Hi,
based on my experience with the MP5073 load switch, which has a soft start pin, for high capacitances a manual discharge of the capacitor is needed, otherwise it would skip a large portion of the ramp up.
I was trying to use the MP2328/38 to implement this feature directly into the switching converter. I have not tested it yet and current availability suggest it will not happen too soon.
I need a fairly large soft start time (around 2 seconds) and so I put a 33uF capacitor on the soft start pin in my design. I think that it will not be automatically discharged on power down (it would take probably 30 minutes or more to discharge through the IC), so are there any simple circuits I can use to discharge the capacitor when the output voltage is 0 (regardless of the reason)?
Currently I have a transistor and mosfet (like in the photo), but physical constraints of the design don’t allow me to place this configuration. Any ideas?
Thanks
(Don’t mind model numbers and power sources too much, this is a snapshot from LTSpice in which I needed something to test. Just look at the actual configuration of the transistor and mosfet)
Hi freex99tv,
Could you explain more about how you are using the MP2328 to do the discharging of the capacitors? Or are you intending to create a discharge circuit for your soft start capacitor. I am not sure I understand correctly.
Do you need to have that transistor? I am assuming you are using your enable and disable. So, could you use a PFET to discharge when enable is low. Will need to consider gate current requirements though.
Regards,
Vinh Tran
Basically I need a circuit that discharges the soft start capacitor whenever the output voltage of the regulator goes to 0. The regulator doesn’t do anything actively. Placing a resistor doesn’t work because the SS pin provides a current source to ramp up the cap voltage and a resistor would take most of the current, changing everything.
Using the enable might work, but I would need to find a P-FET with a Vgs threshold lower than 0.5v which isn’t easy and cheap.
Hi freex99tv,
I do not have any other ideas for discharging right now. I will leave this post up for the sake of discussion.
Regards,
Vinh Tran
Well there is one question that could help me solve this.
Does the soft start current generator stop charging the capacitor at the reference voltage or at its internal vcc? If it goes beyond the reference voltage of 0.5v there are more chances that I can build something cheap and with smaller overall footprint.
