LM5026: Compensation information

1. What does this COMP input impedance given in the datasheet refer to and could you please explain its significance?
   1. Ideal comparator has infinite input impedance (draws no current at input terminals) but in reality there is a measure of input impedance looking into the V+ and V- terminals. The typical input impedance of the LM5026 PWM comparator is 1.7 MΩ. 

1. On what basis the controller determines to go into pulse skipping mode at no load? What factors determine, the slope, period, and amplitude of the pulse skipping mode? 
   1. LM5026 does not have pulse skipping mode.

   2. A controller that has true pulse skipping would do something analogous to switching at a minimum duty cycle and then skipping every nth pulse. Some controllers even change to hysteretic mode to achieve pulse skipping. There are different ways to accomplish this but it's done intentionally and in a controlled manner so that the output stays in regulation. Pulse skipping will also impact output voltage ripple but again, this is done in a controlled and predictable manner.

      When the LM5026 COMP voltage falls below the minimum threshold, pulses will stop until the COMP voltage returns. This is uncontrolled and somewhat random. Pulses are randomly missing as opposed to periodically skipping. What is observed can appear like "pulse skipping" but the LM5026 does not implement true pulse skipping as described here.

Regards,

Steve

a)The input impedance is 1.7k or 1.7M?. In datasheet it is 1.7k but in above message you have mentioned 1.7M.

• Sorry about my typo. the data sheet is correct and the input impedance is 1.7kΩ

b)"When the LM5026 COMP voltage falls below the minimum threshold, pulses will stop until the COMP voltage returns": What is the minimum COMP voltage threshold?

• Depends on the dynamic range of the opto/LM431 but in general, the full dynamic range is set by the full range of your CS signal.

c) In the PWM comparator, what is the value of resistor divider "R" in the PWM comparator section given in Fig. 16. The voltage at V- terminal of the PWM comparator depends on the COMP current and also the value of R?

• "R" is not a measured or specified parameter. The voltage at the inverting input of the PWM comparator is the SS voltage x 1/3 minus 1.4V.

Regards,

Steve

Correct

Steve