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Thursday, November 25th 2010, 9:28pm

Switch power for GSM module with MOSFETs

Q: Type of module? A: Telit GE865
Q: Firmware version? A: do not know, not important for this question
Q: Type of starter kit you have? GE865-QUAD-Starter Kit (STK-RS-GE865)
(S1, S1B, S2, S3, S4, EVK1, EVK2, none, other)

I am developing a LiIon-battery powered device with the Telit GE865 GSM module. Because the GSM module is used only once a day for about 5 minutes, I want to switch off the power supply of the GSM module completely, when it is not used. The GSM module has a low power mode with 60 µA, but this current is too high and I do not want to waste energy.

I thought about a microcontroller based control by means of two MOSFETs, see attached schematic.

So far, I used these MOSFET types:
N: FDY3000NZ: Gate-Source-Threshold 0.6...1.3V, Rds_on = 0.37Ohm @ Vgs=2.5V, Id=500mA
P: IF7555PbF: Vgs_th = -0.6...-1.2V, Rds_on = 0.055Ohm @ Vgs=-4.5V, Id=-4.3A

I built a test circuit:
1. When the microcontroller pin is low (0V) I measure 0V at the P-channel drain-pin.
2. When there is no GSM module and the microcontroller pin is high (3V), I measure the battery voltage at the P-channel drain-Pin. But, when I use a 10 Ohm-resistor for simulating the load (~ 400mA), I only measure about 3.1V at the P-channel drain-Pin. Why don't I measure the battery voltage 4.2V?

Is my circuit not useful for my plan or do I need a P-mosfet with very little Rds_on, e.g. 0.005Ohm?
Do you have further suggestions for this circuit or do you suggest different circuits for switching the power supply of the GSM module?

I would appreciate it very much, if somebody can help. Please answer in English or German.
mcmcontrol has attached the following image:
  • Mosfets.jpg


Friday, November 26th 2010, 9:00am

RE: Switch power for GSM module with MOSFETs

You should use just one FDS6375 and it will work.


Monday, November 29th 2010, 5:13pm

RE: Switch power for GSM module with MOSFETs

Thanks a lot for your reply.

Why is the FDS6375 useful for this task? Which parameters are deciding?

If I understand you correctly, do you suggest, to use only the P channel mosfet FDS6375 and I should remove the N channel mosfet and remove the two resistors?


Monday, November 29th 2010, 6:41pm

RE: Switch power for GSM module with MOSFETs

Exactly that has proven itself in practice.


Which parameters are deciding

Have a look into the data sheet:…LD/FDS6375.html


Monday, January 3rd 2011, 8:37am

RE: Switch power for GSM module with MOSFETs

There is not response since more than 2 weeks. For that reason I will close this thread.
Thank you for understanding.

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