Re: Preventing Voltage Dips
- From: David Brown <david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 02 Dec 2006 21:41:36 +0100
AJ wrote:
Hi
I am trying to power an GSM module from a 3.6V lithium battery via a couple of FETS and I am finding that the modem won't connect when the battery voltage drops to about 3.6V. I have measured the voltage at the modem with my scope and I have noticed that the voltage dips about 800mV during its transmissions bursts. At first I thought it was a problem with my FET's but when I measured right back at the battery it was still dropping around 500mV. I have about 1200uF on the modem and I have noticed that the more capacitance I use, the better it is but I would prefer not too use big capacitors on this design. Does anyone know of any better ways to get around this? The Current spikes are around 2A for about 500uS every 5mS.
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.
Best regards,.
Adrian Hamilton
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