DER-37
16W Adapter using TOP245P
April 27, 2004
The CONTROL pin requires 2 mA to power the internal circuitry of the chip. Any current
beyond that normally would be shunted, and commands the duty cycle to reduce. With
2 mA of current the duty cycle is set to maximum.
Feedback current above ~2mA forward biases Q1 through R10 and pulls up R13 via
R14. The X pin looks like a 1.2V voltage source. The current it is sourcing determines
the output MOSFET peak current limit. As current decreases, the peak current limit is
reduced from maximum (when X-pin current is 170 uA), down to a minimum of 40% of
nominal peak current limit (when X-pin is at 30 uA). Therefore, as the feedback current
increases, the sink current decreases and the primary current limit reduces, thereby
allowing the output voltage feedback loop to control the primary peak current. Resistor
R14 sets the peak current limit (startup and overload). Any feedback current above 2 mA
engages the X pin control, the current into the CONTROL pin is limited to this level and
therefore, the PWM function of the CONTROL pin does not determine the duty cycle.
As the load is reduced, the primary current limit reduces until it reaches 40% of peak
current limit. At this point, another X-pin function is activated. The remote ON/OFF
(inhibit) threshold is reached at an X pin sink current of approximately 27 µA. The supply
then operates with fixed 25% current limit, but with a variable off-time, resulting in a
variable switching frequency. To maintain regulation, as the load is further reduced, the
frequency reduces. This greatly reduces switching losses, maintaining high standby
efficiency and low no-load power consumption.
Slope compensation is provided by a ramp signal generated from the bias winding via
D4, R7 and C9. C9 also serves as a high frequency roll off filter.
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