sg6901asz-f116 Fairchild Semiconductor, sg6901asz-f116 Datasheet - Page 16

• Manufacturer Part Number: sg6901asz-f116
• Description: Pfc/flyback-pwm Controller
• Manufacturer: Fairchild Semiconductor

PFC / Flyback PWM Controller
The peak current of the I
Flyback PWM and Slope Compensation
As shown in Figure 7, 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 improve stability or prevent sub-harmonic oscillation,
a synchronized positive-going ramp is inserted at every
switching cycle.
© System General Corp.
Version 1.0.1 (IAO33.0062.B0)
I
SENSE_peak
FIG. 6 V
=
FIG. 7 Peak Current Control Loop
(I
P
×
RMS
R
R
P
- )
Controlled Current Limiting
S
0.2V
SENSE
------------------
is given by (V
PD-PWM
.
RMS
<1.05V):
(9)
- 16 -
Limited Power Control
Every time the output of power supply is shorted or
overloaded, the FBPWM voltage increases. If the
FBPWM voltage is higher than a designed threshold,
FB
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
overloading
Over-Temperature Protection
SG6901A provides an OTP pin for over-temperature
protection. A constant current is output from this pin. If R
is equal 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 8. When the
OTP voltage drops below V
disabled and does not recover until OTP voltage exceeds
V
Soft-Start
During start-up 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
start-up. In the event of a protected condition and/or
PWM disabled, the SS pin quickly discharges.
Gate Drivers
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
OTP-ON
OPEN-LOOP
(minimum operating voltage), the OPWM is not
(1.4V).
conditions.
(4.5V) for longer than t
www.sg.com.tw • www.fairchildsemi.com
FIG. 8 OTP Function
OTP-OFF
Product Specification
OPEN-PWM
(1.2V), SG6901A is
September 26, 2007
OPEN-PWM-Hiccup
(56ms), the
SG6901A
I