LM5037: Max duty operation of LM5037

Satoshi-san,

Can you clarify, the customer wants to operate at max duty cycle under normal conditions? They do not want to modify the duty cycle for regulation?

With the pin connections noted in (1) above, with FB -> GND, COMP will increase to the max level (~5 V). However, since the RAMP -> GND, there will be no PWM ramp signal to allow for any duty cycle control. So the only way to stop the PWM is from the CS pin - if the peak current exceeds 0.25 V, this will terminate the present PWM cycle, and then start the overload protection (the response depends on the connection to the RES pin, see datasheet sections 7.4.1).

For question (2), in this configuration, there is no RAMP signal for the internal PWM comparator, so there is no way to get any PWM duty cycle control. At startup, during softstart, the COMP level will rise together with SS pin. Since there is a 1-V internal offset between COMP and the PWM comparator, there will be no PWM until COMP & SS reach 1 V. After that, the duty cycle will go straight to ~50% for each PWM output (180 degrees out of phase), because the RAMP pin (which is the other input to the PWM comparator) is at GND.

To get a softstart of the PWM duty cycle from zero to max, you need to add a signal to the RAMP pin. This is shown in sections 7.3.7, 7.3.8, 7.3.9, 7.4.3 and 7.4.4 of the datasheet. The simplest way is to add a resistor from REF to RAMP, and a capacitor from RAMP to GND, this will generate a ramp signal for the PWM on the RAMP pin.


I hope this helps answer your question. If so, please click the "verify answer" button.

Thanks,
Bernard

Satoshi-san

As noted above, with configuration (1), COMP will increase to the max level, so this will set max duty cycle close to 50% on each output. However, as noted above, the softstart function will not be active, since there is no signal on the RAMP pin. To get soft-start, you need to use the RAMP pin as explained above.

If this answers your question, please click the "verify answer" button.

Thanks,
Bernard