FAN7602C Fairchild Semiconductor, FAN7602C Datasheet - Page 10

• Manufacturer Part Number: FAN7602C
• Description: The FAN7602C is a green current-mode PWM controller
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN7602C • Rev. 1.0.0
Application Information
1. Startup Circuit and Soft-Start Block
The FAN7602C contains a startup switch to reduce the
power loss of the external startup circuit of the
conventional PWM converters. The internal startup
circuit charges the V
source if the AC line is connected. The startup switch is
turned off 15ms after IC starts up, as shown in Figure 19.
The soft-start function starts when the V
reaches the start threshold voltage of 12V and ends
when the internal soft-start voltage reaches 1V. The
internal startup circuit starts charging the V
again if the V
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
threshold voltage, the IC starts switching again and the
soft-start block works as well.
During the soft-start, 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 startup switch is turned off when
the soft-start voltage reaches 1.3V.
2. Oscillator Block
The oscillator frequency is set internally and FAN7602C
has a random frequency fluctuation function.
Fluctuation of the switching frequency of a switched
power supply can reduce EMI by spreading the energy
over a wider frequency range than the bandwidth
measured by the EMI test equipment. The amount of
EMI reduction is directly related to the range of the
frequency variation. The range of frequency variation is
fixed internally; however, its selection is randomly
chosen by the combination of external feedback voltage
and internal free-running oscillator. This randomly
chosen switching frequency effectively spreads the EMI
noise nearby switching frequency and allows the use of
a cost-effective inductor instead of an AC input line filter
to satisfy the world-wide EMI requirements.
Figure 19. Startup Current and V
CC
voltage is lowered to the minimum
CC
CC
capacitor with 0.9mA current
voltage reaches the start
CC
Voltage
CC
CC
capacitor
voltage
10
3. Current Sense and Feedback Block
The FAN7602C performs the current sensing for the
current mode PWM and the output voltage feedback
with only one pin, pin 3. To achieve the two functions
with one pin, an internal LEB (leading-edge blanking)
circuit to filter 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 21 shows the current sense and feedback circuits.
R
current. The current sense information is filtered by an
RC filter composed of R
output voltage feedback information, I
stops charging C
zero, C
offset voltage.
Figure 22 shows typical voltage waveforms of the CS/FB
pin. The current sense waveform is added to the offset
voltage, as shown in the Figure 22. 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. Generally, the maximum output power
increases as input voltage increases because the
current slope during switch on-time increases.
S
Comparator
Figure 21. Current Sense and Feedback Circuits
is the current sense resistor to sense the switch
PWM
Figure 20. Frequency Fluctuation Waveform
F
is discharged through R
PWM+
Soft-Start
F
to adjust the offset voltage. If I
Power
Offset
Limit
Plimit
F
3
and C
CS/FB
R
C
FB
F
F
V
F
and R
CC
. According to the
I
FB
R
F
FB
S
www.fairchildsemi.com
to lower the
charges or
R
S
I
sw
FB
is