FA3647P [FUJI]
PWM control IC with light load power saving function For Switching Power Supply Control; PWM控制IC，具有轻载节能功能对于开关电源控制
| 型号： | FA3647P |
| 厂家： | 
FUJI ELECTRIC |
| 描述： | PWM control IC with light load power saving function For Switching Power Supply Control |
| 文件： | 总21页 (文件大小：407K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
PWM control IC with light load power saving function
For Switching Power Supply Control
FA3641P(N), FA3647P(N)
ꢀ Description
ꢀ Dimensions, mm
ꢀ SOP-8
The FA3641P(N) and FA3647P(N) are the PWM type switching
power supply control ICs that can directly drive power MOSFET.
These ICs use a CMOS device with high dielectric strength
(30V) to implement low power consumption. They feature a
number of functions that are equivalent to those of the FA531X
series consisting of bipolar devices. In addition, they have a
function that reduces the oscillation frequency to suppress loss
of the power supply in light load mode and support an
overvoltage protection detecting Vcc voltage. These ICs are
most suitable for high-performance, energy-saving power
supplies that require low input power in standby or no-load
mode.
5
8
1
4
4.9
ꢀ Features
• Uses a newly developed CMOS process with high dielectric
strength (30V) for implementing low power consumption
• Standby current of 2µA or less (at Vcc=14V), and operating
current of 1.9mA (typ.)
0.4±0.1 1.27±0.2
• Automatically reduces the oscillation frequency to suppress
loss of the power supply in light load mode
ꢀ DIP-8
• Overvoltage protection function detecting Vcc voltage
• A drive circuit for connecting a power MOSFET directly
• Output peak current: Source current –500mA
8
1
5
Sink current
+1A
• Pulse-by-pulse overcurrent limiting function
• Overload cutoff function (latch or non-latch mode selectable)
• Output ON/OFF control function by external signal
• Latch mode overvoltage shutdown function
• Undervoltage lockout function
4
9.3
1.5
(16.5V ON / 9V OFF)
• Reference voltage output (5V)
• 8-pin package (DIP/SOP)
7.62
2.54±0.25 0.46±0.1
58
FA3641P(N), FA3647P(N)
ꢀ Block diagram
ꢀ FA3641
ꢀ FA3647
Pin No. Symbol Function
Description
1
RT
Oscillator timing
resistor
Setting oscillation frequency
2
3
FB
Feedback
Input to PWM comparator
IS (+)/ Overcurrent
Input to overcurrent limiting
function
IS (–)
GND
OUT
detection
Ground
Output
4
5
Ground
Output for direct driving a
power MOSFET
6
7
VCC
REF
Power supply
Power supply for IC
Reference voltage Reference voltage output
(5V)
8
CS
Soft-start and
Soft-start, ON/OFF and latch-
mode shutdown operations
ON/OFF control
ꢀꢀAbsolute maximum ratings (Ta=25˚C)
Item
Supply voltage
Symbol Test condition
Rating
Unit
V
VCC1
VCC2
IOL
Low impedance source (Icc >15mA) 30
Internal ZD clamp (Icc < 15mA)
Sink current
Self limiting
V
Output peak current
+1.0
A
IOH
Source current
–0.5
A
FB pin input voltage
REF pin output current
IS pin input voltage
CS pin input current
Total power dissipation
VFB
IREF
VIS
–0.3 to 5.0
–10
V
mA
V
–0.3 to 5.0
ICS
2.0
mA
mW
1
Pd
at Ta =25˚C
DIP
800
*
2
SOP
400
*
Operating temperature
Junction temperature
Storage temperature
Topr
Tj
–30 to +85
˚C
˚C
˚C
125
Tstg
–40 to +150
1
Note:
>
˚C: 8.0mW/˚C
Maximum power dissipation curve
* Derating factor Ta 25
2
>
˚C: 4.0mW/˚C
* Derating factor Ta 25
400mW (SOP)
800mW (DIP)
0
–30
25
85
125
Ambient temperature Ta [˚C]
59
FA3641P(N), FA3647P(N)
ꢀ Recommended operating condition
Item
Symbol
VCC
Min.
10
Typ.
Max.
Unit
V
Supply voltage
28
REF pin bypass capacitor
Soft-start capacitor
Cref
0.1
0.01
30
0.47
µF
CS
1
µF
Oscillation frequency (FB >1.2V)
Min.oscillation frequency at light load mode (FB <1.2V)
fOSC
fOSCL
500
kHz
kHz
10
ꢀ Electrical characteristics (Vcc=18V, RT=47kΩ, Ta=25˚C)
Reference voltage section
Item
Symbol
VREF
VdV
Test condition
Tj=25˚C
Min.
Typ.
Max.
Unit
V
Reference voltage
4.75
5.00
±6
5.25
±20
±20
Voltage variation 1 (Line regulation)
Voltage variation 2 (Load regulation))
Voltage variation 3 (Temperature stability)
VCC=10 to 28V
mV
VdV
IL=0 to 10mA, Vcc=18V
±6
mV
VdT
Ta= –30 to +85°C
±0.5
mV/°C
Oscillator section
Item
Symbol
fOSC
fdV
Test condition
RT=47kΩ, Tj=25˚C
VCC=10 to 28V
Min.
Typ.
100
Max.
Unit
kHz
%
Oscillation frequency
92.6
107.4
Frequency variation 1 (Voltage stability)
Frequency variation 2 (Temperature stability)
±1.6
±0.02
fdT
Ta= –30 to +85°C
% / °C
Pulse width modulation circuit section
Item
Symbol
IFB
Test condition
VFB=0V
Min.
–985
0.95
Typ.
–750
1.03
2.40
70
Max.
Unit
µA
V
FB pin source current
Input threshold voltage (FB pin)
–615
VTH FBO
VTH FBM
DMAX
Duty cycle =0%
Duty cycle =DMAX
VFB=2.5V
V
Maximum duty cycle
66
74
%
Reducing oscillation frequency section
Item
Symbol
VTH FBS
kfS1
Test condition
Min.
Typ.
1.18
16.7
46
Max.
Unit
V
FB pin threshold voltage
Frequency reduction
Minimum oscillation frequency
VFB=1.10 to 1.15V
kHZ
kHZ
foscS2
Overcurrent limiting circuit section
Item
Symbol Test condition FA3641P/N
FA3647P/N
Unit
Min. Typ. Max. Min. Typ. Max.
Input threshold voltage (IS pin)
Input terminal source current (IS pin)
Delay time
VTH IS
I IS
215
235
255
–188 –168 –148 mV
VIS =0V
–5
–20
µA
TPD IS
150
150
ns
Soft-start circuit section
Item
Symbol
ICHG
Test condition
Min.
–4.0
0.95
Typ.
–6.5
1.03
2.40
Max.
Unit
µA
V
Charge current (CS pin)
Input threshold voltage (CS pin)
VCS =1V, Tj=25˚C
Duty cycle =0%
Duty cycle =DMAX
–9.0
VTH CSO
VTH CSM
V
60
FA3641P(N), FA3647P(N)
Output ON/OFF circuit section
Item
Symbol
socs
Test condition
Min.
Typ.
–6.5
0.82
0.68
Max.
–9.0
0.95
Unit
µA
V
CS pinl source current
I
VCS=0V, Tj=25˚C
OFF→ON, Tj=25˚C
ON→OFF, Tj=25˚C
–4.0
OFF-to-ON threshold voltage (CS pin)
ON-to-OFF threshold voltage (CS pin)
VTH ON
VTH OFF
0.50
V
Latch-mode cutoff circuit section
Item
Symbol
ISICS
Test condition
Min.
20
Typ.
35
Max.
50
Unit
µA
V
CS pin sink current
Cutoff threshold voltage (CS pin)
VCS=6.5V, VFB=1V, Tj=25˚C
ON→OFF, Tj=25˚C
OFF→ON, Tj=25˚C
VTH CSF
VTH CSN
VTH HIS
8.0
7.4
8.5
7.9
0.6
9.0
8.4
V
Hysteresis
V
Overload cutoff circuit section
Item
Symbol Test condition
Min.
Typ.
Max.
Unit
Cutoff threshold voltage (FB pin)
VTH FB
2.8
3.0
3.3
V
Overvoltage cutoff circuit section
Item
Symbol
VTH VCC
I VCC
Test condition
Tj=25˚C
Min.
Typ.
32
Max.
Unit
V
Cutoff threshold voltage (Vcc pin)
Cutoff operating supply current (Vcc pin)
Charge current (CS pin)
30
34
Tj=25˚C
13
mA
mA
ISO CS2
VCS=6.5V
–0.5
–0.9
–1.4
Undervoltage lockout circuit section
Item
Symbol
VCC ON
VCC OFF
VHYS
Test condition
Tj=25˚C
Min.
15.5
8.5
Typ.
16.5
9.0
Max.
17.5
10.0
8.2
Unit
V
OFF-to-ON threshold voltage
ON-to-OFF threshold voltage
Hysteresis
Tj=25˚C
V
Tj=25˚C
6.8
7.5
V
Output section
Item
Symbol
Test condition
IO=100mA
Min.
Typ.
0.7
16.5
50
Max.
Unit
V
L-level output Voltage
H-level output Voltage
Rise time
VOL
VOH
tr
1.5
IO= –100mA, VCC=18V
OUT=1000pF
15
V
ns
ns
Fall time
tf
OUT=1000pF
40
Overall device
Item
Symbol
ICC STB
ICC ST
ICC OP
ICCOF
ICCL
Test condition
VCC=14V
Min.
Typ.
Max.
2
Unit
µA
Standby current
Startup current
VCC=VCCON
No load
12
30
µA
Operating-state supply current
OFF-state supply current
Cutoff-state supply current
1.9
100
45
2.5
mA
µA
VCC=17V, Cs=0V
VCC=10V
100
µA
61
FA3641P(N), FA3647P(N)
ꢀꢀCharacteristic curves (Ta=25˚C)
Oscillation frequency (fosc) vs.
timing resistor resistance (RT)
Oscillation frequency (fosc) vs. supply voltage (Vcc)
105
104
103
102
101
1000
100
100
99
98
97
VCC = 18V
FB = 1.5V
OUT = No load
= 47 kΩ
Rt
FB = Open, CS = 3V
96
95
10
1
10
100
1000
10
15
25
30
20
Rt [kΩ]
Vcc [V]
Oscillation frequency (fosc) vs.
junction temperature (Tj)
L-level output vltage (VOL) vs. supply voltage (Vcc)
105
1
= 47kΩ
Rt
FB = 2.5V
104
103
102
101
100
99
0.8
0.6
0.4
0.2
= 100 mA
Io
98
97
0
100
150
10
15
25
30
–50
0
50
20
Vcc [V]
Tj [°C]
Supply current (Icc) vs. supply voltage (Vcc)
Supply current (Icc) vs. junction temperature (Tj)
Operating mode
Operating mode
2
2
1.95
1.95
1.9
1.9
1.85
1.85
1.8
1.8
= 47kΩ
Vcc = 18V
FB = 0V
Rt
= 47kΩ
Rt
FB = 0V
1.75
10
1.75
–50
25
30
100
150
15
0
20
50
Vcc [V]
Tj [°C]
62
FA3641P(N), FA3647P(N)
Supply current (Icc) vs. supply voltage (Vcc)
Supply current (Icc) vs. supply voltage (Vcc)
Latch mode
Latch mode
200
3000
2500
160
2000
1500
1000
120
80
40
0
500
0
25
30
10
15
20
16
18
20
10
12
14
Vcc [V]
Vcc [V]
Supply current (Icc) vs. supply voltage (Vcc)
Supply current (Icc) vs. supply voltage (Vcc)
OFF mode
OFF mode
200
160
3000
2500
2000
1500
1000
120
80
40
0
500
0
16
18
20
10
12
14
25
30
10
15
20
Vcc [V]
Vcc [V]
UVLO OFF-to-ON thrreshold voltage (Vcc on) vs.
junction temperature (Tj)
UVLO ON-to-OFF thrreshold voltage (Vcc off) vs.
junction temperature (Tj)
1.7
9.2
16.8
9.1
9
16.6
16.4
8.9
8.8
16.2
16
100
150
–50
0
50
100
150
–50
0
50
Tj [°C]
Tj [°C]
63
FA3641P(N), FA3647P(N)
CS terminal current (Ics) vs. CS terminal voltage (Vcs)
CS terminal current (Ics) vs. CS terminal voltage (Vcs)
20
50
FB = Open
FB = OV
15
40
10
5
30
20
10
0
–5
0
–10
–10
10
6
8
12
150
3
0
2
4
10
6
8
12
0
2
4
Vcs [V]
Vcs [V]
CS terminal charge current (Ichg) vs.
junction temperature (Tj)
FB terminal source current (IFB) vs.
FB terminal voltag (VFB)
0
–100
–200
–300
–5
–6
–7
–8
–400
–500
–600
–9
Vcc =18V
CS = 0V
–700
–800
–10
5
3
4
100
0
1
2
–50
0
50
Tj [°C]
VFB [V]
IS (+) terminal current (IIS (+)) vs.
IS (+) terminal voltage (VIS (+))
FA3641
IS (–) terminal current (IIS (–)) vs.
IS (–) terminal voltage (VIS (–))
FA3647
5
0
0
–0.1
–5
–10
–0.2
–0.3
–0.4
–15
–20
–25
1.5
2
2.5
1.5
IS (–) [V]
0
0.5
–0.5
0
0.5
1
2.5
1
2
3
V
IS (+) [V]
V
64
FA3641P(N), FA3647P(N)
ꢀ Description of each circuit
1. Oscillator
The oscillator generates a triangular waveform by charging and
discharging the built-in capacitor. A desired oscillation
frequency can be set by the value of the resistor connected to
the RT pin (See Figure 1).
The built-in capacitor voltage oscillates between about 3V and
1V, with almost the same charging and discharging gradients
(Figure 2). You can set the desired oscillation frequency by
changing the gradients using the resistor connected to the RT
pin. (Large Rt = low frequency, small Rt = high frequency) The
oscillation frequency is automatically lowered when output duty
cycle is small (FB ꢀ about 1.18V) in light load mode. For more
information, see item 2, “Reducing oscillation frequency circuit
in light-load mode.”
Fig. 1 Oscillator
The relationship between Rt and the fixed oscillation frequency
is approximately given by:
4880
Rt + 1.4
Fig. 2 Oscillator output
f0 [kHz] ꢀ
..................................................... (1)
4880
f0
Rt [kΩ] ꢀ
– 1.4
.......................................... (2)
f
O
: Fixed frequency [kHz]
Rt: Timing resistance [kΩ]
The oscillator waveform cannot be observed from the
outside because a pin for this purpose is not provided.
The oscillator output is connected to a PWM comparator.
The RT pin is 2.5V DC in normal fixed frequency operation
mode. When the frequency is lowered, the voltage also
decreases linearly to about 1V.
2. Reducing oscillation frequency circuit in light-load mode
To reduce the loss of the power supply in standby mode, this
IC has a feature that automatically lowers the oscillation
frequency when the load is light. When the load is light, with
the result that the IC output pulse width narrows below about
10% and the FB pin voltage decreases below about 1.18V,
the oscillation frequency begins to decreases linearly until
the output pulse width becomes 0. When the output pulse
width is 0, the oscillation frequency is about 46% of normal
fixed frequency (Figure 3). Even while the oscillation
frequency is decreasing, the built-in capacitor voltage
oscillates between about 3V and 1V.
Fig. 3 Oscillation frequency
The frequency reduction rate (46%) can be adjusted from
the outside. (See “Design advice” for more information.)
3. PWM comparator
Fig. 4 PWM comparator
The PWM comparator has four inputs as shown in Figure 4.
Oscillator output ቢ is compared with CS pin voltage ባ, FB
pin voltage ቤ, and DT voltage ብ. The lowest of three inputs
ባ, ቤ, and ብ has priority and is compared with output ቢ.
While the voltage is lower than the oscillator output, the
comparator output is high. While the voltage is higher than
the oscillator output, the PWM comparator output is low (see
Figure 5). The IC OUT pin voltage is high while the PWM
comparator output is low.
When the IC is powered up, CS pin voltage ባ controls soft
start operation. The output pulse then begins to widen
gradually. During normal operation, the output pulse width is
determined within the maximum duty cycle (70%) set by DT
voltage ብ under the condition set by FB pin voltage ቤ, to
stabilize the output voltage.
Fig. 5 PWM comparator timing chart
65
FA3641P(N), FA3647P(N)
4. CS pin circuit
As shown in Figure 6, capacitor Cs is connected to the CS pin.
The CS pin voltage varies depending on the charging voltage of
this capacitor Cs. When the power is turned on, the constant
current source (6.5µA) begins to charge capacitor. Accordingly,
the CS pin voltage rises as shown in Figure 7. The CS pin
voltage is connected to the PWM comparator, which is
characterized to make output based on the lowest of input
voltages. The device enters soft-start mode while the CS pin
voltage is between 1.0V and 2.4V. During normal operation, the
CS pin is clamped at 4.0V by internal zener diode.
If the output voltage drops due to an overload and the FB
voltage rises to 3V or more, the clamp voltage 4.0V is canceled
and the CS pin voltage rises to 9.5V. The CS pin is also
connected to latch comparator C2. If the CS pin voltage rises
to 8.5V or more, comparator C2 toggles to turn off the 5V REF
circuit, thereby shutting the output down. Since the CS pin is
also connected to comparator C1, the 5V REF circuit can be
turned off to shut the output down by dropping the CS pin
voltage below 0.68V. In this way, comparator C1 can be used
for output on-off control.
Fig. 6 CS pin circuit
As explained above, the CS pin can be used for soft-start,
overload output shutdown, and output on-off control by varying
the voltage.
Further details on the above three major functions of the CS pin
are given below.
4.1 Soft start function
Figure 8 shows the soft start circuit. Figure 9 is a soft-start
operation timing chart. The CS pin is connected to capacitor
Cs. When the power is turned on, the constant current source
(6.5µA) begins to charge the capacitor. As shown in the timing
chart, the CS pin voltage rises slowly in accordance with the
capacitor Cs charging current. The CS pin is also connected to
the IC internal PWM comparator, which has such
characteristics that the voltage is determined to output on the
basis of the lowest of input voltages. The comparator output
pulse slowly widens to cause a soft start as shown in the timing
chart.
Fig. 7 CS pin waveform
The soft start period can be approximately estimated by the
period t , from the time the IC is activated to the time the output
S
pulse width widens to 30%. The period is given by the following
equation:
ts [ms] ꢀ 250 ꢁ Cs ...................................................... (3)
Cs : Soft start capacitor [µF]
Fig. 8 Soft-sart circuit
Fig. 9 Soft-sart timing chart
66
FA3641P(N), FA3647P(N)
4.2 Overload shutdown function
Figure 10 shows the overload shutdown circuit, and Figure 11
is a timing chart that illustrates overload shutdown operation.
If the output voltage drops due to an overload or short circuit,
the FB pin output voltage rises. If the FB pin voltage exceeds
the reference voltage (3.0V) of comparator C3, the output of
comparator C3 goes low to turn off the switch. With the switch
off, the CS pin voltage clamped at 4.0V by zener diode in
normal operation is unclamped, and the constant current
source (6.5µA) begins to charge capacitor Cs again and the CS
pin voltage rises. When the CS pin voltage exceeds the
reference voltage (8.5V) of comparator C2, the output of
comparator C2 toggles to turn off the 5V REF circuit. The IC
then enters the latched mode and shuts down the output. IC
current consumption for shutdown is 45µA (typ) (Vcc = 10V).
This current must be supplied through the startup resistor. The
IC enters output off (low voltage) state.
The overload shutdown operation can be reset by lowering the
supply voltage Vcc to below the OFF threshold voltage (9.0V)
or forcing the CS pin voltage below 7.9V.
Fig. 10 Overload shutdown circuit
The period tOL from the time the output is short-circuited to the
time the output circuit goes off is given by the following
equation:
tOL [ms] ꢀ 690 ꢁ Cs .................................................. (4)
Cs: Soft start capacitor [µF]
When you want to disable the overload shutdown function, see
item 11 in “Design advice”
4.3 Output ON/OFF control function
The IC can be turned on or off via an external signal applied to
the CS pin. Figure 12 shows the output on/off control circuit,
and Figure 13 is a timing chart.
The IC is turned off when the CS pin voltage is externally made
to drop below 0.68V (typ). The output of comparator C1 goes
high to turn the 5V REF circuit. This shuts the output down.
The IC enters output off (low voltage) state. Required IC
current consumption during shutdown is 100µA (typ) (Vcc =
17V). This current must be supplied through the startup
resistor. The IC goes on when the CS pin is opened and the
CS pin voltage exceeds 0.82V (typ). This turns on the 5V REF
circuit and results in automatic soft start. The power supply
then restarts operation.
Fig. 11 Overload shutdown timing chart
Fig. 13 Output ON/OFF control circuit timing chart
Fig. 12 External output ON/OFF control circuit
67
FA3641P(N), FA3647P(N)
5. Overcurrent limiting circuit
The overcurrent limiting circuit detects the peak value of every
drain current pulse (pulse by pulse method) of the main
switching MOSFET to limit the overcurrent. The detection
threshold voltage is +0.235V for FA3641 or –0.168V for FA3647
with respect to the ground as shown in Figure 14.
The drain current of the MOSFET is converted to voltage by
resistor Rs and fed to the IS pin of the IC. If the voltage
exceeds the reference voltage +0.235V (FA3641) or –0.168V
(FA3647) of comparator C4, comparator C4 works to set flip-
flop output Q to high. The output is immediately turned off to
shut off the current. Flip-flop output Q is reset on the next cycle
to turn on the output again. This operation is repeated to limit
the overcurrent.
If the overcurrent limiting circuit malfunctions due to noise,
place an RC filter between the IS pin and MOSFET as shown in
Figure 14. (See item 14 in “Design advice.”)
Figure 15 is a timing chart that illustrates overcurrent-limiting
operations.
6. Vcc overvoltage protection circuit
The IC contains a Vcc overvoltage protection circuit to protect
the IC from damage by overvoltage. Figure 16 shows the
overvoltage protection circuit. Figure 17 is a timing chart that
illustrates overvoltage protection operations.
Overvoltage is detected if the supply voltage Vcc rises to 32V
(Icc = 13mA) or more and current flows in the built-in zener
diode. The output of comparator C5 then goes high and the
constant current source (0.9mA) raises the CS pin voltage.
When the CS pin voltage exceeds 8.5V, the output of
comparator C2 goes high to turn off the 5V REF circuit. The IC
then enters the latched mode and the IC output is put in the off
(low voltage) state. When latched mode, the IC current
consumption is 45µA (typ) (Vcc = 10V). This current must be
supplied through the startup resistor.
Fig. 14 Overcurrent limiting circuit
The overvoltage shutdown operation can be reset by lowering
the supply voltage to below 9.0V or forcing the CS pin voltage
below 7.9V.
(When you want to enable Vcc overvoltage shutdown at a
desired voltage, see item 7 in “Design advice.”
Fig. 15 Overcurrent timing chart
Fig. 16 Overvoltage shutdown circuit
Fig. 17 Overvoltage shutdown timing chart
68
FA3641P(N), FA3647P(N)
7. Undervoltage lockout circuit (U.V.L.O.)
The IC incorporates a circuit that prevents the IC from
malfunctioning when the supply voltage drops. When the
supply voltage is raised from 0V, the IC starts operation with
Vcc = 16.5V (typ). If the supply voltage drops, the output is
shut down when Vcc = 9.0V (typ). When the undervoltage
lockout circuit operates, the outputs of the OUT and CS pins go
low to reset the IC.
8. Output circuit
The IC contains a push-pull output stage and can directly drive
the MOSFET. The maximum peak current of the output stage is
a sink current of 1A and a source current of 0.5A. If the circuit
operation stops when the undervoltage lockout circuit operates,
the OUT pin voltage goes low to shut down the MOSFET.
Fig. 18 Oscillator circuit
ꢀ Design advice
1. Externally setting the oscillation frequency in the light-
load mode
As explained in item 2 in “Description of each circuit,” the IC has
a function that automatically lowers the oscillation frequency
when the load is light, to reduce the loss of the power supply in
standby mode. The oscillation frequency goes down to about
46% without adjustment by external circuit.
To further lower the frequency to below 46%, connect
adjustment resistor Rr between the RT and REF pins as shown
in Figure 18. Then the fixed frequency determined by Rt also
falls. The relationship between the external resistance and
oscillation frequency is outlined below:
2.35
3.35A – B
2.35
A – B
............................ (5)
Rt [kΩ] ꢀ
f0 [kHz] ꢀ
Rr [kΩ] ꢀ
4880
RtRr
Rr – Rt
2500
fr [kHz] ꢀ
....... (6)
RtRr
+ 6
+ 1.4
Rr – 3.35Rt
fO: Fixed frequency [kHz]
fr: Minimum frequency in variable mode [kHz]
Rt: Timing resistance [kΩ]
Rr:Adjustment resistance [kΩ]
f0
fr
A =
B =
4880 – 1.4f0
2500 – 6fr
Select Rt and Rr so that the relationship between the two
satisfies the following:
• Rt < 0.3 Rr .................................................................... (7)
• Set the minimum frequency in light-load mode to 10 kHz or
more.
Failure to keep the above relationship may disturb the
operation.
Note that the above expressions determine approximate values.
Note also that the minimum frequency in light-load mode
depends on such conditions as the power supply efficiency.
Therefore, check the operation using a practical circuit to make
a final decision.
Calculation example
To set the fixed frequency f = 100kHz and minimum frequency
O
in light-load mode to fr = 20kHz, the following can be obtained
from expressions (5).
Rt ꢀ 37.7 [kΩ]
Rr ꢀ 185.1 [kΩ]
Decrease Rr to permit the frequency to vary in a wider range.
69
FA3641P(N), FA3647P(N)
2. Deciding the startup circuit
These ICs, which use CMOS process, consume less current,
and therefore can use larger startup resistance than the
conventional bipolar type of IC.
To decide the startup resistance, the following conditions must
be satisfied:
(a) The IC is started when the power is turned on.
(b) The IC consumption current is supplied during latch
mode operation to maintain the latch state.
(c) The IC consumption current is supplied during the off state
under the on/off function to maintain the off state.
However, these are the minimum conditions for using the IC.
The startup time required for the power supply must also be
decided on.
2.1 Connecting a startup resistor before rectification
(AC line)
When the startup resistor is connected before rectification (AC
line) as shown in Figure 19, the voltage applied to the startup
resistor forms a half-wave rectified waveform of the AC input
voltage.
Fig. 19 Startup circuit (1)
Startup resistor R1 must satisfy the three equations shown
below. Select a smaller-side value for R1 in consideration of the
temperature characteristics.
(a) To supply startup current 30µA at ON threshold voltage
17.5V (max.) of UVLO:
2
π
ꢀ Vac – 17.5
........................................ (8)
R1 [kΩ] <
0.03
(b) To supply IC consumption current 100µA (max.) (Vcc =10V)
in latch mode:
2
π
ꢀ Vac – 10
....................................... (9)
R1 [kΩ] <
0.1
(c) To supply IC consumption current 200µA (max.) (Vcc =17V)
in the off state under the on/off function:
Fig. 20 Startup circuit (2)
2
π
ꢀ Vac – 17
......................................(10)
R1 [kΩ] <
0.2
R1: Startup resistance [kΩ]
Vac: Effective value of AC input voltage [V]
If neither the latch mode operation nor the on/off functions are
used, only the expression in (8) needs to be satisfied.
In this method, the supply current to the IC via the start-up
resistor is stopped when AC input is shut down.Therefore, after
latch mode operation, shutting the AC input down resets the
latch mode in a very short period of time.
2.2 Connecting the startup resistor after rectification
(DC line)
When the startup resistor is connected after rectification (DC
line) as shown in Figure 20, the voltage applied to the startup
resistor becomes the peak value of the AC input voltage.
Startup resistor R1 must satisfy the three equations shown
below. Select a smaller-side value for R1 in consideration of
temperature characteristics.
(a) To supply startup current 30µA at ON threshold voltage
17.5V (max.) of UVLO:
2 ꢀ Vac – 17.5
..................................(11)
R1 [kΩ] <
0.03
70
FA3641P(N), FA3647P(N)
When the capacitor value is adequate
(b) To supply IC consumption current 100µA (max.) (Vcc =10V)
in latch mode:
2 ꢀ Vac – 10
...................................(12)
R1 [kΩ] <
0.1
(c) To supply IC consumption current 200µA (max.) (Vcc = 17V)
in the off state under the on/off function:
2 ꢀ Vac – 17
...................................(13)
R1 [kΩ] <
0.2
R1: Startup resistance [kΩ]
Vac: Effective value of AC input voltage [V]
Fig. 21 Vcc voltage at startup with an adequate capacitor
When the capacitor value is inadequate
If neither the latch nor the on/off functions are used, only the
expression in (11) needs to be satisfied.
In this method, after latch mode operation, smoothing capacitor
C1 in the main circuit supplies current to the IC via the startup
resistor even if the AC input is shut down. Therefore, some time
must elapse before the latch mode is reset.
3. Determining the Vcc capacitor value
To properly start the power supply, a certain value is required
for the capacitor connected to the VCC pin.
Figure 21 shows the Vcc voltage at start-up when a proper
value is given to the capacitor.
When the input power is turned on, the capacitor connected to
the VCC pin is charged via the startup resistor and the voltage
increases. The IC is then in standby state and almost no
current is consumed. (Icc < 2µA)
Fig. 22 Vcc voltage at startup with an inadequate capacitor
Thereafter, Vcc reaches the ON threshold voltage of UVLO and
the IC begins operation.
When the IC begins operation to make output, the IC operates
based on the voltage from the auxiliary winding. When the IC is
just starting up, however, it takes time for the voltage from the
auxiliary winding to rise enough, and Vcc drops during this
period.
Determine the Vcc capacitor value so that Vcc will not drop
down to the OFF threshold voltage of UVLO during this period.
If the Vcc capacitor value is too small, Vcc will drop to the OFF
threshold voltage of UVLO before the auxiliary winding voltage
rises enugh. If so, Vcc repeatedly goes up and down between
the UVLO threshold voltages, and the power supply cannot
start up. (Figure 22)
Fig. 23 Startup circuit (3)
4. Shortening the startup period
Increasing the resistance of the startup resistor to reduce loss
prolongs the startup period. Figure 23 shows a circuit for
shortening the startup period. The C2 capacitance is
decreased to shorten the startup period and, after the IC starts
up, power is supplied from C3.
5. Setting soft start period and OFF latch delay independently
Figure 24 shows a circuit for setting the soft start period and
OFF latch delay independently. In this circuit, capacitance CS
determines the soft start period, and capacitance CL
determines the OFF latch delay.
If the overload shutdown or overvoltage shutdown functions
raise the CS pin voltage to around 5V, zener diode Zn becomes
conductive to charge capacitor CL. The OFF latch delay can be
thus prolonged by capacitance CL.
Fig. 24 Independent setting of soft start period and
OFF latch delay
71
FA3641P(N), FA3647P(N)
6. Overvoltage protection using the VCC pin
These ICs contain an overvoltage protection function detecting
the Vcc voltage using internal ZD (See item 6 in “Description of
each circuit”). If Vcc voltage exceed about 32V, the current of
13mA flows through the internal ZD and the overvoltage
protection function operates.
After this protection function operates, the IC continues to
consume the large current if high voltage continues to be
applied to the Vcc pin. Mind that total IC loss does not exceed
the rating.
If the voltage source applied to Vcc pin has relatively high
impedance and cannot supply the current of 13mA, overvoltage
protection function does not operate. But the internal ZD
maintains the Vcc voltage 32V or less and protects the IC.
7. Overvoltage protection using CS pin
These ICs contain the overvoltage protection function detecting
Vcc voltage. However, the threshold voltage is fixed. Adding a
circuit to CS pin enables the overvoltage protection detecting
desired voltage.
Fig. 25 Overvoltage shutdown circuit (1)
7.1 Detecting on secondary side
Figure 25 shows the overvoltage shutdown circuit based on the
signal from the secondary side. The optocoupler output
transistor is connected between the CS and Vcc pins. When
the output voltage is put in the overvoltage state, the
optocoupler output transistor goes on to raise the CS pin
voltage via resistor R2. When the CS pin voltage exceeds the
reference voltage (8.5V) of comparator C2, the output of the
comparator C2 goes high to turn off the 5V REF circuit.
Accordingly, the IC enters the OFF latch mode and shuts the
output down. The IC consumes current 45µA (typ) (Vcc = 10V)
in latch mode. This current must be supplied via startup
resistor R1.
Fig. 26 Overvoltage shutdown circuit (2)
The overvoltage protection circuit can be reset by lowering the
supply voltage Vcc to below 9.0V or forcing the CS pin voltage
below 7.9V.
In normal operation, the CS pin voltage is clamped by the 4V
zener diode with maximum sink current 50µA . Therefore, to
raise the CS pin voltage to 8.5V or more, 50µA or a higher
current needs to be supplied from the optocoupler. Set the
current input to the CS pin to 1mA or less.
7.2 Detecting on primary side (detecting Vcc voltage)
To attain overvoltage protection, the CS pin voltage is forcibly
raised from outside the IC until it exceeds the reference voltage
(8.5V) of the internal comparator C2. When the reference
voltage is exceeded, the IC enters latch mode and shuts the
output down. Connect a zener diode (ZD) and resistor between
the Vcc and CS pins as shown in Figure 26. When the Vcc
voltage exceeds about ZD voltage + 8.5V, the ICs enter the
OFF latch mode and shut the output down. If Vcc remains high
even after shutdown and current is input to the CS pin, set the
current to 1mA or lower.
Fig. 27 Overvoltage shutdown circuit (3)
Set the zener voltage of the ZD connected to the CS pin higher
than the UVLO ON threshold voltage. Startup is disabled below
this voltage.
Figure 27 shows another circuit for enabling latch mode
shutdown by detecting a desired Vcc voltage using the CS pin.
In this circuit, overvoltage shutdown works when the Vcc
voltage is about the same as the ZD voltage.
For this circuit also, use a ZD voltage higher than the UVLO ON
threshold voltage. Set the current flowing into the CS pin to
1mA or lower.
72
FA3641P(N), FA3647P(N)
8. Feedback pin circuit
Figure 28 gives an example of connection in which a feedback
signal is input to the FB pin.
If this circuit causes power supply instability, connect R3 and
C4 as shown in Figure 28 to decrease the frequency gain. Set
R3 between several tens of ohms to several kilohms and C4
between several thousand picofarads to one microfarad. Be
especially careful in light load mode, in which the frequency
drops, thereby increasing the probability of power supply
instability being triggered.
9. REF characteristics
If noise is applied to the VCC pin from the outside, it may
appear at the REF pin without attenuation depending on the
noise frequency. The noise causes no problems in normal IC
operation, but must be taken into consideration when the REF
voltage is used for an external circuit. If the noise appearing at
the REF pin causes any problems, use the REF pin as shown
in Figure 29.
Fig. 28 FB pin circuit
10. Simple voltage control on the primary side
In a flyback type power supply, the output voltages of the power
supply and auxiliary winding are almost proportional to the
number of winding turns of the transformer. This characteristic
can be used in the circuit shown in Figure 30, where the output
voltage can easily be made constant by detecting the voltage of
the auxiliary winding voltage.
Fig. 29 REF pin circuit
However, this is an easy output voltage control method, and the
output voltage precision and regulation are therefore not as
good.
To reduce output pulse width completely to 0%, the FB pin
voltage must fall below 0.95V and R5 must be set below about
960Ω from the characteristics of the FB pin voltage and source
current. When using this method, also keep in mind the
characteristics of REF in item 9.
11. Disabling the overload shutdown function
As shown in Figure 31, connect a 8.2kΩ resistor R6 between
the FB pin and the ground. The FB pin voltage then does not
rise sufficiently high to reach the shutdown threshold voltage
when an overload occurs so that IC does not enter OFF latch
mode. Even with this connection, the overvoltage shutdown
function is available.
Since resistor R6 limits the upper voltage of the FB pin, the
maximum duty cycle may be limited to about 65%, if a 5%
precision resistor is used. To not limit the maximum duty cycle,
use a 2% or better-precision resistor for R6.
Fig. 30 Simple voltage control circuit
12. Polarities for overcurrent detecting and their
characteristics
The FA3641 uses positive polarity detection for overcurrent
limiting (number 3 pin of IS pin) and the FA3647 uses negative
polarity detection. The characteristics of positive and negative
polarity detection are summarized below. Select one in
accordance with the circuit used. (See item 5 in “Description of
each circuit.”)
Positive detection (FA3641)
Fig. 31 Disabling overload shutdown function
• Wiring is easy because the ground can be shared by the
main circuit and IC peripherals.
• It is easy to correct the overload detecting current, which is
used to detect overload, against the input voltage.
Negative detection (FA3647)
• The MOSFET drive current does not flow to the current
detection resistor and therefore it hardly affects overcurrent
detection.
73
FA3641P(N), FA3647P(N)
13. Correcting overload detection current (FA3641 only)
If the power supply output is overloaded, the IC overcurrent
limiting function restricts the output power and the overload
shutdown function stops the IC. The output current when an
overload occurs varies depending on the input voltage; the
higher the input voltage, the more the overload detection current
may increase.
If any problems occur as a result of the appearance of this
symptom, connect resistor R8 between current detection
resistor Rs and the IS (+) pin and add resistor R7 for correction
as shown in Figure 32. The standard resistance of R8 is several
hundred ohms, and that of R7 is from several hundred kilohms
to several megohms.
Note that the above correction slightly lowers the output current
when overload even where the input voltage is low. This
correction is available only for the FA3641 that uses positive
polarity for overcurrent detection.
Fig. 32 Correcting overload detecting current circuit
14. Preventing malfunction caused by noise
The IS pin for overcurrent limiting function detects the MOSFET
current converted to the voltage.The parasitic capacitor and
inductor of the MOSFET, transformer, wiring, etc. cause a noise
in switching operation. If this switching noise causes a
malfunction of overcurrent limitimg function, insert the RC filter
into IS pin as shown in Figure 14.
Also, connect a noise prevention capacitor (0.1µF or more) to
the REF pin that outputs the reference voltage for each
component.
15. Preventing malfunction caused by negative voltage
applied to a pin
Fig. 33 Protection of OUT pin against the negative voltage
When large negative voltage is applied to each IC pin, a
parasitic element in the IC may operate and cause malfunction.
Be careful not to allow the voltage applied to each pin to drop
below –0.3V.
Especially for the OUT pin, voltage oscillation caused after the
MOSFET turns off may be applied to the OUT pin via the
parasitic capacitance of the MOSFET, causing the negative
voltage to be applied to the OUT pin. If the voltage falls below
–0.3V, add a Schottky diode between the OUT pin and the
ground. The forward voltage of the Schottky diode can suppress
the voltage applied to the OUT pin.
Fig. 34 Gate circuit
Use the low forward voltage of the Schottky diode. Similarly, be
careful not to cause the voltages at other pins to fall below –0.3V.
16. Gate circuit configuration
To adjust switching speeds or prevent oscillation at gate
terminals, resistors are normally inserted between the power
MOSFET gate terminal to be driven and the OUT pin of the IC.
You may prefer to decide on the drive current independently, to
turn the MOSFET on and off. If so, connect the MOSFET gate
terminal to the OUT pin of the IC as shown in Figure 34.
In the circuit shown in Figure 34, Rg1 and Rg2 restrict the
current when the MOSFET is turned on, and only Rg1 restricts
the current when it is turned off.
74
FA3641P(N), FA3647P(N)
17. Loss calculation
IC loss must be confirmed to use the IC within the ratings.
Since it is hard to directly measure IC loss, some examples of
calculating approximate IC loss are given below.
17.1 Calculation example 1
Suppose the supply voltage is Vcc, IC current consumption is
lccop, the total gate charge of the power MOSFET is Qg, and
the switching frequency is fSW. Total IC loss Pd can be
calculated by:
Pd = Vcc ꢁ (Iccop + Qg ꢁ fsw) ................................(14)
This expression calculates an approximate value of Pd, which is
normally a little larger than the actual loss. Since various
conditions such as temperature characteristics apply,
thoroughly verify the appropriateness of the calculation under
all applicable conditions.
Fig. 35 Output stage
Example:
When Vcc = 18V, lccop = 2.5mA (max.) is obtained from the
specifications. Suppose Qg = 80nC and fsw = 100kHz.
Pd ꢀ 18V ꢁ (2.5mA + 80nC ꢁ 100kHz)
= 189mW
17.2 Calculation example 2
The IC loss consists of the loss caused by operation of the
control circuit and the loss caused at the output circuit to drive
the power MOSFET.
(1) Loss at the control circuit
The loss caused by operation of the IC control circuit is
calculated by the supply voltage and IC current consumption.
When the supply voltage is Vcc and IC current consumption is
lccop, loss Pop at the control circuit is:
Pop = Vcc ꢁ Iccop ....................................................... (15)
Example:
When Vcc = 18, lccop = 1.9mA (typ) is obtained from the
specifications. The typical IC loss is given by:
Pop = 18V ꢁ 1.9mA = 34.2mW
(2) Loss at the output circuit
The output circuit of the IC is a MOSFET push-pull circuit.
When the ON resistances of MOSFETs making up the output
circuit are Ron and Roff, the resistances can be determined as
shown below based on Vcc = 18V and Tj = 25˚C obtained from
the output characteristics included in the specifications:
Ron = 15Ω (typ)
Roff = 7Ω (typ)
When the total gate charge of the power MOSFET is Qg, the
switching frequency is fSW, the supply voltage is Vcc, and
gate resistance is Rg, the loss caused at the IC output circuit
is given by:
1
2
Ron
Rg ꢂ Ron
Roff
Rg ꢂ Roff
Pdr = ꢀ Vcc ꢀ Qg ꢀ fsw ꢀ
ꢂ
....... (16)
(
)
75
FA3641P(N), FA3647P(N)
When gate resistance differs between ON and OFF as shown
in Figure 36, the loss is given by:
1
2
Ron
Roff
Pdr = ꢀVccꢀQgꢀfswꢀ
ꢂ
... (17)
(
)
Rg1ꢂRg2ꢂRon Rg1ꢂRoff
Example:
When Vcc = 18V, Qg = 80nC, fsw = 100kHz, and Rg = 10Ω, the
typical IC loss is given by:
1
2
15Ω
7Ω
Pdr = ꢀ 18V ꢀ 80nC ꢀ 100kHz ꢀ
ꢂ
(
)
10Ω ꢂ 15Ω 10Ω ꢂ 7Ω
=72.8mW
Fig. 36 Gate circuit
(3) Total loss
The total loss (Pd) of the IC is the sum of the control circuit loss
(Pop) and the output circuit loss (Pdr) calculated previously:
Pd = Pop + Pdr .............................................................. (18)
Example:
The standard IC loss under the conditions used in (1) and (2)
above are:
Pd = Pop + Pdr = 34.2mW + 72.8mW = 107mW
76
FA3641P(N), FA3647P(N)
ꢀ Application circuit
ꢀ FA3641
ꢀ FA3647
Parts tolerances characteristics are not defined in the circuit design
sample shown above. When designing an actual circuit for a
product, you must determine parts tolerances and characteristics for
safe and economical operation.
77
FA3641P(N), FA3647P(N)
ꢀ Electrical characteristics of application circuit
Input power vs. input voltage
Input power vs. output power
Condition:No-load
10
0.6
0.5
0.4
0.3
0.2
1
230V AC
100V AC
0.1
0
0.1
0.01
250
150
200
300
0
50
100
0.1
1
10
Input voltage [Vac]
Output power [W]
Oscillation frequency vs. output power
90
80
100V AC
70
60
50
230V AC
40
30
20
10
0
40
0
10
20
30
60
50
Output power [W]
78
相关型号：
FA38SA50LCPBF
Power Field-Effect Transistor, 38A I(D), 500V, 0.13ohm, 1-Element, N-Channel, Silicon, Metal-oxide Semiconductor FET, SOT-227, 4 PIN
INFINEON
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