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FR9888
fitipower integrated technology lnc.
85T
23V, 3.5A, 340KHz Synchronous Step-Down
DC/DC Converter
Description
Features
The FR9888 is a synchronous step-down DC/DC
converter that provides wide 4.5V to 23V input
voltage range and 3.5A continuous load current
capability.
The FR9888 fault protection includes cycle-by-cycle
current limit, input UVLO, output over voltage
protection and thermal shutdown. Besides,
adjustable soft-start function prevents inrush current
at turn-on. This device uses current mode control
scheme which provides fast transient response.
Internal Compensation function reduces external
compensation components and simplifies the design
process. In shutdown mode, the supply current is
less than 1uA.
The FR9888 is available in an 8-pin SOIC package,
provides a very compact system solution and good
thermal conductance.
● High Efficiency Up to 96%
● Low Rds(on) integrated Power MOSFET
● Internal Compensation Function
● Wide Input Voltage Range: 4.5V to 23V
● Adjustable Output Voltage Range: 0.925V to
20V
● 3.5A Output Current
● Fixed 340KHz Switching Frequency
● Current Mode Operation
● Adjustable Soft-Start
● Cycle-by-Cycle current limit
● Input Under Voltage Lockout
● Over-Temperature Protection With Auto
Recovery
● <1uA Shutdown Current
●SOP-8 Exposed Pad Package
Applications
● Set-Top-Box (STB)
● Televisions
● Distributed Power Systems
● XDSL Modems
Pin Assignments
Ordering Information
FR9888□□□
SP Package (SOP- 8 Exposed pad )
TR: Tape / Reel
BOOST
1
8
SS
VIN
2
7
LX
GND
3
4
6
SHDN
NC
5
FB
G: Green
Package Type
SP: SOP-8(Exposed Pad)
Figure 1. Pin Assignment of FR9888
FR9888- 1.0-APR-2011
1
FR9888
fitipower integrated technology lnc.
85T
Typical Application Circuit
C4
10nF~0.1uF
R3
10kΩ~100kΩ
7
1
SHDN BOOST
2
VIN
LX 3
VIN
L1
4.7uH~15uH
VOUT
3.3V
4.5V to 23V
C1
10uF/25V
CERAMIC x 2
FR9888
6 NC
FB
GND
4
R1
26.1kΩ
5
SS
C6
(optional)
C2
22uF/6.3V
CERAMIC x 2
R2
10kΩ
8
C3
10nF ~ 0.1uF
Figure 2. CIN /COUT use Ceramic Capacitors Application Circuit
C4
10nF~0.1uF
R3
10kΩ~100kΩ
7
1
SHDN BOOST
2
VIN
LX
VIN
3
L1
4.7uH~15uH
VOUT
3.3V
4.5V to 23V
C1
100uF/25V
EC x 1
C5
0.1uF/25V
CERAMIC x 1
FR9888
6
NC
GND
4
FB
5
SS
R1
26.1kΩ
C6
(optional)
C2
100uF/6.3V
EC x 1
R2
10kΩ
8
C3
10nF ~ 0.1uF
Figure 3. CIN /COUT use Electrolytic Capacitors Application Circuit
VOUT
1.2V
1.8V
2.5V
3.3V
5V
1.2V
1.8V
2.5V
3.3V
5V
R1
3kΩ
9.53kΩ
16.9kΩ
26.1kΩ
44.2kΩ
3kΩ
9.53kΩ
16.9kΩ
26.1kΩ
44.2kΩ
R2
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
C6
200pF~1nF
200pF~1nF
200pF~1nF
200pF~1nF
200pF~1nF
------
L1
4.7uH
4.7uH
10uH
10uH
10uH
4.7uH
4.7uH
10uH
10uH
10uH
COUT
22uF MLCC x2
22uF MLCC x2
22uF MLCC x2
22uF MLCC x2
22uF MLCC x2
100uF EC x1
100uF EC x1
100uF EC x1
100uF EC x1
100uF EC x1
Table 1. Recommended Component Values
FR9888- 1.0-APR-2011
2
FR9888
fitipower integrated technology lnc.
85T
Functional Pin Description
I/O
Pin Name
Pin No.
Pin Function
I
FB
5
I
VIN
2
I
SHDN
7
I
GND
4
Ground Pin. Connect this pin to exposed pad.
O
LX
3
Power Switching Output. It is the output pin of internal high side NMOS which
is the switching to supply power.
O
SS
8
O
BOOST
1
O
NC
6
Voltage Feedback Input Pin. Connect FB and VOUT with a resistive voltage
divider. This IC senses feedback voltage via FB and regulates it at 0.925V.
Power Supply Input Pin. Drive this pin by 4.5V to 23V voltage to power on the
chip.
Enable Input Pin. This pin provides a digital control to turn the converter on or
off. Connect VIN with a 100KΩ resistor for self-startup.
Soft-Start Pin. This pin controls the soft-start period. Connect a capacitor
from SS to GND to set the soft start period.
High Side Gate Drive Boost Pin. A 10nF or greater capacitor must be
connected from this pin to LX. It can boost the gate drive to fully turn on the
internal high side NMOS.
No connection. Keeps this pin floating.
Block Diagram
VIN
ISEN
UVLO
&
POR
SHDN
Internal
Regulator
VCC
OTP
VCC
1M
OVP
Oscillator
BOOST
6µA
High-Side
MOSFET
S
FB
Current
Comp
SS
R
OTP
OVP
UVLO
PWM
Control
Driver
Logic
LX
Low-Side
MOSFET
0.925V
Current
Limit
GND
Figure 4. Block Diagram of FR9888
FR9888- 1.0-APR-2011
3
FR9888
fitipower integrated technology lnc.
85T
Absolute Maximum Ratings (Note1)
● Supply Voltage VIN-------------------------------------------------------------------------------------- -0.3V to +25V
● Enable Voltage VSHDN --------------------------------------------------------------------------------- -0.3V to +25V
● LX Voltage VLX (50ns)---------------------------------------------------------------------------------- -1V to VIN+0.3V
● Boost Trap Voltage VBOOST---------------------------------------------------------------------------- VLX-0.3V to VLX+6V
● All Other Pins Voltage---------------------------------------------------------------------------------- -0.3V to +6V
● Maximum Junction Temperature (TJ)-------------------------------------------------------------- +150
● Storage Temperature (TS)----------------------------------------------------------------------------- -65
to +150
● Lead Temperature (Soldering, 10sec.) ----------------------------------------------------------- +260°C
● Power Dissipation @TA=25
, (PD) (Note2)
SOP-8 (Exposed Pad )-------------------------------------------------------------------- 2.08 W
● Package Thermal Resistance, (
JA):
SOP-8 (Exposed Pad )-------------------------------------------------------------------- 60°C/W
● Package Thermal Resistance, (
JC):
SOP-8 (Exposed Pad )-------------------------------------------------------------------- 15°C/W
Note1:Stresses beyond this listed under “Absolute Maximum Ratings" may cause permanent damage to the device.
Note2:PCB heat sink copper area = 10mm2.
Recommended Operating Conditions
● Supply Voltage VIN------------------------------------------------------------------------------------ +4.5V to +23V
● Enable Voltage VSHDN -------------------------------------------------------------------------------
0V to VIN
● Operation Temperature Range--------------------------------------------------------------------- - 40°C to + 85°C
FR9888- 1.0-APR-2011
4
FR9888
fitipower integrated technology lnc.
85T
Electrical Characteristics
(VIN=12V, TA=25
, unless otherwise specified.)
Parameter
Symbol
Conditions
Min
Typ
Unit
23
V
VIN Input Supply Voltage
VIN
VIN Quiescent Current
IDDQ
VSHDN =1.8V, VFB=1.0V
VIN Shutdown Supply Current
ISD
VSHDN =0V
Feedback Voltage
VFB
4.5V≦VIN≦23V
Feedback OVP Threshold Voltage
VOVP
1.5
V
High-Side MOSFET RDS(ON) (Note3)
RDS(ON)
110
mΩ
Low-Side MOSFET RDS(ON) (Note3)
RDS(ON)
80
mΩ
High-Side MOSFET Leakage Current
ILX(leak)
VSHDN =0V, VLX=0V
High-Side MOSFET Current Limit
(Note3)
ILIMIT(HS)
Minimum Duty
Low-Side MOSFET Current Limit
(Note3)
ILIMIT(LS)
From Drain to Source
Error Amplifier Voltage Gain
4.5
Max
FOSC
Short Circuit Oscillation Frequency
FOSC(short)
Maximum Duty Cycle
DMAX
Minimum On Time (Note3)
TMIN
Input UVLO Threshold
Lockout
0.9
VUVLO(Vth)
Threshold
0.925
mA
1
μA
0.95
V
10
4
(Note3)
Oscillation frequency
Under
Voltage
Hysteresis
2.5
290
uA
5
A
1.5
A
400
V/V
340
420
KHz
VFB=0V
110
KHz
VFB=0.8V
90
%
100
ns
4.3
V
250
mV
VIN Rising
VUVLO(HYS)
Soft-Start Current
ISS
VSS=0V
6
uA
Soft-Start Period
TSS
CSS=0.1uF
15
ms
SHDN Input Low Voltage
VSHDN
(L)
SHDN Input High Voltage
VSHDN
(H)
ISHDN
SHDN Input Current
Thermal Shutdown Threshold
(Note3)
TSD
0.4
2
VSHDN =2V
V
V
2
uA
170
℃
Note3:Not production tested.
FR9888- 1.0-APR-2011
5
FR9888
fitipower integrated technology lnc.
85T
Typical Performance Curves
VIN = 12V, VOUT = 3.3V, C1 =10uF x 2, C2 = 22uF x 2, L1 = 10uH, TA = +25℃, unless otherwise noted.
VOUT = 1.2V
Figure 5. Efficiency vs. Load Current
VOUT = 3.3V
Figure 6. Efficiency vs. Load Current
VOUT = 5V
Figure 7. Efficiency vs. Load Current
Figure 8. Current Limit vs. Temperature
Figure 9. Feedback Voltage vs. Temperature
Figure 10. Switching Frequency vs. Temperature
FR9888- 1.0-APR-2011
6
FR9888
fitipower integrated technology lnc.
85T
Typical Performance Curves
VIN = 12V, VOUT = 3.3V, C1 = 10uF x 2, C2 = 22uF x 2, L1 = 10uH, TA = +25℃, unless otherwise noted.
IOUT=0A
IOUT=3.5A
VIN 10mV/div. (AC)
VIN 100mV/div. (AC)
VOUT 20mV/div. (AC)
VOUT 20mV/div. (AC)
IL 1A/div.
IL 1A/div.
VLX 5V/div.
VLX 5V/div.
2us/div.
2us/div.
Figure 11. Steady State Waveform
IOUT=0A
Figure 12. Steady State Waveform
IOUT=3.5A
VIN 5V/div.
VIN 5V/div.
VOUT 1V/div.
VOUT 1V/div.
IL 1A/div.
IL 1A/div.
VLX 5V/div.
VLX 5V/div.
40ms/div.
40ms/div.
Figure 13. Power On through VIN Waveform
IOUT=0A
Figure 14. Power On through VIN Waveform
IOUT=3.5A
VIN 10V/div.
VIN 10V/div.
VOUT 1V/div.
VOUT 1V/div.
IL 1A/div.
IL 1A/div.
VLX 5V/div.
VLX 5V/div.
20ms/div.
Figure 15. Power Off through VIN Waveform
FR9888- 1.0-APR-2011
10ms/div.
Figure 16. Power Off through VIN Waveform
7
FR9888
fitipower integrated technology lnc.
85T
Typical Performance Curves
VIN = 12V, VOUT = 3.3V, C1 = 10uF x 2, C2 = 22uF x 2, L1 = 10uH, TA = +25℃, unless otherwise noted.
IOUT=3.5A
IOUT=0A
VSHDN 5V/div.
VSHDN 5V/div.
VOUT 1V/div.
VOUT 1V/div.
IL 1A/div.
IL 1A/div.
VLX 5V/div.
VLX 5V/div.
4ms/div.
Figure 17. Power On through SHDN Waveform
4ms/div.
Figure 18. Power On through SHDN Waveform
IOUT=3.5A
IOUT=0A
VSHDN 5V/div.
VSHDN 5V/div.
VOUT 1V/div.
VOUT 1V/div.
IL 1A/div.
IL 1A/div.
VLX 5V/div.
VLX 5V/div.
4ms/div.
Figure 19. Power Off through SHDN Waveform
80us/div.
Figure 20. Power Off through SHDN Waveform
VOUT 1V/div.
VOUT 200mV/div.
IL 2A/div.
IL 1A/div.
400us/div.
Figure 21. Load Transient Waveform
FR9888- 1.0-APR-2011
40us/div.
Figure 22. Short Circuit Test
8
FR9888
fitipower integrated technology lnc.
85T
Function Description
The FR9888 is a high efficiency, internal
compensation, and constant frequency current mode
step-down synchronous DC/DC converter. It has
integrated high-side (110mΩ, typ) and low-side
(80mΩ, typ) power switches, and provides 3.5A
continuous load current. It regulates input voltage
from 4.5V to 23V, and down to an output voltage as
low as 0.925V.
Input Under Voltage Lockout
Control Loop
Short Circuit Protection
Under normal operation, the output voltage is sensed
by FB pin through a resistive voltage divider and
amplified through the error amplifier. The voltage of
error amplifier output is compared to the switch
current to control the RS latch. At the beginning of
each clock cycle, the high-side NMOS turns on when
the oscillator sets the RS latch, and turns off when
current comparator resets the RS latch. Then the
low-side NMOS turns on until the clock period ends.
The FR9888 provides short circuit protection
function to prevent the device damage from
short condition. When the short condition
occurs and the feedback voltage drops lower
than 0.4V, the oscillator frequency will be
reduced to 110KHz to prevent the inductor
current increasing beyond the current limit. In
the meantime, the current limit will also be
reduced to lower the short current. Once the
short condition is removed, the frequency and
current limit will return to normal.
Enable
The FR9888 SHDN pin provides digital control to
turn on/turn off the regulator. When the voltage of
SHDN exceeds the threshold voltage, the regulator
starts the soft start function. If the SHDN pin
voltage is below than the shutdown threshold voltage,
the regulator will turn into the shutdown mode and
the shutdown current will be smaller than 1uA. For
auto start-up operation, connect EN to VIN through a
100KΩ resistor.
Soft Start
The FR9888 employs adjustable soft start function to
reduce input inrush current during start up. When
the device turns on, a 6uA current begins charging
the capacitor which is connected from SS pin to
GND. The equation for the soft start time is shown
as below:
TSS  ms  =
CSS  nF   VFB
ISS  uA 
The VFB voltage is 0.925V and the ISS current is 6uA.
If a 0.1uF capacitor is connected from SS pin to
GND, the soft start time will be 15ms.
Output Over Voltage Protection
When the FB pin voltage exceeds 1.5V, the output
over voltage protection function will be triggered and
turn off the high-side/low-side MOSFET.
FR9888- 1.0-APR-2011
When the FR9888 is power on, the internal
circuits are held inactive until VIN voltage
exceeds the input UVLO threshold voltage.
And the regulator will be disabled when VIN is
below the input UVLO threshold voltage. The
hysteretic of the UVLO comparator is 250mV
(typ).
Over Current Protection
The FR9888 over current protection function is
implemented using cycle-by-cycle current limit
architecture.
The inductor current is
monitored by measuring the high-side
MOSFET series sense resistor voltage. When
the load current increases, the inductor current
also increases. When the peak inductor current
reaches the current limit threshold, the output
voltage starts to drop. When the over current
condition is removed, the output voltage returns
to the regulated value.
Over Temperature Protection
The FR9888 incorporates an over temperature
protection circuit to protect itself from
overheating. When the junction temperature
exceeds the thermal shutdown threshold
temperature, the regulator will be shutdown.
And the hysteretic of the over temperature
protection is 60℃ (typ).
Internal Compensation Function
The stability of the feedback circuit is controlled
through internal compensation circuits. This
internal compensation function is optimized for
most applications and this function can reduce
external R, C components.
9
FR9888
fitipower integrated technology lnc.
85T
Application Information
Output Voltage Setting
The output voltage VOUT is set using a resistive
divider from the output to FB. The FB pin regulated
voltage is 0.925V. Thus the output voltage is:
 R1 
VOUT=0.925V   1+

 R2 
Table 2 lists recommended values of R1 and R2 for
most used output voltage.
Table 2 Recommended Resistance Values
VOUT
R1
R2
5V
44.2kΩ
10kΩ
3.3V
26.1kΩ
10kΩ
2.5V
16.9kΩ
10kΩ
1.8V
9.53kΩ
10kΩ
1.2V
3kΩ
10kΩ
Place resistors R1 and R2 close to FB pin to prevent
stray pickup.
A low ESR capacitor is required to keep the
noise minimum.
Ceramic capacitors are
better, but tantalum or low ESR electrolytic
capacitors may also suffice.
When using
tantalum or electrolytic capacitors, a 0.1uF
ceramic capacitor should be placed as close to
the IC as possible.
Output Capacitor Selection
The output capacitor is used to keep the DC
output voltage and supply the load transient
current. When operating in constant current
mode, the output ripple is determined by four
components:
VRIPPLE  t   VRIPPLE(C)  t   VRIPPLE(ESR)  t 
 VRIPPLE(ESL)  t   VNOISE
The following figures show the form of the ripple
contributions.
VRIPPLE(ESR)(t)
Input Capacitor Selection
The use of the input capacitor is filtering the input
voltage ripple and the MOSFETS switching spike
voltage. Because the input current to the step-down
converter is discontinuous, the input capacitor is
required to supply the current to the converter to
keep the DC input voltage. The capacitor voltage
rating should be 1.25 to 1.5 times greater than the
maximum input voltage. The input capacitor ripple
current RMS value is calculated as:
+
VRIPPLE(ESL) (t)
+
VRIPPLE(C) (t)
(t)
(t)
IIN RMS =IOUT  D  1-D 
Where D is the duty cycle of the power MOSFET.
This function reaches the maximum value at D=0.5
and the equivalent RMS current is equal to IOUT/2.
The following diagram is the graphical representation
of above equation.
+
VNOISE
(t)
3.5A
3A
2.5A
=
VRIPPLE(t)
2A
(t)
FR9888- 1.0-APR-2011
10
FR9888
fitipower integrated technology lnc.
85T
Application Information
VRIPPLE(ESR)=IL  R(ESR)
VRIPPLE(ESL)=IL  L(ESL)  FOSC 
VRIPPLE(C)=IL 
IL 
1
D 1  D 
1
8  COUT  FOSC
VOUT
 VOUT 
 1
FOSC  L 
VIN 
VOUT
Where Δ IL is the peak-to-peak inductor ripple
current, FOSC is the switching frequency, L is the
inductance value, VIN is the input voltage, VOUT is the
output voltage, R(ESR) is the equivalent series
resistance value of the output capacitor, L(ESL) is the
equivalent series inductance value of the output
capacitor and the COUT is the output capacitor. The
following diagram is an example to graphical
represent Δ IL equation.
L=4.7uH
L=6.8uH
L=10uH
VOUT=3.3V, FOSC=340KHz
Low ESR capacitors are preferred.
Ceramic,
tantalum or low ESR electrolytic capacitors can be
used depending on the output ripple requirement.
When using the ceramic capacitors, the ESL
component is usually negligible.
It is important to use the proper method to eliminate
high frequency noise when measuring the output
ripple. The figure shows how to locate the probe
across the capacitor when measuring output ripple.
Removing the scope probe plastic jacket in order to
expose the ground at the tip of the probe. It gives a
very short connection from the probe ground to the
capacitor and eliminating noise.
FR9888- 1.0-APR-2011
Probe Ground
GND
Ceramic Capacitor
Output Inductor Selection
The output inductor is used for storing energy
and filtering output ripple current. But the
trade-off condition often happens between
maximum energy storage and the physical size
of the inductor. The first consideration for
selecting the output inductor is to make sure
that the inductance is large enough to keep the
converter in the continuous current mode.
That will lower ripple current and result in lower
output ripple voltage. The inductance value
should be determined to set the peak-to-peak
inductor ripple current Δ IL around 20% to 50%
of the maximum load current.
Then the
inductance can be calculated with the following
equation:
IL=  0.2~0.5   IOUT(MAX)
VOUT


L   VIN-VOUT   

 FOSC  ΔIL  VIN 
To guarantee sufficient output current, peak
inductor current must be lower than the FR9888
high-side MOSFET current limit. The peak
inductor current is as below:
IPEAK=IOUT(MAX)+
ΔIL
2
11
FR9888
fitipower integrated technology lnc.
85T
Application Information
Feedforward Capacitor Selection
PCB Layout Recommendation
Internal compensation function allows users saving
time in design and saving cost by reducing the
number of external components. The use of a
feedforward capacitor C6 in the feedback network is
recommended to improve the transient response or
higher phase margin.
The device’s performance and stability is
dramatically affected by PCB layout. It is
recommended to follow these general
guidelines show as below:
VOUT
R1
FR9888
C6
FB
R2
1. Place the input capacitors and output
capacitors as close to the device as
possible.
Trace to these capacitors
should be as short and wide as possible to
minimize
parasitic
inductance
and
resistance.
2. Place feedback resistors close to the FB
pin.
3. Keep the sensitive signal (FB) away from
the switching signal (LX).
For optimizing the feedforward capacitor, knowing the
cross frequency is the first thing.
The cross
frequency (or the converter bandwidth) can be
determined by using a network analyzer. When
getting the cross frequency with no feedforward
capacitor identified, the value of feedforward
capacitor C6 can be calculated with the following
equation:
C6 
1
1
1 1


2  FCROSS
R1 R1 R2
4. The exposed pad of the package should be
soldered to an equivalent area of metal on
the PCB. This area should connect to the
GND plane and have multiple via
connections to the back of the PCB as well
as connections to intermediate PCB layers.
The GND plane area connecting to the
exposed pad should be maximized to
improve thermal performance.
5. Multi-layer PCB design is recommended.
C6
R1
R3
Where FCROSS is the cross frequency.
R2
C3
To reduce transient ripple, the feedforward capacitor
value can be increased to push the cross frequency
to higher region.
Although this can improve
transient response, it also decrease phase margin
and cause more ringing. In the other hand, if more
phase margin is desired, the feedforward capacitor
value can be decreased to push the cross frequency
to lower region.
8
7
6
5
Exposed
GND
Pad
GND
–
C1
+
–
C5
1
2
3
VIN
C4
C2
+
4
LX
L1
VOUT
External Boost Diode Selection
For 5V input applications, it is recommended to add
an external boost diode. This helps improving the
efficiency. The boost diode can be a low cost one
such as 1N4148.
Figure 23. FR9888 SOP-8(Exposed Pad)
package CIN/COUT with EC capacitors
Recommended PCB Layout Diagram
D1
1N4148
VIN
5V
VIN
BOOST
FR9888
C4
LX
FR9888- 1.0-APR-2011
12
FR9888
fitipower integrated technology lnc.
85T
Outline Information
SOP- 8 (Exposed Pad) Package (Unit: mm)
SYMBOLS
UNIT
DIMENSION IN MILLIMETER
MIN
MAX
A
1.25
1.70
A1
0.00
0.15
A2
1.25
1.55
B
0.31
0.51
D
4.80
5.00
D1
1.82
3.35
E
3.80
4.00
E1
1.82
2.41
e
1.20
1.34
H
5.80
6.20
L
0.40
1.27
Note:Followed From JEDEC MO-012-E.
Carrier dimensions
Life Support Policy
Fitipower’s products are not authorized for use as critical components in life support devices or other medical systems.
FR9888- 1.0-APR-2011
13
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