MAX8540, MAX8541

MAX8540, MAX8541
19-2830; Rev 0; 4/03
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
The MAX8540/MAX8541 pulse-width-modulated (PWM)
controllers for forward/flyback isolated DC-to-DC converters provide maximum flexibility to power-supply designers and reduce external component count. These
controllers allow common PC board layout for currentmode (MAX8540) and voltage-mode (MAX8541) designs.
Both controllers feature adjustable switching frequency
and external synchronization from 200kHz to 1MHz.
Active-high and active-low enable, undervoltage protection (UVP), and overvoltage protection (OVP) reduce
external component count.
Maximum duty cycle is adjustable, and the feed-forward
function scales the maximum duty cycle with input voltage to limit the maximum volt-seconds applied to the
transformer primary. The MAX8540 allows the user to
select the value of slope compensation to further optimize magnetics design. The MAX8541 features useradjustable ramp magnitude for the PWM comparator.
A cycle-by-cycle current-limit function controls the peak
primary current during overload and short circuit. Both
controllers can be set to latch off or to hiccup when a
short circuit is detected. The number of current-limited
cycles to initiate the hiccup mode and number of
cycles skipped are user selectable to allow startup with
high-capacitance loads and protect the converter
against sustained short circuits.
The MAX8540/MAX8541 feature 9Ω internal gate drivers for low-power applications and are compatible
with external gate drivers for high-power applications.
Both devices are available in the space-saving 16-pin
QSOP package.
The MAX8541 EV kit evaluates the MAX8540 and the
MAX8541 to speed designs. Design examples are also
available in application notes: 50W Voltage-Mode
Forward Converter Design with the MAX8541 and 50W
Current-Mode Forward Converter Design with the
MAX8540.
Applications
Isolated DC-to-DC Modules (Bricks)
Cellular Base Stations
Telecom and Network Systems
High-Performance Off-Line AC/DC Converters
Features
♦ 200kHz to 1MHz Adjustable Switching Frequency
♦ Synchronization to External Clock
♦ Programmable, Constant Maximum Volt-Second
Simplifies Transformer Design
♦ Programmable Hiccup/Skip Cycles or Latch-Mode
Protection
♦ Clean Startups with High-Capacitance Loads
♦ Programmable Under/Overvoltage Protection
♦ Current-Mode, Adjustable Slope Compensation
(MAX8540)
♦ Voltage-Mode, Adjustable Ramp Magnitude
(MAX8541)
♦ 10mA, 5V Regulator
♦ 75mV to 1.25mV Adjustable Current Limit
Reduces External Components
♦ Internal Gate Driver for Low-Power Applications
♦ External Gate Driver for High-Power Applications
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX8540EEE
-40°C to +85°C
16 QSOP
MAX8541EEE
-40°C to +85°C
16 QSOP
Pin Configurations
TOP VIEW
REF 1
16 VCC
OV 2
15 DRV
UV 3
14 GND
FREQ/SYNC 4
MAX8540EEE
SS 5
13 SKTON
12 CS
MAXDTY 6
11 SKTOFF
EN 7
10 ILIM
SCOMP 8
9
OPTO
QSOP
Functional Diagrams and Typical Operating Circuit appear
at end of data sheet.
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX8540/MAX8541
General Description
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
ABSOLUTE MAXIMUM RATINGS
VCC to GND ............................................................-0.3V to +20V
OPTO, UV, OV, EN, PRAMP, ILIM, REF to GND ......-0.3V to +6V
SCOMP, FREQ, CS, SKTON, SKTOFF, SS,
MAXDTY to GND .....................................-0.3V to VREF + 0.3V
DRV to GND ................................................-0.3V to VVCC + 0.3V
Continuous Power Dissipation (TA = +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0,
RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
18.0
V
VCC
Operating Voltage Range
Undervoltage Lockout
(Note 1)
7.6
VCC rising
7.6
8.0
8.4
VCC falling
6.6
7.0
7.4
0.9
1.0
Undervoltage Lockout Hysteresis
Supply Current
VREF is active,
IREF = 0
Shutdown Current
V EN = 3V
No switching
Switching with no load
V
V
3.5
4.2
5
6.3
0.5
0.9
5.0
5.1
V
20
50
mV
mA
mA
REF
Output Voltage
IREF = 0
Load Regulation
IREF = 0 to 10 mA
4.9
Line Regulation
VVCC = 8.4V to 18V
5
20
mV
Pulldown Resistor in Shutdown
V EN = 3V
10
30
Ω
UNDERVOLTAGE/OVERVOLTAGE (UV/OV)
UVLO Threshold
VUV rising, 120mV typical hysteresis
1.200
1.250
1.300
VUV falling
1.083
1.128
1.173
-0.5
+0.01
+0.5
Input Bias Current
OVLO Threshold
VOV rising, 130mV typical hysteresis
VOV falling
2.9
3.021
3.1
2.778
2.894
3.010
V
µA
V
EN
Input Low Threshold
VIL
Input High Threshold
VIH
Pullup Current
V EN = 2.6V
1.2
2.6
5
V
V
17
µA
CURRENT LIMIT
VILIM Range
0.075
CS Current-Limit to DRV Turn-Off
Includes leading-edge blanking time
Delay
VCS Cycle-by-Cycle Threshold
Voltage
2
110
Accuracy when VILIM = 0.075V
VILIM = 1.250V
1.250
V
150
ns
±10
1.175
1.250
_______________________________________________________________________________________
%
1.330
V
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
(VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0,
RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
ILIM Leakage Current
MIN
TYP
-1
Hiccup Skip Off-Time
Hiccup Skip Off-Charging
CSKTOFF = 0.1µF (Note 3)
Hiccup Skip On-Time
CSKTON = 0.01µF (Note 3)
MAX
+1
90
0.9
1.5
UNITS
µA
ms
2.1
9
µA
ms
Hiccup Skip On-Charging
Current
1.5
2.5
3.5
µA
Hiccup SKTOFF Voltage
Threshold
2.3
2.5
2.7
V
OPTO
Input Bias Current
VOPTO = 3.5V
30
µA
Minimum Input Voltage
Minimum voltage for duty-cycle control
0.6
V
Maximum Input Voltage
Maximum voltage for duty-cycle control
2.6
V
RMAXDTY = 24.3kΩ, VUV = 1.3V
15
DUTY CYCLE
Maximum Duty Cycle
RMAXDTY = 130kΩ, VUV = 1.3V
80
VUV = 1.3V, RMAXDTY = 97.6kΩ
60
%
Internally programmed limit for maximum duty cycle at
all frequencies
79
80
82
VSS = 1V
4.5
5.5
6.5
Pulldown Resistance in Shutdown ISS = 10mA, V E N = 3V
10
30
Time
440
SOFT-START
Source Current
µA
Ω
ms/µF
FREQUENCY
RFREQ = 32.4kΩ
Initial Accuracy
Switching Frequency Range
280
300
RFREQ = 48.7kΩ
200
RFREQ = 8.87kΩ
1000
320
kHz
kHz
SCOMP (MAX8540)
Slope Compensation
RSCOMP = 14kΩ
1.5
RSCOMP = 204kΩ
0.1
RPRAMP = 12.4kΩ
2.5
V
PRAMP (MAX8541)
Ramp Voltage Amplitude
RPRAMP = 21kΩ
1
RPRAMP = 42kΩ
0.5
CDRV = 500pF, 10% to 90% of VVCC
30
CDRV = 2000pF, 10% to 90% of VVCC
57
V
DRV (MOSFET DRIVER)
Rise Time
ns
_______________________________________________________________________________________
3
MAX8540/MAX8541
ELECTRICAL CHARACTERISTICS (continued)
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0,
RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
Fall Time
CONDITIONS
MIN
TYP
CDRV = 500pF, 90% to 10% of VVCC
27
CDRV = 2000pF, 90% to 10% of VVCC
53
MAX
UNITS
ns
On-Resistance Pullup
9
Ω
On-Resistance Pulldown
7
Ω
SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION)
Frequency Range
Frequency syncs ±30% of frequency set by external R
on FREQ pin
FREQ Input Threshold
(Note 4)
0.2
1.0
MHz
2
3
V
THERMAL SHUTDOWN
Thermal Shutdown
TA rising, 20°C hysteresis
°C
170
ELECTRICAL CHARACTERISTICS
(VVCC = 12V, CREF = 1µF, VUV = VOV = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ,
RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40°C to +85°C,
unless otherwise noted.) (Note 5)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
VCC
Operating Voltage Range
Undervoltage Lockout
(Note 1)
7.6
18.0
VCC rising
7.6
8.4
VCC falling
6.6
7.4
Undervoltage Lockout Hysteresis
0.9
Supply Current
VREF = is active,
IREF = 0
Shutdown Current
V EN = 3V
V
V
No switching
4.2
Switching with no load
6.3
0.9
mA
mA
REF
Output Voltage
IREF = 0
5.1
V
Load Regulation
IREF = 0 to 10mA
4.9
50
mV
Line Regulation
VVCC = 8.4V to 18V
20
mV
Shutdown Resistor
V EN = 3V
30
Ω
UNDERVOLTAGE/OVERVOLTAGE (UV/OV)
UVLO Threshold
VUV falling, 120mV typical hysteresis
1.200
1.300
VUV rising
1.083
1.173
-0.5
+0.5
VOV falling, 130 mV typical hysteresis
2.901
3.142
VOV rising
2.778
3.010
Input Bias Current
OVLO Threshold
4
_______________________________________________________________________________________
V
µA
V
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
(VVCC = 12V, CREF = 1µF, VUV = VOV = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ,
RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40°C to +85°C,
unless otherwise noted.) (Note 5)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
EN
VIL
Input Threshold
VIH
Pullup Current
V EN = 2.6V
1.2
2.6
5
V
µA
CURRENT LIMIT
VILIM Range
0.075
CS Current Limit to DRV Delay
Includes leading-edge blanking time
VCS Cycle-by-Cycle Threshold Voltage
Accuracy when VILIM = 0.075V
VILIM = 1.250V
1.25
V
150
ns
±10
%
1.130
1.375
V
ILIM Leakage Current
-1
+1
µA
Hiccup Skip Off-Charging Current
0.9
2.1
µA
Hiccup Skip On-Charging Current
1.5
3.5
µA
Hiccup SKTOFF Voltage Threshold
2.3
2.7
V
DUTY CYCLE
Duty-Cycle Range
VUV = 1.3V
15
80
%
Maximum Duty Cycle
Internal programmed max duty at all frequencies
79
82
%
Source Current
VSS = 1V
4.5
6.5
µA
Pulldown Resistance in Shutdown
ISS = 10mA, V EN = 3V
30
Ω
280
320
kHz
0.2
1.0
MHz
2
3
V
SOFT-START
FREQUENCY
Initial Accuracy
RFREQ = 32.4kΩ
SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION)
Frequency Range
Frequency syncs ±30% of frequency set by
external R on FREQ pin
FREQ Input Threshold
(Note 4)
This is the VCC operating range after clearing the rising UVLO.
Guaranteed by design.
See the SKTON and SKTOFF section.
The minimum FREQ pulse must be 200ns in duration with a minimum magnitude of 3V plus VF of the external diode and
have a maximum duty cycle of 55%.
Note 5: Specifications to -40°C are guaranteed by design and not production tested.
Note 1:
Note 2:
Note 3:
Note 4:
_______________________________________________________________________________________
5
MAX8540/MAX8541
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VCC = 12V, TA = +25°C, unless otherwise specified.)
580
560
540
5.2
520
5.0
4.8
500
460
4.4
440
4.95
420
VEN = 0
4.0
VEN = 5V
400
-20
0
20
40
60
100
80
4.90
-40
-20
0
20
40
60
80
100
-20
0
20
40
60
80
TA (°C)
TA (°C)
MAXIMUM DUTY CYCLE
vs. TEMPERATURE
FREQUENCY
vs. TEMPERATURE
CURRENT-SENSE THRESHOLD
vs. TEMPERATURE
FREQUENCY (kHz)
80
RFREQ = NONE
75
70
65
130
800
125
RFREQ = 10kΩ
120
VCS (mV)
85
1000
600
400
RFREQ = 36.5kΩ
115
110
RFREQ = 20kΩ
60
100
MAX8540 toc06
MAX8540 toc04
90
200
105
55
RFREQ = 50kΩ
100
0
50
-40
-20
0
20
40
60
-40
100
80
-20
0
20
40
60
80
-20
0
20
4.0
180
MAX8540 toc08
190
60
OV/UV THRESHOLD VOLTAGE
vs. TEMPERATURE
MAX8540 toc07
200
40
TA (°C)
CURRENT-LIMIT DELAY TIME
vs. TEMPERATURE
3.5
3.0
170
OV/UV VOLTAGE (V)
DELAY TIME (ns)
-40
100
TA (°C)
TA (°C)
160
150
140
130
VOV
2.5
2.0
1.5
1.0
120
VUV
0.5
110
100
0
-40
-20
0
20
40
TA (°C)
6
-40
TA (°C)
MAX8540 toc05
-40
5.00
480
4.6
4.2
5.05
VREF (V)
5.4
IVCC (µA)
IVCC (mA)
5.6
5.10
MAX8540 toc02
5.8
REFERENCE VOLTAGE
vs. TEMPERATURE
600
MAX8540 toc01
6.0
SHUTDOWN VCC CURRENT
vs. TEMPERATURE
MAX8540 toc03
VCC NO-LOAD SWITCHING CURRENT
vs. TEMPERATURE
DMAX (%)
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
60
80
100
-40
-20
0
20
40
60
TA (°C)
_______________________________________________________________________________________
80
100
80
100
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
SKTON/SKTOFF TIME
vs. CAPACITANCE
FREQUENCY
vs. INPUT VOLTAGE
PROGRAMABLE LATCH-OFF DELAY
800
1000
VDRV
800
TIME (ms)
RFREQ = 10kΩ
600
400
600
400
2V/div
VCS
RFREQ = 20kΩ
200
200
RFREQ = 50kΩ
CSKTON = 0.01µF
0
0
9.7
11.8
13.8
15.9
0
18.0
0.2
0.4
0.6
1.0
0.8
4ms/div
CAPACITANCE (µF)
INPUT VOLTAGE (V)
REFERENCE VOLTAGE
vs. LOAD CURRENT
REFERENCE VOLTAGE vs. VCC
5.06
5.08
5.06
MAX8540 toc14
5.10
MAX8540 toc13
5.04
5.04
5.02
5.02
VREF (V)
7.6
VREF (V)
FREQUENCY (kHz)
1000
MAX8540 toc11
MAX8540 toc10
MAX8540 toc12
5.00
5.00
4.98
4.98
4.96
4.94
4.96
4.92
4.90
4.94
0
2
4
6
8
10
12
7.6
LOAD CURRENT (mA)
9.7
11.8
13.8
15.9
18.0
VCC (V)
HICCUP-MODE OVERCURRENT WAVEFORMS
LATCH-OFF OVERCURRENT WAVEFORMS
MAX8540 toc16
MAX8540 toc15
VDRV
VDRV
VSKTON
1V/div
VSKTOFF
5V
CSKTON = 0.01µF
10ms/div
1V/div
VSKTON
VSKTOFF
1V/div
CSKTON , CSKTFOF = 0.01µF
4ms/div
_______________________________________________________________________________________
7
MAX8540/MAX8541
Typical Operating Characteristics (continued)
(VCC = 12V, TA = +25°C, unless otherwise specified.)
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
Pin Description
PIN
MAX8540 MAX8541
8
NAME
FUNCTION
1
1
REF
5V Reference Voltage Output. Bypass to ground with a 1µF capacitor. REF is pulled to GND
during shutdown.
2
2
OV
System Input Overvoltage Monitoring. The OV threshold is 3.021V with 130mV hysteresis.
Connect to the center of a resistor-divider from the system input to GND to set the overvoltage
trip point (see the Overvoltage Threshold section).
System Input Undervoltage Monitoring and Feed-Forward Input. The UV threshold is 1.25V with
120mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to
set the undervoltage trip point (see the Undervoltage Threshold section). This input can be used
as a disable input by pulling it below 1.2V. UV remains active during shutdown.
3
3
UV
4
4
FREQ/
SYNC
Programmable Frequency and Sync Input. Connect a resistor in parallel with RC (10kΩ, 0.01µF) from
FREQ/SYNC to GND to set the switching frequency or apply an external clock signal through a diode
to synchronize to an external frequency.
5
5
SS
Soft-Start Capacitor Connection. Connect a capacitor from SS to GND to set the soft-start time period.
Programmable Maximum Duty-Cycle Input. Connect a resistor from MAXDTY to GND to set the
maximum duty cycle for minimum system input voltages. The maximum duty cycle is inversely
MAXDTY proportional to the system input voltage. The higher the input voltage, the less the maximum
duty cycle. The maximum duty cycle is internally limited to 80% at any switching frequency.
MAXDTY is high impedance during shutdown. Bypass MAXDTY with a 2200pF capacitor.
6
6
7
7
EN
8
—
SCOMP
Programmable Slope Compensation Input. Connect a resistor from SCOMP to GND to set the
slope compensation.
—
8
PRAMP
Programmable Voltage-Mode Control-Ramp Input. Connect a resistor from PRAMP to GND to
set the magnitude of the voltage ramp.
9
9
OPTO
Isolated Feedback Input. Connect to an optocoupler for an isolated power converter or connect to the
output of a voltage-error amplifier for a nonisolated power converter for a feedback-error signal.
10
10
ILIM
Current-Limit Threshold Input. Connect to the center of a resistor-divider from REF to GND to set
the current-limit threshold voltage.
11
11
SKTOFF
Current-Limit Off-Time Input. Connect a capacitor from SKTOFF to GND to set the current-limit off-time.
Pull SKTOFF to REF through a 10kΩ resistor to latch off the IC after an overcurrent event.
12
12
CS
Current-Sense Input. Provides current-sense feedback for cycle-by-cycle current limit and is
also the current-mode input for the MAX8540. Connect a current-sense resistor from CS to GND.
13
13
SKTON
14
14
GND
Ground
15
15
DRV
Gate Drive for External N-Channel Power MOSFET. Connect to the gate of an external MOSFET
for low-power applications. Connect to the input of an external gate driver for high-power
applications.
16
16
VCC
Voltage Supply for the IC. Operating input voltage range is 7.6V to 18V. Bypass VCC with a
ceramic capacitor to GND.
Active-Low Enable Input. Drive EN to GND to enable the IC. Drive high to place the IC in
shutdown mode.
Current-Limit On-Time Input. Connect a capacitor to ground to set the current-limit on-time.
_______________________________________________________________________________________
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
Overvoltage Threshold
The MAX8540/MAX8541 include an overvoltage protection (OVP) feature that turns off the external MOSFET
when the input voltage exceeds the user-set threshold.
Connect a resistor-divider from the system input to
GND with OV connected to the center to set the OVP
trip point. The threshold voltage for OV is 3.021V (typ):
⎛ R1 + R2 ⎞
VIN(MAX) = ⎜
⎟ × VOV
⎝ R2 ⎠
where VOV is the OV threshold, VIN(MAX) is the overvoltage trip point, R1 is the resistor from the system input
to OV, and R2 is the resistor from OV to GND.
Undervoltage Threshold
The MAX8540/MAX8541 also include an undervoltage
(UV) sensing input. The IC holds the external MOSFET
low until UV reaches its threshold (1.25V typ). Once the
threshold has been reached, the circuit enters soft-start
and brings the output into regulation. Connect a resistordivider from the system input to GND with UV at the center to set the undervoltage protection (UVP) trip point.
⎛ R3 + R4 ⎞
VIN(MIN) = ⎜
⎟ × VUV
⎝ R4 ⎠
where VUV is the UV threshold, VIN(MIN) is the UV trip
point, R3 is the resistor from the system input to UV,
and R4 is the resistor from UV to GND.
An alternate method used in the application circuits for
setting the OV and UV trip points is demonstrated in
Figure 1. Use 36.5kΩ for the bottom resistor (RC). RA
and RB are calculated as follows:
⎛ VOV × VIN(MIN)
RB = RC × ⎜
⎝ VUV × VIN(MAX)
RA =
RC × VIN(MIN)
VUV
−
RB
−
−
⎞
1⎟
⎠
RC
where VIN(MIN) is the UV trip point, VIN(MAX) is the OV
trip point, VUV is the UV threshold (1.25V typ), and VOV
is the OV threshold (3.021V typ). RA should consist of
two equal-value resistors in series to protect against
single-point failure.
Table 1. Typical Specifications of UV/OV
and the Actual On/Off Hysteresis of Input
Voltage
UV VOLTAGE
SPECIFICATIONS
(V)
VIN OFF
WINDOW (LTP)
(V)
MIN
1.083
29.76
TYP
1.128
31.00
MAX
1.173
32.24
MIN
1.200
TYP
1.250
MAX
1.300
OV VOLTAGE
SPECIFICATIONS
(V)
32.97
VIN OFF
WINDOW (UTP)
(V)
2.901
79.70
3.021
83.00
MAX
3.142
86.32
2.778
2.894
MAX
3.010
34.34
36.00
TYP
TYP
—
—
MIN
MIN
VIN ON
WINDOW (UTP)
(V)
VIN ON
WINDOW (LTP)
(V)
—
76.32
—
79.50
82.68
Assume that 1.250V at UV/OV is scaled to 34.34V for a 48V bus.
Table 2. Switching Frequency Selection
RFREQ/SYNC (kΩ)
SWITCHING FREQUENCY (kHz)
48.7
200
19.1
500
15.8
600
11.0
800
8.87
1000
Switching Frequency and Synchronization
The MAX8540/MAX8541 oscillator operates in two
modes: stand-alone or synchronized (sync). A single
input, FREQ/SYNC, doubles as the attachment point for
the frequency-programming resistor and as the synchronization input. The mode recognition is automatic,
based on the signal applied to FREQ/SYNC.
In stand-alone mode, an external resistor connected
from FREQ/SYNC to GND sets the operating frequency.
A 1.25V source is internally applied to FREQ/SYNC and
the oscillator frequency is proportional to the current
out of FREQ/SYNC through the programming resistor.
_______________________________________________________________________________________
9
MAX8540/MAX8541
Detailed Description
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
The MAX8540/MAX8541 also synchronize with an
external oscillator. Drive FREQ/SYNC with a square
wave through a series diode with a positive pulse width
of at least 200ns and a minimum pulse amplitude of 3V,
plus the VF of the external diode. Alternatively, a 1.5V
level-shifted external clock can be applied without the
series diode. The maximum duty cycle of the external
signal allowed is 55%. The MAX8540/MAX8541 synchronize to frequencies between 200kHz and 1MHz;
however, the signal must be within ±30% of the frequency set by the external resistor at FREQ/SYNC.
VIN
36V TO 76V
C1
RMAXDTY =
DMAX × (97.6 × 103 )
60
where DMAX is the desired maximum duty cycle.
The range of valid resistor values for RMAXDTY is from
24.3kΩ to 130kΩ.
N-Channel MOSFET Driver
The DRV output drives an N-channel MOSFET in lowpower applications. In high-power applications, the gate
driver internal to the MAX8540/MAX8541 may not be
capable of driving the external MOSFET efficiently and
an external gate driver may be required. In this situation,
connect DRV to the input of the external gate driver.
Slope Compensation (MAX8540)
The MAX8540 is a current-mode device and requires
slope compensation for proper operation. To provide
slope compensation, connect a resistor from SCOMP to
GND (RSCOMP). The value of RSCOMP is determined as
follows.
For applications using a synchronous rectifier in the
output, set the slope compensation equal to the negative slope of the output inductor. RSCOMP is equal to:
SCF = N
RSCOMP =
10
VOUT
RCS
L1
dVRAMP
R
× SUM
2SCF
dt
C3
RA
Maximum Duty Cycle
Set the maximum duty cycle at the minimum system
input voltage (V IN(MIN) ) connecting a resistor from
MAXDTY to GND. The maximum duty cycle is inversely
proportional to the voltage at UV. As the voltage on UV
increases, the duty cycle decreases. The maximum
duty cycle is internally limited to 80% at all switching
frequencies. The MAXDTY resistor is determined as:
C2
RB
OV
RC
UV
MAX8541
Figure 1. A Method to Set UV and OV Thresholds
where dVRAMP/dt = 2.5V(fS), RSUM = 25kΩ, N is the
turns ratio of the primary to secondary, L1 is the output
inductance, VOUT is the output voltage, fS is the switching frequency, and RCS is the current-sense resistance.
For applications where a diode is used in the output
instead of the synchronous rectifier, the slope compensation resistor is then equal to:
SCF = N
RSCOMP =
VOUT + VF
RCS
L1
dVRAMP
R
× SUM
2SCF
dt
where dVRAMP/dt = 2.5V(fS),RSUM = 25kΩ, N is the turns
ratio of the primary to secondary, L1 is the output inductance, VOUT is the output voltage, VF is the diode voltage,
fS is the switching frequency, and RCS is the currentsense resistance.
Voltage-Ramp Amplitude (MAX8541)
The MAX8541 is a voltage-mode device and features
adjustable voltage ramp. Connect a resistor from
PRAMP to GND (R PRAMP ) to set the voltage-ramp
magnitude, VM.
VM = 1.25V
25kΩ
RPRAMP
______________________________________________________________________________________
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
Soft-Start
The soft-start feature allows converters built using the
MAX8540/MAX8541 to apply power to the load in a
controllable soft ramp, thus reducing startup surges
and stresses. It also determines power-up sequencing
when several converters are used.
Upon power turn-on, the soft-start pin acts as a current
sink to discharge any capacitance connected to it. Once
the voltage at VCC has exceeded its lockout value, softstart then charges the external capacitor (CSS), allowing
the converter output voltage to ramp up. Full output voltage is reached in approximately 440ms/µF.
Current Limit
The MAX8540/MAX8541 utilize two current-limit
schemes: cycle-by-cycle current limit and short-circuit
current limit. Set the current-limit threshold using a
resistor-divider from REF to GND with ILIM connected to
the center. The current-limit threshold is determined as:
VILIM =
R5
× VREF
R5 + R6
where R5 is the resistor from ILIM to GND and R6 is the
resistor from REF to ILIM. Use 240kΩ for R16 and vary
R23 to change the threshold.
The CS signal provides feedback on the current ramp
through the main external MOSFET. The voltage on CS
is monitored by the IC. The cycle-by-cycle current limit
abbreviates the on-time of the external MOSFET in the
event that the voltage at CS is greater than the threshold voltage set by ILIM.
The current-limit feature protects against a hard short
or overcurrent fault at the output by one of two selected
protection modes: by latching off the output, or pulsing
the output to reduce the average output current
(hiccup mode).
To select latched mode, connect SKTOFF to REF. In
this mode, if the hard short exists for the time period set
by the capacitance at SKTON, the output is latched off.
To unlatch the output, toggle EN or cycle the input
power to VCC.
To select hiccup mode, connect capacitors to SKTON
and SKTOFF to program the hiccup mode on- and offtimes. When a cycle-by-cycle event is detected, the IC
charges the capacitor at SKTON. The capacitor continues to charge as long as the CS voltage is greater than
the ILIM threshold voltage. Once the voltage on SKTON
reaches its threshold voltage, the MAX8540/MAX8541
begin skipping switching cycles for a time determined
by the capacitance connected to SKTOFF. Once this
time period has elapsed, the IC begins to switch for the
time period set by the capacitance connected to
SKTON. This process continues until the output short is
removed. See the SKTON and SKTOFF section for
details on setting the hiccup-mode periods.
Connect SKTOFF to REF and SKTON to GND to disable
the latched-mode and hiccup-mode protection, and
operate continuously in cycle-by-cycle current-limit.
PMW Comparator
The PWM comparator of the MAX8540 transforms the
optocoupled error voltage (VOPTO) into a duty cycle by
comparing the opto feedback-error voltage with a
summed voltage. The summed voltage is the sum of
the programmable slope compensation and the current-sense voltages. When the summed voltage
exceeds the opto feedback error voltage, the gatedrive logic turns off the external MOSFET.
The PWM comparator in the MAX8541 compares
VOPTO with the programmable voltage ramp. When the
voltage ramp exceeds V OPTO, the gate-driver logic
turns off the external MOSFET.
SKTON and SKTOFF
The capacitance, CSKTON, determines the time period
allowed before the short-circuit current limit initiates.
Once the CS voltage exceeds the ILIM threshold, the
capacitor at SKTON begins to charge. The capacitor
continues to charge until the SKTON threshold voltage
is reached or the overcurrent event is removed. This
feature allows for the higher currents required during
startup with high-capacitance loads. Set CSKTON to
allow sufficient time for startup. The required capacitance at SKTON is determined as:
CSKTON = tON / 103
where tON is in ms and CSKTON is in µF. The allowable
range for CSKTON is 100pF to 0.01µF.
The capacitance at SKTOFF determines the time period
that the external MOSFET is turned off during an overcurrent event. Once the SKTON time period is exceeded,
the SKTOFF capacitor charges. Once VSKTOFF reaches
its threshold, the IC begins to switch again. CSKTOFF is
determined as:
CSKTOFF = tOFF / 103
where tOFF is in ms and CSKTOFF is in µF. The allowable range for CSKTOFF is 1000pF to 1µF.
______________________________________________________________________________________________________
11
MAX8540/MAX8541
where RPRAMP is in kΩ. The ramp-voltage magnitude is
independent of frequency. The range of values for
RPRAMP is from 12.4kΩ to 42kΩ.
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
Pull VSKTOFF to VREF through a 10kΩ pullup resistor to
enable the latch-off feature. In this mode, once the
SKTON time has elapsed, the IC is latched off. The circuit remains off until EN is toggled, or the input power is
toggled.
Pin Configurations (continued)
TOP VIEW
Soft-Start Capacitor Selection
REF 1
16 VCC
During startup, the capacitor at soft-start is charged
using a 5µA current source. Once the voltage at softstart reaches the threshold voltage (2V typ), the IC
switches normally. Use a low-ESR ceramic capacitor
placed as close as possible to the IC at soft-start. The
value is determined as follows:
OV 2
15 DRV
CSS =
tSS
530
where tSS is the desired soft-start period in ms and CSS
is in µF.
Applications Information
Refer to the following application notes for the
MAX8540/MAX8541 application circuits and applications information: 50W Current-Mode Forward
Converter Design with the MAX8540 and 50W VoltageMode Forward Converter Design with the MAX8541.
12
UV 3
FREQ/SYNC 4
14 GND
MAX8541EEE
SS 5
13 SKTON
12 CS
MAXDTY 6
11 SKTOFF
EN 7
10 ILIM
PRAMP 8
9
OPTO
QSOP
Chip Information
TRANSISTOR COUNT: 2704
PROCESS: BiCMOS
______________________________________________________________________________________
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
VCC UVLO
COMPARATOR
VCC
VCC
TEMP OK
MAX8540
8V
SS_RESET
5V
REFERENCE
REF
OVERVOLTAGE
COMPARATOR
3.02V
ENABLE
OV
UNDERVOLTAGE
COMPARATOR
CLK
ENABLE
DRIVE LOGIC
DRV
UV
1.25V
EN
SLOPE
COMPENSATION
CS
SCOMP
1.2V
SKTOFF
SKTON
SHORT-CIRCUIT
TIMER
PWM
COMPARATOR
OPTO
CS
80% MAX
ILIM
GND
SS
MAX
TON
5µA
SS_RESET
OSC
DECODER RAMP
MAXDTY
CLK
REF
FREQ/SYNC
Figure 2. MAX8540 Functional Diagram
______________________________________________________________________________________
13
MAX8540/MAX8541
Functional Diagrams
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
MAX8540/MAX8541
Functional Diagrams (continued)
VCC UVLO
COMPARATOR
VCC
VCC
TEMP OK
MAX8541
8V
SS_RESET
5V
REFERENCE
REF
OVERVOLTAGE
COMPARATOR
3.02V
ENABLE
OV
UNDERVOLTAGE
COMPARATOR
CLK
ENABLE
DRIVE LOGIC
DRV
UV
1.25V
EN
VOLTAGE
RAMP
CLK
PRAMP
1.2V
SKTOFF
SKTON
SHORT-CIRCUIT
TIMER
PWM
COMPARATOR
OPTO
CS
80% MAX
ILIM
GND
SS
MAX
TON
5µA
SS_RESET
OSC
DECODER RAMP
CLK
REF
FREQ/SYNC
Figure 3. MAX8541 Functional Diagram
14
______________________________________________________________________________________
MAXDTY
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
INPUT
36V TO 76V
OUTPUT
2.5V AT 20A
OUT
IN
FB
MAX8515
VIN
PGND GND
OPTIONAL
SYNCHRONIZATION
OV
VCC
UV
FREQ/SYNC
SS
U1
REF
MAX8540
DRV
CS
MAXDTY
ILIM
SKTOFF
SCOMP
SKTON
OFF
OPTO
EN
GND
ON
______________________________________________________________________________________
15
MAX8540/MAX8541
Typical Operating Circuit
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.
QSOP.EPS
MAX8540/MAX8541
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
21-0055
F
1
1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2003 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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