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Please suggest me circuit to limit power on peak current

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Member level 5
Aug 6, 2005
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I have designed a power supply of 4v/500mA & there is a supercap of 550mF
accross load.During power on initial peak current of supply is about 2A due to supercap.Which circuit can I use to suppress this power on peak current.


What kind of power supply is it and what control circuit are you using?

Did you use a control IC for your power converter? If yes, does it have a soft-start functionality? This would normally be a pin named SS or something like that and you just put a capacitor there. What the IC does is to charge this capacitor through and internal current source of a known value. This essentially ramps up the operation of the device so you gracefully turn everything on. Large the capacitor ... longer the soft start time.

I think internally what happens is that the reference voltage is ramped up from 0 to 1.2V or whatever it is.

Best regards,

Thanks for your reply.
I am using max1776 dc-dc converter ic it does not have have soft start provision or external control of soft start.

This chip has internal current limiting, so there is no danger of it failing.
The only problem may be the external cap.
If you are exceeding its rating (hard to believe), then you can reconfigure the ILIM, ILIM2 pins on the MAX1776, to achieve lower current limit.

For example, connecting ILIM to Vin and ILIM2 to GND, the peak current will be limited to 600mA. That means the load current will be limited to roughly 500mA. Note that this value depends heavily on the inductor used and the IC's tolerances.
In fact, the peak current will be limited to anywhere from 400 to 800mA. Depending on the inductor, that could mean about 300 to 700mA of output current.

Try the above, if you are happier with these limits. I understand that the current may be less than your designed 500mA, but perhaps this is less critical than ensuring a safe inrush for the supercap.
OTherwise, you can change the IC, to say MAX1744. This needs an external P-CH MOSFET (so the circuit is more expensive), but the current limit is controlled via an external resistor, so you can more easily adjust it. I have had good experience with this latter chip. The chip's tolerance is only 15% for the current sense input, plus say 1% for the resistor, it's about 16%. Note, however, that the resistor is a low-ohm one, which is also expensive (about $0.30).

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