FAN7602B Fairchild Semiconductor, FAN7602B Datasheet - Page 9

• Manufacturer Part Number: FAN7602B
• Description: The FAN7602B is a green current-mode PWM controller
• Manufacturer: Fairchild Semiconductor

FAN7602B Rev. 1.0.0
© 2007 Fairchild Semiconductor Corporation
Applications Information
1. Start-up Circuit and Soft Start Block
The FAN7602B contains a start-up switch to reduce the
power loss of the external start-up circuit of the conven-
tional PWM converters. The internal start-up circuit
charges the V
the AC line is connected. The start-up switch is turned off
15ms after IC starts up, as shown in Figure 19. The soft-
start function starts when the V
start threshold voltage of 12V and ends when the internal
soft-start voltage reaches 1V. The internal start-up circuit
starts charging the V
is lowered to the minimum operating voltage, 8V. The
UVLO block shuts down the output drive circuit and
some blocks to reduce the IC operating current and the
internal soft-start voltage drops to zero. If the V
age reaches the start threshold voltage, the IC starts
switching again and the soft-start block works as well.
During the soft-start, the pulse-width modulated (PWM)
comparator compares the CS/FB pin voltage with the
soft-start voltage. The soft-start voltage starts from 0.5V
and the soft-start ends when it reaches 1V and the soft-
start time is 10ms. The start-up switch is turned off when
the soft-start voltage reaches 1.5V.
2. Oscillator Block
The oscillator frequency is set internally. The switching
frequency is 65kHz.
3. Current Sense and Feedback Block
The FAN7602B performs the current sensing for the cur-
rent-mode PWM and the output voltage feedback with
only one pin, pin3. To achieve the two functions with one
pin, an internal leading edge blanking (LEB) circuit to fil-
ter the current-sense noise is not included because the
external RC filter is necessary to add the output voltage
feedback information and the current-sense information.
Figure 20 shows the current-sense and feedback cir-
cuits. R
current. The current-sense information is filtered by an
RC filter composed of R
voltage feedback information, I
1.5V
12V
0.5V
Figure 19. Start-up Current and V
1V
8V
S
is the current-sense resistor to sense the switch
CC
Time (10ms)
capacitor with 0.9mA current source if
Soft Start
CC
Soft Start
Voltage
V
CC
capacitor again if the Vcc voltage
F
and C
Start-up
Current
5ms
F
. According to the output
CC
FB
voltage reaches the
charges or stops
CC
Voltage
CC
t
volt-
9
charging C
is discharged through R
Figure 21 shows typical voltage waveforms of the CS/FB
pin. The current-sense waveform is added to the offset
voltage, as shown in Figure 21. The CS/FB pin voltage is
compared with PWM+ that is 1V - Plimit offset. If the CS/
FB voltage meets PWM+, the output drive is shut off. If
the feedback offset voltage is low, the switch on time is
increased. If the feedback offset voltage is high, the
switch on time is decreased. In this way, the duty cycle is
controlled according to the output load condition. In gen-
eral, the maximum output power increases as the input
voltage increases because the current slope during
switch on-time increases.
To limit the output power of the converter constantly, a
power-limit function is included. Sensing the converter
input voltage through the Latch/Plimit pin, the Plimit off-
set voltage is subtracted from 1V. As shown in Figure 21,
the Plimit offset voltage is subtracted from 1V and the
switch on-time decreases as the Plimit offset voltage
increases. If the converter input voltage increases, the
switch on-time decreases, keeping the output power
constant. The offset voltage is proportional to the Latch/
Plimit pin voltage and the gain is 0.16; if the Latch/Plimit
voltage is 1V, the offset voltage is 0.16V.
Figure 20. Current-Sense and Feedback Circuits
Comparator
Figure 21. CS/FB Pin Voltage Waveforms
PWM
PWM+
CS/FB
PWM+
CS/FB
GND
GND
1V
1V
F
PWM+
to adjust the offset voltage. If I
(a) Low-Power Limit Offset Case
Start
Soft
(b) High-Power Limit Offset Case
Power
Offset
Limit
Plimit
On Time
On Time
F
and R
3
CS/FB
S
to lower offset voltage.
R
C
FB
F
Vcc
Power Limit
Offset
FB
Power Limit
Offset
I
Offset
FB
Offset
FB
R
FB
F
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is zero, C
R
S
Isw
F