Fuji FA7701V Fuji power supply control ic Datasheet

Quality is our message
FUJI Power Supply Control IC
FA7700V/01V
Application Note
Dec -2000
Fuji Electric Co., Ltd.
Matsumoto Factory
1
Quality is our message
WARNING
1.This Data Book contains the product specifications, characteristics, data, materials, and structures as of Dec
2000. The contents are subject to change without notice for specification changes or other reasons. When
using a product listed in this Data Book, be sure to obtain the latest specifications.
2. All applications described in this Data Book exemplify the use of Fuji's products for your reference only. No right
or license, either express or implied, under any patent, copyright, trade secret or other intellectual property right
owned by Fuji Electric Co., Ltd. is (or shall be deemed) granted. Fuji makes no representation or warranty,
whether express or implied, relating to the infringement or alleged infringement of other's intellectual property
rights which may arise from the use of the applications described herein.
3. Although Fuji Electric is enhancing product quality and reliability, a small percentage of semiconductor products
may become faulty. When using Fuji Electric semiconductor products in your equipment, you are requested to
take adequate safety measures to prevent the equipment from causing a physical injury, fire, or other problem if
any of the products become faulty. It is recommended to make your design fail-safe, flame retardant, and free
of malfunction.
4.The products introduced in this Data Book are intended for use in the following electronic and electrical
equipment which has normal reliability requirements.
• Computers • OA equipment • Communications equipment (terminal devices)
• Measurement equipment • Machine tools • Audiovisual equipment • Electrical home appliances
• Personal equipment • Industrial robots etc.
5.If you need to use a product in this Data Book for equipment requiring higher reliability than normal, such as for
the equipment listed below, it is imperative to contact Fuji Electric to obtain prior approval. When using these
products for such equipment, take adequate measures such as a backup system to prevent the equipment
from malfunctioning even if a Fuji's product incorporated in the equipment becomes faulty.
• Transportation equipment (mounted on cars and ships)
• Trunk communications equipment
• Traffic-signal control equipment
• Gas leakage detectors with an auto-shut-off feature
• Emergency equipment for responding to disasters and anti-burglary devices
• Safety devices
6. Do not use products in this Data Book for the equipment requiring strict reliability such as (without limitation)
• Space equipment
• Aeronautic equipment
• Atomic control equipment
• Submarine repeater equipment
• Medical equipment
7. Copyright © 1995 by Fuji Electric Co., Ltd. All rights reserved. No part of this Data Book may be reproduced in
any form or by any means without the express permission of Fuji Electric.
8. If you have any question about any portion in this Data Book, ask Fuji Electric or its sales agents before using
the product. Neither Fuji nor its agents shall be liable for any injury caused by any use of the products not in
accordance with instructions set forth herein.
2
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CONTENTS
page
1.
Description
・・・・・・・・・・・・・・・・・・・
4
2.
Features
・・・・・・・・・・・・・・・・・・・
4
3.
Outline
・・・・・・・・・・・・・・・・・・・
4
4.
Block diagram
・・・・・・・・・・・・・・・・・・・
5
5.
Pin assignment
・・・・・・・・・・・・・・・・・・・
5
6.
Ratings and characteristics
・・・・・・・・・・・・・・・・・・・
6
7.
Characteristics curves
・・・・・・・・・・・・・・・・・・・
9
8.
Description of each circuit
・・・・・・・・・・・・・・・・・・・
13
9.
Design advice
・・・・・・・・・・・・・・・・・・・
17
Application circuit
・・・・・・・・・・・・・・・・・・・
18
10.
Note
• Parts tolerance and characteristics are not defined in all application described in this Data book. When design an
actual circuit for a product, you must determine parts tolerances and characteristics for safe and stable operation.
3
Quality is our message
1.
. Description
FA7700V/FA7701V are the PWM type DC to DC converter control ICs with 1ch output that can directly drive power
MOSFETs. CMOS devices with high breakdown voltage are used in these ICs and low power consumption is
achieved. These ICs have the many functions equivalent to those which our conventional version bipolar ICs –
FA76XX series- have,and have merits of output ON/OFF control function,directly driving Nch/Pch MOSFETs,low
power consumption , higher frequency operation, and less external discrete components.
2.
. Features
・Wide range of supply voltage.: VCC=2.5 to 20V
・FA7700V—for boost, flyback converter (Maximum output duty cycle is 80%)
FA7701V—for buck converter (Maximum output duty cycle is 100%)
・output stage consist of CMOS push-pull circuit, and achieves a high speed switching of external MOSFETs.
(FA7700V: for Nch-MOSFET driving, FA7701V: for Pch-MOSFET driving)
・High accuracy reference voltage (Error amplifier): 0.88V±2%
・Soft start function.
・Adjustable built-in timer latch for short-circuit protection.
・Output ON/OFF control function
・Less external discrete components needed (2 components less than conventional version of the equivalent
products)
・Low power consumption
Stand-by current: 40μA(typ.)
Operating current: 1.2mA(typ.) (including error amplifier output current and oscillator current)
・High frequency operation: 50kHz to 1MHz
・Package: TSSOP-8(thin and small)
3.
. Outline
units:mm
4
Quality is our message
4.
. Block diagram
FA7700V
RT
1
UVLO
VREF
0.3V
VREF
REF
2
2.2V
OSC
5.5V
1.5V
S.C.DET
BIAS
1.5V
+
-
Power Good Signal
IN−
3
0.88V
CS
7
VCC
6
OUT
5
GND
PWM
ER.AMP
FB
OFF
ON/OFF
+
‑
+
+
+
-
ON/OFF
+
S.C.P
+
2.2V
8
4
FA7701V
RT
1
UVLO
VREF
0.3V
VREF
REF
2
2.2V
OSC
5.5V
1.5V
S.C.DET
BIAS
1.5V
+
-
Power Good Signal
IN−
3
0.88V
+
-
+
‑
+
ON/OFF
+
S.C.P
+
2.2V
OFF
ON/OFF
8
CS
7
VCC
6
OUT
5
GND
PWM
ER.AMP
FB
4
5.
. Pin assignment
Pin
No.
Pin Name
1
RT
2
REF
Internal bias voltage
3
IN(-)
Error amplifier inverting input
4
FB
Error amplifier output
5
GND
Ground
6
OUT
Output for driving switching device
7
VCC
Power supply
8
CS
Function
Oscillator timing resistor
ON/OFF, Soft start, Timer latched short circuit protection
5
Quality is our message
6.
. Ratings and Characteristics
(1) Absolute maximum ratings
Item
Power supply voltage
REF terminal output current
Symbol
Vcc
IREF
ISOpeak
ISOcont
ISIpeak
ISIcont
VRT,VREF
VIN-,VFB
OUT terminal source current
OUT terminal sink current
RT,REF,IN−,FB
terminal voltage
CS terminal voltage
Ratings
20
2
-400(peak)
-50(continuos)
+150(peak)
+50(continuos)
+2.5(max.)
-0.3(min.)
Self Limiting≒5.5(max.)
-0.3(min.)
200
250(Ta≦25℃)
-30〜+85
+125
-40〜+150
VCS
CS terminal sink current
Power dissipation
Operating ambient temperature
Operating junction temperature
Storage temperature
ICS
Pd
Ta
Tj
Tstg
Units
V
mA
mA
mA
V
V
μA
mW
℃
℃
℃
Maximum power dissipation curve
Maximum power dissipation
[mW]
300
250
200
150
100
50
0
-30
0
30
60
90
Ambient temperature [℃]
120
150
(2) Recommended operating conditions
Item
Symbol
MIN.
TYP.
MAX.
Supply voltage
VCC
2.5
6
18
DC feedback resistor
100
RNF
of error amplifier
VCC terminal capacitance
CVCC
0.1
REF terminal capacitance
CREF
0.047
0.1
1
CS terminal capacitance
CS
0.01
10
CS terminal sink current
Icsin
1 (*1)
50
Oscillation frequency
fosc
50
1000
(*1)Lower Limit of ICSIN does not include leak current “IL” for capacitor Cs. Set a resistor
“Rcs[MΩ]” connected between VCC terminal and CS terminal to satisfy the following equation.
VCC − 1.5
VCC − 1.5
< RCS[ MΩ] <
50uA + IL
1uA + IL
6
Units
V
kΩ
μF
μF
μF
μA
kHz
Quality is our message
(3) Electrical characteristics
(Unless otherwise standard, Ta=25℃,Vcc=6V,RT=22kΩ)
(1)Internal Bias Section (REF terminal voltage)
Item
Symbol
Conditions
REF terminal source current
Output Voltage
VREF
IREF = 0mA
Line Regulation
VLINE
Vcc = 2.5 to 20V,IREF = 0mA
Load Regulation
VLOAD
IREF = 0 to 2mA
VTC1
Ta = -30 to 25℃
Variation
with temperature
VTC2
Ta = 25 to 85℃
(2)Oscillator Section (Frequency set by RT terminal )
Item
Symbol
Conditions
Oscillation
fosc
RT = 22kΩ
frequency
Line Regulation
fLINE
Vcc = 2.5 to 20V
f
TC1
Ta = -30 to 25℃, 50k to 1MHz
Variation
with temperature
fTC2
Ta = 25 to 85℃, 50k to 1MHz
(3)Error Amplifier Section (IN- terminal , FB terminal )
Item
Symbol
Conditions
IN- terminal, FB terminal
Reference Voltage
VB
:shorted (Voltage Follower)
Input current
IIN“VB”
Vcc = 2.5 to 20V
VBLINE
Line Regulation
VBTC1
Ta = -30 to 25℃
“VB” variation
with temperature
VBTC2
Ta = 25 to 85℃
Open Loop Gain
AVO
Unity Gain
fT
Bandwidth
Source
IOHE
FB terminal = VREF- 0.5V
Output
Current
FB terminal = 0.5V
Sink
IOLE
MIN.
TYP.
MAX.
Units
2.16
2.23
2.30
V
±2
±2
±0.3
±0.3
±14
±12
mV
mV
%
%
MIN.
TYP.
MAX.
Units
155
185
215
kHz
±0.1
±2
±3
MIN.
TYP.
MAX.
Units
0.863
0.880
0.897
V
+500
nA
±5
mV
-500
±1
7
±0.3
±0.3
%
%
dB
1.5
MHz
70
-220
3
(4)Pulse Width Modulation (PWM) Section (FB terminal voltage and Duty Cycle)
Item
Symbol
Conditions
MIN.
FB 0% threshold
VFB0
Duty Cycle = 0%
0.560
FB 50% threshold
VFB50
Duty Cycle = 50%
DMAX1
85
RT = 100kΩ,f≒ 50kHz
Maximum FA7700
DMAX2
83
RT = 22kΩ,f≒185kHz
Duty
DMAX3
80
RT = 3kΩ,f≒1MHz
Cycle
100
DMAX
FA7701
(5)Under Voltage Lock-Out Section (VCC terminal voltage)
Item
Symbol
Conditions
ON threshold
VCCON
OFF threshold
VCCOF
Hysteresis Voltage
VCCHY
Ta = -30 to 25℃
Variation with
VCCHY
temperature
Ta = 25 to 85℃
%
%
%
MIN.
1.60
0.04
-160
6
-100
12
μA
mA
TYP.
0.660
0.880
90
88
86
MAX.
0.760
Units
V
V
%
%
%
%
TYP.
2.07
1.93
0.14
+0.2
−0.2
MAX.
2.30
95
93
92
0.24
Units
V
V
V
mV/℃
mV/℃
Quality is our message
(6)ON/OFF Section (CS terminal voltage)
Item
Symbol
ON/OFF threshold
VONOF
Threshold Variation
with temperature
VONTC
Conditions
MIN.
0.150
MIN.
0.560
(8)Timer Latched Short circuit Protection Section (FB terminal, CS terminal)
Item
Symbol
Conditions
MIN.
CS terminal
Clamped Voltage
MAX.
0.450
+0.5
Ta = -30 to 85℃
(7)Soft Start Section (CS terminal voltage)
Item
Symbol
Conditions
Threshold Voltage 1
VCS0
Duty Cycle = 0%
Threshold Voltage 2
VCS50
Duty Cycle = 50%
Short Detection
Threshold Voltage
Latched Mode
Threshold Voltage
Latched Mode
Reset Voltage
Latched Mode
Hysteresis
TYP.
0.300
Units
V
mV/℃
TYP.
0.660
0.880
MAX.
0.760
Units
V
V
TYP.
MAX.
Units
VFBTH
FB terminal voltage
1.350
1.500
1.650
V
VCSTH
CS terminal voltage
2.050
2.200
2.350
V
VCSRE
CS terminal voltage
1.700
2.030
2.300
V
VCSHY
CS terminal voltage
50
170
350
mV
1.400
1.500
1.600
V
4.500
5.500
6.500
V
MIN.
TYP.
10
18
5
5
20
25
45
40
MAX.
20
36
10
10
Units
Ω
Ω
Ω
Ω
ns
ns
ns
ns
MIN.
TYP.
MAX.
Units
40
100
μA
0.9
1.5
mA
1.2
2.0
mA
0.9
1.5
mA
VCSCL1
VCSCL2
FB terminal<1.35V
CS sink current = +1μA
FB terminal>1.65V
CS sink current = +150μA
(9)Output Stage Section (OUT terminal)
Item
Symbol
Conditions
VCC = 6V, Source Current = -50mA
RONH
High Side
On Resistance
VCC = 2.5V,Source Current = -50mA
RONH
VCC = 6V,
Sink Current = +50mA
RONL
Low Side
On Resistance
VCC = 2.5V, Sink Current = +50mA
RONL
330pF Load to GND terminal
FA7700
Rise Time
tr
330pF Load to VCC terminal
FA7701
330pF Load to GND terminal
FA7700
Fall Time
tf
330pF Load to VCC terminal
FA7701
(10)Overall Section (Supply Current to VCC terminal)
Item
Symbol
Conditions
OFF mode
CS terminal=0V
ICCST1
Supply Current
Duty Cycle = 0%, OUT:open
ICC0
IN- =0V, FB:open
Operating mode
Supply Current
Duty Cycle = 50%, OUT:open
ICC1
IN-, FB:shorted
Latched mode
CS terminal >2.35V
ICCLAT
Supply Current
IN- = 0V, FB:open
8
Quality is our message
7.
. characteristics curve
Oscillation frequency vs. ambient temperature
Timing resistor vs. Oscillation frequency
Oscillation frequency variation[%]
Oscillation frequency [kHz]
10000
1000
100
10
1
10
5
4
3
2
1
0
-1
-2
-3
-4
-5
fosc=1MHz
fosc=185kHz
fosc=50 kHz
-40
100
FB terminal voltage vs. Duty cycle
FA7700
80
80
Duty cycle [%]
Duty cycle [%]
90
fosc=1MHz
60
50
40
30
20
60
80
100
fosc=1MHz
60
50
40
30
fosc=185kHz
10
0
0
0.5
0.7
0.9
1.1
FB terminal voltage [V]
0.5
1.3
FB terminal voltage vs. Duty cycle
FA7701
100
100
90
90
80
80
70
fosc=1MHz
60
0.7
0.9
1.1
CS terminal voltage [V]
1.3
CS terminal voltage vs. Duty cycle
FA7701
Duty cycle [%]
Duty cycle [%]
40
70
20
fosc=185kHz
10
20
CS terminal voltage vs. Duty cycle
FA7700
100
90
70
0
Ambient temperature Ta [℃]
Timing resistor R T [kΩ]
100
-20
50
40
30
70
fosc=1MHz
60
50
40
30
20
20
fosc=185kHz
10
10
fosc=185kHz
0
0
0.5
0.7
0.9
1.1
FB terminal voltage [V]
0.5
1.3
9
0.7
0.9
1.1
CS terminal voltage [V]
1.3
Quality is our message
Maximum Duty cycle vs. ambient temperature
FA7700
Error Amp. Reference voltage vs.
ambient temperature
0.90
94
92
Reference voltage [V]
Maximum Duty cycle [%]
fosc=50 kHz
90
88
fosc=185kHz
86
fosc=1MHz
84
0.89
0.88
0.87
82
0.86
80
-40
-20
0
20
40
60
Ambient temperature Ta [℃]
80
-40
100
Internal bias voltage vs. ambient temperature
80
100
2.20
VCC terminal ON/OFF threshold
Internal bias voltage [V]
0
20
40
60
Ambient temperature Ta [℃]
Under voltage lock-out vs. ambient temperature
2.28
2.26
2.24
2.22
2.20
2.18
2.15
V CCON
2.10
2.05
2.00
1.95
V CCOFF
1.90
1.85
1.80
-40
-20
0
20
40
60
Ambient temperature Ta [℃]
80
100
-40
CS terminal ON/OFF threshold vs.
ambient temperature
0.40
-20
0
20
40
60
Ambient temperatureTa [℃]
80
100
CS terminal voltage vs.CS terminal sink current
200
CS terminal sink current [uA]
CS terminal ON/OFF threshold [V]
-20
0.35
0.30
0.25
0.20
0.15
-40
-20
0
20
40
60
Amient temperature Ta [℃]
80
180
Ta=-30℃
160
Ta=85℃
140
120
Ta=25℃
100
80
60
FB<1.35V
FB>1.65V
40
20
0
100
0
10
1
2
3
4
5
CS terminal voltage [V]
6
7
Quality is our message
Vcc vs. Operating mode supply current
Duty=50%
IN(-)-FB:shorted
3.0
Operating mode supply current [mA]
2.0
Operating mode supply current [mA]
Vcc vs. operating mode supply current
fosc=1MHz
1.5
1.0
fosc=185kHz
0.5
Duty=50%
IN(-)-FB:shorted
2.5
fosc=1MHz
2.0
fosc=185kHz
1.5
1.0
0.5
0.0
0.0
0
0.5
1
1.5
Vcc [V]
2
2.5
4
3
CS=0V
10
12
14
Vcc [V]
16
18
20
RT=22kΩ
1.5
Operating mode supply current [mA]
OFF mode supply current [uA]
60
55
Vcc=20V
45
40
Vcc= 6V
35
Vcc=20V (Duty=50%)
1.4
1.3
1.2
1.1
Vcc= 6V (Duty=50%)
1.0
0.9
0.8
Vcc= 6V (Duty=0%)
0.7
0.6
30
-40
-20
0
20
40
60
Temperature Ta [℃]
80
-40
100
Vcc=6V
RT=22kΩ
CS>2.35V
0.95
0.90
0.85
0.80
0.75
3.0
Operating mode supply current [mA]
1.00
-20
0
20
40
60
Temperature Ta [℃]
Oscillation frequency vs.
operating mode supply current
Latched mode supply current vs. temperature
Latched mode supply current [mA]
8
Operating mode supply current vs. temperature
OFF mode supply current vs. temperature
50
6
80
100
VCC=6V
Duty=50%
2.5
2.0
1.5
1.0
0.5
0.0
0.70
-40
-20
0
20
40
60
Temperature Ta [℃]
80
10
100
11
100
Oscillation frequency [kHz]
1000
Quality is our message
OUT terminal High side voltage vs.
source current
OUT terminal Low side voltage vs. sink current
200
400
350
OUT terminal sink current [mA]
OUT terminal source current [mA]
450
Vcc=20V
300
Vcc=12V
250
Vcc= 6V
200
150
100
Vcc=2.5V
50
0
0
5
10
15
20
25
150
100
50
0
OUT terminal voltage [V]
0
Error Amplifier Gain and Phase vs. frequency
12
0.5
1
OUT terminal voltage [V]
1.5
Quality is our message
8.
. Description of each circuit
(1) Reference Voltage Circuit
This circuit consists of the reference voltage circuit using band gap reference, and also serves as the power
supply of the internal circuit. The precision of output is 2.23V±3%. It is stabilized under the supply voltage of
2.5V or over. The precision of reference voltage of error amplifier circuit is 0.88V±2%, and the reference
voltage circuit is connected to the non-inverting input of the error amplifier circuit.
(2) Oscillator
The oscillator generates a triangular waveform by charging and discharging the
built-in capacitor. A desired oscillation frequency can be determined by the value
of the resistor “RT”connected to the RT terminal (Fig. 1). The built-in capacitor
voltage oscillates between approximately 0.66V and 1.1V with almost the same
charging and discharging gradients. You can set the desired oscillation frequency
by changing the gradients using the resistor connected to the RT terminal. (Large
RT: low frequency, small RT: high frequency) The oscillator waveform cannot be
observed from the outside because a
RT value : small
1.1V
terminal for this purpose is not provided.
The oscillator output is connected to the
PWM comparator.
0.66V
OSC
1 RT
RT
Fig.1
RT value : large
fig.2
(3) Error Amplifier Circuit
The IN(-) terminal (Pin3) is an inverting input terminal. The nonInverting input is internally connected to the reference voltage
(0.88V±2%; 25℃). The FB terminal (Pin4) is the output of the
error amplifier. Gain setting and phase compensation setting is
done by connecting a capacitance and a resistor between the FB
terminal and the IN(-) terminal. Vout which is the output voltage of
DC to DC converter can be calculated by:
Vout = VB ×
Vout
RNF
R1
Er.AMP
3
4
IN(‑)
FB
+
R2
R1 + R 2
R2
VB
(0.88V)
PWM
fig.3
Gain AV between the Vout and the FB terminal can be calculated by:
AV = −
RNF
R1
(4) PWM comparator
The PWM comparator has 4 input
terminals. (Fig. 4) The oscillator output
① is compared with the CS terminal
voltage ② , and the error amplifier
voltage ③ ,then, the lower voltage
between ② and ③ is preferred.
While the preferred voltage is lower
than the oscillator output, the PWM
comparator output is LOW. While the
preferred voltage is higher than the
oscillator output, the PWM comparator
output is HIGH(Fig. 5). When the IC
starts, the capacitor connected to the
①Oscillation output
②CS terminal voltage
③Error Amplifier output
④DT voltage
‑
+
+
+
③Error Amplifier output
①Oscillation output
PWM output
fig.4
②CS terminal voltage
④DT voltage
PWM
output pulse
fig.5
13
Quality is our message
CS terminal is charged through the resistor connected to the power supply , and then the output pulses begin to
widen gradually as the operation of soft start. In steady operation, the pulse width is determined based on the
voltage of the error amplifier③, and then the output voltage is stabilized. The Dead Time control voltage (④DT
voltage) of FA7700 and FA7701 has different characteristics to adjust the ICs to various types of power supply
circuits being controlled and also to reduce external discrete components as many as possible. FA7700 is
developed for fly-back circuits, and boost circuits, and the DT voltage is set in the IC so that the maximum output
duty cycle is fixed to 80%(min.). (Maximum output duty cycle changes according to operation frequencies.—
See P10 “Maximum output duty vs. temperature”.) It prevents magnetic saturation of the transformer or the like
when a short-circuit in the output circuit occurs. FA7701 is developed for buck circuits, and it is designed for the
maximum output duty cycle of 100%. The timing chart of PWM comparator is described in Fig. 5.
VCC
REF OFF
+
Rcs
C3
0.3 V
CS
5.5V
C1
1.5V
FB
Output
off
S.C.P
+
Cs
ON/OFF
8
2.2V
C2
+
(5) Soft start function
As described in Fig. 6, RCS is connected
between CS terminal and VCC terminal, and
Cs is connected between CS terminal and
GND. The voltage of CS terminal rises
when starting the power supply, because Cs
is charged by Vcc through Rcs. The soft
start function starts by charging a capacitor
Cs connected to PWM comparator. To
estimate the soft start period, the time(ts)
between the start and the moment when the
width of output pulse reaches 50% is
calculated by:
S.C.DET
1.5V
Vcc
ts [ms]≒Cs × Rcs × ln(
)
Vcc − 0.88
fig.6
Cs: Capacity of Cs [μF]
Rcs: resistance of Rcs [kΩ]
Vcc: supply voltage [V]
The maximum current flowing in Rcs should be within the recommended value(50μA max.).
VCC − 1.5
VCC − 1.5
< RCS[ MΩ] <
50uA + IL
1uA + IL
(IL: leak current of capacitor Cs)
Note) This IC operates ON/OFF function by the CS terminal(CS<0.3V typ. :OFF), then it turns off the internal
bias voltage VREF (off mode). Therefore, you can not connect the resistor “Rcs” between CS terminal and REF
terminal, and can connect the resistor only to VCC terminal.
(6) ON/OFF circuit
The ON/OFF function can be controlled by external signal to the CS
terminal,the IC becomes off mode .When the CS terminal voltage is
below 0.30V(typ.), the output of ON/OFF comparator C3 is set to LOW,
and the internal power source VREF is shut off, then the IC is switched
to the off mode. The power consumption in the off mode is 40μA(typ.).
A sample circuit is given in Fig. 7.
Vcc
CS
ON/OFF
Cs
fig.7
14
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CS terminal voltage [V]
(7) Timer latch short-circuit protection circuit
The short-circuit protection
6
circuit
consists
of
two
Lower value of either 5.5V or Vcc terminal voltage
comparators C1, C2(Fig. 6). In
5
steady operation, the output
of S.C.DET comparator C2 is
4
set to High, and the CS terminal
Start-up
3
is clamped by the 1.5V Zener
2.2V
diode,because the output of
2
error amplifier is about 1V. If the
1.5V
short circuit protection
tp
converter output voltage drops
1
momentary short circuit
due to a short-circuit etc, when
short circuit
soft
start
0
the output voltage of error
Time
amplifier rises. excesses 1.5V,
fig.8
the
output
of
S.C.DET
comparator C2 is set to Low,
and then the clamp of Zener diode is turned off. As a result, the voltage of CS terminal rises up to the lower
value of either 5.5 V or the voltage of VCC terminal. If the voltage of CS terminal excesses 2.2V, the output of
S.C.P comparator C1 is set to High,and the circuit shuts down the output circuit of the IC. When it occurs, the
current consumption of the IC is 0.9mA(typ.) because the IC is set to OFF latch mode. The period (tp) between
the occurrence of a short-circuit in the converter output and the triggering of the short-circuit protection function
can be calculated by the following expression:
Vcc − 1.5
tp [ms]≒Cs × Rcs × ln(
)
Vcc − 2.2
Cs: capacitance of Cs[μF]
Rcs: Resistance of Rcs[kΩ]
Vcc: supply voltage [V]
Note) When the IC is used in a product with low VCC voltage, the period (tp) of the triggering of the shortcircuit protection described above fluctuates significantly. Therefore, sufficient care should be taken in such
cases.
Ex.) When Rcs=750kΩ, Cs=0.1μF:
Vcc=2.5V: tp≒90ms
Vcc=3.6V: tp≒30ms
You can reset the off latch mode operation of the short-circuit protection by either of the following ways: lowering
the CS voltage below 2.03V(typ.); lowering the Vcc voltage below the OFF threshold voltage of Under voltage
Lock out ; 1.93V(typ.); lowering the voltage of FB terminal below 1.5V(typ.)
The off latch mode action cannot be triggered by externally applying voltage of over 2.2 V forcibly to the CS
terminal (1.5V,ZD clamped) Characteristics of the current and the voltage of CS terminal is shown in the
characteristics curve [CS terminal voltage vs. CS terminal sink current] in page 10. Be sure to use the IC up to
the recommended CS terminal current of 50μA.
15
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(8) Output circuit
The IC contains a push-pull output stage and can directly drive MOSFETs (FA7700: N ch, FA7701: P ch). The
maximum peak current of the output stage is a sink current of +150mA, and a source current of - 400mA. The
IC can also drive NPN, and PNP transistors. The maximum peak current in such cases is ± 50mA. Be sure to
design the output current considering the rating of power dissipation.
(9) Power good signal circuit/ Undervoltage lockout circuit
The IC contains a protection circuit against Undervoltage malfunctions to protect the circuit from the damage
caused by malfunctions when the supply voltage drops. When the supply voltage rises from 0V, the circuit starts
to operate at VCC of 2.07V(typ.) and outputs generate pulses. If a drop of the supply voltage occurs, it stops
output at VCC of 1.93V(typ.). when it occurs, the CS terminal is turned to LOW level and then it is reset. The
power good signal circuit monitors the voltage of REF terminal, and stops output until the voltage of REF
terminal excesses approximately 2V to prevent malfunctions.
16
Quality is our message
9.
. Design Advice
(1) Setting the oscillation frequency
As described in Section 8(1), “Description of Each Circuit,” a desired oscillation frequency can be determined by
the value of the resistor connected to the RT terminal. When designing an oscillation frequency, you can set any
frequency between 50kHz and 1MHz. You can roughly obtain the oscillation frequency from the characteristic
curve “Oscillation frequency(fosc) vs. timing resistor resistance(RT)” or the value can be calculated by the
following expression.
fosc = 3000 × RT −0.9
 3000 

RT = 
 fosc 
1.11
fosc: oscillation frequency [kHz]
RT: timing resistor [kΩ]
This expression, however, can be used for rough calculation, the value obtained is not guaranteed. The
operation frequency varies due to the conditions such as tolerance of the characteristics of the ICs, influence of
noises, or external discrete components. When determining the values, be sure to verify the effectiveness of the
values of the components in an actual circuit.
(2) Operation around the maximum or the minimum output duties
As described in P9 of characteristic curves of “FB terminal voltage (VFB) vs.output duty cycle” and “CS terminal
voltage (Vcs) vs. output duty cycle”, the linearity of the output duty of this IC drops around the minimum output
duty and the maximum output duty(FA7701 only). This phenomena are conspicuous when operating in a high
frequency(when the pulse width is narrow). Therefore be careful when using high frequency.
(3) Restriction of external discrete components
To achieve a stable operation of the ICs, the value of external discrete components connected to Vcc, REF, CS,
FB terminals should be within the recommended operational conditions.
(4) Loss Calculation
Since it is difficult to measure IC loss directly, the calculation to obtain the approximate loss of the IC connected
directly to a MOSFET is described below.
When the supply voltage is Vcc, the current consumption of the IC is Icc, the total input gate charge of the driven
MOSFET is Qg, the switching frequency is fsw, the total loss Pd of the IC can be calculated by:
Pd ≒ Vcc×(Icc+Qg×fsw).
The values in this expression is influenced by the effects of the dependency of supply voltage, the
characteristics of temperature, or tolerance. Therefore,be sure to verify appropriateness of the value considering
the factors above under all applicable conditions.
Example)
When VCC=6V, in the case of a typical IC, from the characteristic curve, Icc=1.2mA. When operating in
Qg=6nC, fsw=500kHz, Pd should be:
Pd≒6×(1.2mA+6nC×500kHz)≒25.2mW
17
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10.
.Application
Vin
2.5〜11V
Vout
12V/0.2A
8
CS
7
VCC
ON /OFF
6
OUT
5
GND
FA7700
RT
1
REF
2
IN‑
3
FB
4
Vin
Vout
8
CS
ON /OFF
7
VCC
6
OUT
5
GND
FA7700
RT
1
REF
2
IN‑
3
18
FB
4
Quality is our message
Vin
7〜18V
Vout
5V/0.5A
8
CS
ON /OFF
7
VCC
6
OUT
5
GND
FA7701
RT
1
REF
2
IN‑
3
19
FB
4
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