SG6901 Fairchild Semiconductor, SG6901 Datasheet - Page 17

• Manufacturer Part Number: SG6901
• Description: The highly integrated SG6901A is designed for power supplies with boost PFC and flyback PWM
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6901A • Rev. 1.0.2
The peak current of the ISENSE is given by
(V
Flyback PWM and Slope Compensation
As shown in Figure 29, peak-current-mode control is
utilized for flyback PWM. The SG6901A inserts a
synchronized 0.5V ramp at the beginning of each
switching cycle. This built-in slope compensation
ensures stable operation for continuous current-mode
operation.
When the IPWM voltage, across the sense resistor,
reaches the threshold voltage (0.9V), the OPWM is
turned off after a small propagation delay t
To
oscillation, a synchronized positive-going ramp is
inserted at every switching cycle.
I
SENSE_peak
RMS
improve
<1.05V):
Figure 28. V
Figure 29. Peak Current Control Loop
=
(I
P
×
stability
R
RMS
R
P
- )
S
Controlled Current Limiting
0.2V
or
prevent
sub-harmonic
PD-PWM
.
(8)
17
Limited Power Control
Every time the output of the power supply is shorted or
overloaded, the FBPWM voltage increases. If the
FBPWM voltage is higher than a designed threshold,
FBOPEN-LOOP (4.5V) for longer than t
(56ms), the OPWM is turned off.
As long as the voltage on the VDD pin is larger than
V
enabled. This protection is reset every t
interval. A low-frequency hiccup mode protection
prevents the power supply from being overheated
under overload conditions.
Over-Temperature Protection
The OTP pin provides for over-temperature protection.
A constant current is output from this pin. If R
to 24KΩ, the magnitude of the constant current is
100µA. An external NTC thermistor must be connected
from this pin to ground, as shown as Figure 30. When
the OTP voltage drops below V
is disabled and does not recover until OTP voltage
exceeds V
Soft Start
During startup of PWM stage, the SS pin charges an
external capacitor with a constant current source. The
voltage on FBPWM is clamped by the SS voltage
during startup. In the event of a protected condition
and/or PWM is disabled, the SS pin quickly discharges.
Gate Driver
SG6901A output stage is a fast totem-pole gate driver.
The output driver is clamped by an internal 18V Zener
diode to protect the external power MOSFET.
DD-OFF
(minimum operating voltage), the OPWM is not
OTP-ON
Figure 30. OTP Function
(1.4V).
OTP-OFF
(1.2V), SG6901A
OPEN-PWM-Hiccup
www.fairchildsemi.com
I
OPEN-PWM
is equal