PVIN
ITH
PVIN
VRNG
PVIN
PGOOD
PVIN
SGND
1
2
3
4
5
6
7
8
9 10 11 12 13 14
COUT: TDKC3225XROJ107M
L1: VISHAY IHLP2525-R47
C5: TAIYO YUDEN JMK316BJ226ML-T
= PGND
R1
15k
1%
25
24
C1
DB
CMDSH-3
RON
102k
1%
(OPTIONAL)
23
22
21
CON
0.01µF
20
R2
30.1k
1%
(OPTIONAL)
C2
VOUT
VIN
(OPTIONAL)
19
R5
5.76k
18
CC1
1500pF
17
16
15
RPG1
100k
CC2
100pF
INTVCC
SGND
VOUT
CB1
0.22µF
INTVCC
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
26
SGND
51
NC
LTC3608
VON
50
ION
SW
RUN/SS
49
CIN
10µF
25V
3×
SW
BOOST
48
VFB
SGND
47
VIN
VIN
5V TO 18V
SW
NC
46
NC
SW
45
SW
PVIN
44
(OPTIONAL)
GND
NC
PVIN
43
SW
PVIN
+
SGND
PVIN
COUT1
100µF
×2
SW
PVIN
C5
22µF
6.3V
42
PVIN
41
L1
0.47µH
PVIN
VOUT
1.8V AT
8A
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
VOUT
RSS1
510k
CSS
0.1µF
0.1µF
VIN
(OPTIONAL)
3608 TA04
= SGND
Transient Response
Efficiency vs Load Current
90
80
EFFICIENCY (%)
IL
5A/DIV
VOUT
200mV/DIV
20µs/DIV
LOAD STEP 1A-5A
VIN = 12V
VOUT = 1.8V
FCB = 0V
3608 TA04a
DCM
CCM
70
60
50
40
30
100
VIN = 12V
1000
LOAD CURRENT (mA)
10000
3608 TA04b
3608fc
23
LTC3608
Package Description
WKG Package
52-Lead QFN Multipad (7mm × 8mm)
(Reference LTC DWG # 05-08-1768 Rev Ø)
SEATING PLANE
A
7.00
BSC
0.00 – 0.05
2.625 REF
41
B
PAD 1
CORNER
4
2.90 REF
0.50 BSC
40
bbb M C A B
7
8.00
BSC
PIN 1 ID
52
1
3.40 REF
3.20 ± 0.10
2.025
± 0.10
2.925 ± 0.10
3.90 ± 0.10
3.40 REF
33
8
32
9
1.00 REF
10
aaa C 2x
NX b
TOP VIEW
0.90 ± 0.10
NX
0.08 C
// ccc C
8
7.50 ± 0.05
2.90 REF
0.50 BSC
1.35
± 0.10
1.775
REF
15
0.25 ± 0.05
BOTTOM VIEW
(BOTTOM METALLIZATION DETAILS)
MLP52 QFN REV Ø 0807
NOTE:
1. DIMENSIONING AND TOLERANCING CONFORM TO ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS, ANGLES ARE IN DEGREES (°)
3. N IS THE TOTAL NUMBER OF TERMINALS
4
2.025
± 0.10
3.40 REF
2.925 ± 0.10
3.90 ± 0.10
8.50 ± 0.05
4.275 ± 0.10
PACKAGE
OUTLINE
0.40 ± 0.10
1.775
REF
1.35
± 0.10
THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING
CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002
5. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY
6. NJR REFER TO NON JEDEC REGISTERED
7
DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED
BETWEEN 0.20mm AND 0.30mm FROM THE TERMINAL TIP. IF THE TERMINAL
HAS THE OPTIONAL RADIUS ON THE OTHER END OF THE TERMINAL, THE
DIMENSION b SHOULD NOT BE MEASURED IN THAT RADIUS AREA.
8
COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE
METALLIZATION
9
DRAWING SHOWN ARE FOR ILLUSTRATION ONLY
1.00 REF
0.25 ± 0.05
0.40 ± 0.10
19
2.625 REF
PIN 1
2.25 ± 0.10
14
26
9
3.40 REF
2.25 ± 0.10
27
0.580 ± 0.10
aaa C 2x
3.20 ± 0.10
4.275 ± 0.10
SYMBOL TOLERANCE
aaa
0.15
bbb
0.10
ccc
0.10
RECOMMENDED SOLDER PAD LAYOUT
TOP VIEW
3608fc
24
LTC3608
Revision History
(Revision history begins at Rev C)
REV
DATE
DESCRIPTION
C
06/10
Updated SW voltage range in Absolute Maximum Ratings.
PAGE NUMBER
2
Note 4 updated.
4
3608fc
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
25
LTC3608
Typical Application
14V to 18V Input to 12V/5A at 500kHz
CVCC
4.7µF, 6.3V
PGND
INTVCC
SW
SGND
42
43
44
45
46
47
NC
SGND
SVIN
EXTVCC
INTVCC
SW
PGND
PGND
PGND
PGND
PGND
VFB
SW
ION
SW
NC
LTC3608
SW
48
SGND
PVIN
CIN: TAIYO YUDEN TMK432BJ106MM
COUT: SANYO 16SVP180MX
L1: CDEP85NP-4R3MC-88
PVIN
PGOOD
PVIN
SGND
2
3
4
5
6
7
8
9 10 11 12 13 14
25
24
DB
CMDSH-3
R1
3.16k
1%
C1
22
21
CON
0.01µF
20
R5
24.9k
18
17
16
15
90.9k
90
EFFICIENCY (%)
20µs/DIV
LOAD STEP 1A-8A
VIN = 18V
VOUT = 12V
FCB = 0V
VOUT
VIN
CC1
3300pF
INTVCC
RPG1
100k
INTVCC
CC2
100pF
SGND
INTVCC
(OPTIONAL)
VIN
CVON
(OPTIONAL)
RUN/SS
3608 TA05
100
95
3608 TA05a
C2
Efficiency Curve
Transient Response
VOUT
200mV/DIV
(OPTIONAL)
10k
= SGND
IL
5A/DIV
R2
60.4k
1%
(OPTIONAL)
19
RSS1
510k
CSS
0.1µF
RON
1M
1%
(OPTIONAL)
23
CB1
0.22µF
INTVCC
= PGND
VON
1
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
26
SGND
VRNG
RUN/SS
ITH
PVIN
BOOST
52
PVIN
SGND
50
FCB
NC
49
51
(OPTIONAL)
SW
SW
+
NC
PVIN
C6
10µF
35V
SW
PVIN
CIN
10µF
25V
3×
GND
NC
PVIN
VIN
VIN
14V TO 18V
SGND
SW
PVIN
(OPTIONAL)
GND
SW
PVIN
L1
4.3µH
PVIN
+
PVIN
COUT1
180µF
16V
INTVCC
41
C5
22µF
25V
PGND
PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
VOUT
12V AT
5A
EXTVCC
C4
0.01µF
VIN2
RF1
CF
0.1µF 1Ω
25V
85
80
DCM
CCM
75
70
65
60
55
50
100
VIN = 18V
FREQ = 500kHz
1000
1000
LOAD CURRENT (A)
10000
3608 TA05b
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LTC1778
No RSENSE Current Mode Synchronous Step-Down Controller
Up to 97% Efficiency, VIN: 4V to 36V, 0.8V ≤ VOUT ≤ (0.9)(VIN), IOUT Up
to 20A
LTC3414
4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter
95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64µA, ISD:
<1µA, TSSOP20E Package
LTC3418
8A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter
95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, Thermally
Enhanced 38-Lead QFN Package
LTC3610
12A Current Mode Monolithic Synchronous Step-Down
Converter
Up to 24V Input (28V Maximum). Current Mode Extremely Fast
Transient Response
LTM4600HV
10A Complete Switch Mode Power Supply
92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode
Control, Ultrafast Transient Response
LTM4601HV
12A Complete Switch Mode Power Supply
92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode
Control, Ultrafast Transient Response
LTM4603HV
6A Complete Switch Mode Power Supply
93% Efficiency, VIN: 4.5V to 28V, with PLL, Output Tracking and
Margining with Ultrafast Transient Response
3608fc
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
l
FAX: (408) 434-0507 l www.linear.com
LT 0610 REV C • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2008
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