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?2008 Fairchild Semiconductor Corporation
 
www.fairchildsemi.com
SG6901A " Rev. 1.0.2
17
Figure 28. V
RMS
 Controlled Current Limiting
The   peak   current   of   the   ISENSE   is   given   by
(V
RMS
<1.05V):
S
P
P
SENSE_peak
R
0.2V
-
)
R
(I
 
 
I
?/DIV>
=
 
(8)
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
PD-PWM
.
To    improve    stability    or    prevent    sub-harmonic
oscillation,   a   synchronized   positive-going   ramp   is
inserted at every switching cycle.
Figure 29. Peak Current Control Loop
 
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
OPEN-PWM
 
(56ms), the OPWM is turned off.
As long as the voltage on the VDD pin is larger than
V
DD-OFF
 (minimum operating voltage), the OPWM is not
enabled. This protection is reset every t
OPEN-PWM-Hiccup
 
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
I
 is equal
to 24K&, the magnitude of the constant current is
100礎. An external NTC thermistor must be connected
from this pin to ground, as shown as Figure 30. When
the OTP voltage drops below V
OTP-OFF
 (1.2V), SG6901A
is disabled and does not recover until OTP voltage
exceeds V
OTP-ON
 (1.4V).
 
Figure 30. OTP Function
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.
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