Preventing Voltage Dips

GSM modems typically take up to 2A or 1A pulses (depending on frequency range in use), and it's not really the best thing to try and power directly from a battery. If you have a single Li+ battery, you should look at a regulator that can operate across the range and still provide the output you need. I have GSM / GPRS devices in my products and this is something that drives the design of the regulators feeding them. Some modem app notes suggest adding lots of capacitance to avoid power droop, incidentally (certainly Sony-Ericcson M2M, [now part of Wavecom] does so).

What FETs are you using, by the way?

TI offers appropriate regulators, as well as the other usual suspects (National, Maxim, AD)

Cheers

PeteS

On a sunny day (Thu, 23 Nov 2006 11:35:30 GMT) it happened "AJ" wrote in :

You lose 300mV at 2A in the FETS, so that is a Ron of .15 Ohm, that can be done better I think. Because Q = C.U = i.t and say if you only allowed 100mV drop, the C would have to be: C = i.t / U = (2 x .0005) / .1 = .001 / .1 = 10mF or 10000uF. Caps would not be that big a size, as the voltage is rather low. Those caps only cost a few $

Thanks Jan

I have used caps it the past, I found that 3000uF lowered the drop to around

regards

AJ.

Hi Pete

That is exactly what I am looking for, thanks a lot. I knew this would be a fairly common problem and there must have been a chip out there that did what I wanted but I wasn't sure what too look for. I was looking at some SEPIC converters but this is perfect for my application and has a nice low part count.

Regards

AJ

Supercap? 1 F at 5 V?

If you can't afford much voltage drop, then caps alone are of limited help - you'll need a great deal to make a difference. For example, if you need to supply 2A for 500 us, and can't afford more than 200 mV drop, you need a capacitor of (2 * 0.0005) / 0.200 = 5 mF. That's fairly big.

If you can start with a higher voltage, however, life gets much easier. Add another battery, so you start with 4.8 V, and use a low drop-out regulator (preferably switch mode, but even linear would help). Then you've got something like 1000 mV to play with, giving you 1 mF capacitance. Doubling the battery to 7.2 V and using a switch-mode buck regulator, and you are down to about 200 uF.