GSM modem with USB Interface

[cesare]

Hi All

Can anyone help me?

I'm designing a GSM modem with USB Interface. The problem is with the power supply because the modem must be a "High power bus powered function": it means that the maximum peak current is 500mA.

I saw several devices like this and all of them uses a small LDO (typically 200 mA)with a big capacitor (2700 uF) to provide the high current pulses (up to 2A) required from the GSM engine.

The LDO is chosen so that the short circuit current is limited to less than 500 mA.

Now the peak current that the LDO must be able to supply in order to recharge the reseirvor capacitor usually is not well specificated and so what i want to do is to use a generic LDO and design an external current limiting loop.

I tried with a TPS77501 (500 mA , 2A peak) but when the current limiting loop comes up to limit the current it causes an oscillation : the current is effectively limited but the GSM modem does not like this oscillation and it does not work.

Does anyone know a circuital configuration suitable to limit LDO current without oscillation.

Thanks in advance!!

 

[jourval]

I am not familiar with this LDO, but,
often series shottky diode between
LDO and big capacitor prevent from
such kind oscilations.
Of course you'll pay in additional
voltage drop, but any other device
will need it too.

 

[redfred]

Hi!

Check w?w.falcon.de
They are producing USB GSM engines.
denesg