Hey guys,
I'm working on a GSM project using the SIM900A module, and yeah I <Mod: Obsecenity deleted> I totally forgot that SIM900A needs max 4.5V on the VBAT pin.
So I went ahead and designed my whole circuit thinking everything is 5V powered, including SIM900A.
Luckily, I had already added a diode for reverse voltage protection between USB power and external 5V (HLK-10M05L) input. And by pure luck, this diode has around 0.5V forward drop at 3A, so now my SIM900 is getting around 4.5V max.
I know this ain't the most professional way to regulate voltage but I'm not going for long-term use — just want it to work reliably enough to send some data via GSM.

My Questions:
Will this diode drop hack work fine for powering SIM900A short-term?
Do I need to add a 470uF cap close to VBAT pin or is it overkill?
Could someone check my schematic (attached) and let me know if I missed anything else stupid?

I’m using an SK36 3A 60V Schottky diode and have a P-MOSFET (DMG3415UFY4Q) to auto switch between USB and HLK-10M05L power.
I attached the schematic here
Thanks in advance any feedback, roast or tips welcome.

 

diode has around 0.5V forward drop at 3A, so now my SIM900 is getting around 4.5V max.
I know this ain't the most professional way to regulate voltage

What is the voltage tolerance of the SIM900 and what is its current draw range?

Using a diode to drop the voltage defeats the purpose of having a regulated voltage because the voltage drop of the diode depends on current.

 

What is the voltage tolerance of the SIM900 and what is its current draw range?

Using a diode to drop the voltage defeats the purpose of having a regulated voltage because the voltage drop of the diode depends on current.

I checked the SIM900A datasheet, the VBAT absolute max is 4.5V, recommended is 3.4V to 4.4V, and the peak current draw can go up to 2A during transmission bursts.

The SK36 3A 60V Schottky diode according to its datasheet, has a typical forward voltage of 0.5V and a maximum of 0.75V at 3A

 

I checked the SIM900A datasheet, the VBAT absolute max is 4.5V, recommended is 3.4V to 4.4V, and the peak current draw can go up to 2A during transmission bursts.

Those specs are pretty much meaningless.

What is the minimum current draw? That, along with the specs for the diode at that current, will give you the worst-case maximum voltage.

If you read the SIM900A datasheet carefully, you'll likely find that the device isn't guaranteed to survive the absolute maximum voltage.

The SK36 3A 60V Schottky diode according to its datasheet, has a typical forward voltage of 0.5V and a maximum of 0.75V at 3A

At what current is the 0.5V drop? What is it at the minimum current draw of the SIM900A?

Also, regarding the "typical" device ratings in datasheets. Most devices will be "typical", but the min and max specifications are given to encompass all devices.

 

At what current is the 0.5V drop? What is it at the minimum current draw of the SIM900A?

ok thanks, so the only issue now is the diode, right? should i just go with one that has a higher minimum voltage drop like a silicon diode with around 0.7V? or do you suggest a better solution for this case?

for the SIM900A : Operating Current, Idle: ~20mA, Active: ~200mA, Peak: 2A

 

SIM900A : Operating Current, Idle: ~20mA, Active: ~200mA, Peak: 2A

To keep the voltage no greater than 4.5V, I would suggest using one 3A silicon junction diode, since the Schottky diode forward drop will likely only be about 200mV with the module idle current of 20mA.
A junction diode should have a voltage drop of at least 0.5V, even down to the Sleep Mode current of 1.5mA.