hi, I need some help with my dc voltage. I have a 5V output Regulator and my GSM modem works under 3.6V- 4V. I tried to find a regulator with
3.6V but I cannot find. Does somebody know a regulator with ouput of 3.6V or can advice me what I can do with my regulator to get 3.6V.
Use a 3.3V regulator and lift the ground led with a schottky diode (adds
0.4V).Meindert
Use a variable regulator like the ZR431 or LM2941 depending on current required. You'll need acouple of resistors too.
Paul Burke
Use adjustable regulators, such as the Texas TL317. You choose the voltage, from 1.2 V to 32 V, with a pair of external resistors.
You could put two silicon diodes in series.
Leon
Use an adjustable low drop regulator, e.g. lt1585.
-- Dipl.-Inform(FH) Peter Heitzer, peter.heitzer@rz.uni-regensburg.de HTML mails will be forwarded to /dev/null.
LDO or diodes would be fine if your modem does not draw too much power. However, it would be more efficient if you can regulate it from a higher input. Do you have access to the input of the 5V regulator, if so, what voltage?
With GSM modems you need to be concerned about regulator response times, because of the pulsing current demand. Some cheaper LDOs may not cope very well. I have used the LT1528 successfully on a couple of commercial designs, with a potential divider on the feedback pins of (I think) 6R2 (top) and 33R (bottom).
The low value of these resistors reduces the dynamic range of the current and improves the regulation, at some expense in power. AFAIR they are not needed for bias current reasons.
The connections from the regulator to the modem must be very low resistance and the regulator capacitors of suitable value and quality.
HTH Richard D.
Spamtrapped at ngsATradixDASHdesignDOTcoDOTuk
Following up my own postings: how sad is that?
In fact the bias current of the LT1528 is fairly high at 125uA, so a reasonably low resistance feedback divider is of course required.
-RD
Richard D.
Spamtrapped at ngsATradixDASHdesignDOTcoDOTuk
As Richard noted, GSM modems draw large impulse currents. I am currently using a Sony-Ericsson part and it draws 1A pulses for GPRS slots and up to 2A in voice [at 3.6V nominal]. A simple solution is the LT1767 (adjustable version) and plenty of output capacitance as the loop compensation won't handle the speed of the impulse, which is true of most regulators anyway (perhaps 1000 uF or so should do it).
Other options are some of the newer parts from TI which have the loop compensation internal to the regulator. You'll still need to put a fair amount of output capacitance on it though.
You'll notice I ignore linear regulators as most of them really don't do well in this sort of situation. If you want to use a linear (the average current is quite low - typically in the 20-40 mA range) regulator, then make sure it can handle the response times and the load changes. Many LDOs can't handle ultra low esr caps on their outputs, but that's exactly what you need in this case.
Cheers
PeteS
Generally, all LDOs based on PNP or NPN output devices have large and variable reference currents because this pin carries the base drive for the output device. As the output current varies, the reference pin current must vary, roughly, proportionately. LDOs based on mosfet outputs have less DC reference current variation, but still must pull dynamic gate loads through this pin. But a capacitor to ground can reduce the effect of that.
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