3.3 - 4.5 V Power supply

[justin_t]

Hi

I am trying to interface an Atmel chip with a GPRS modem chip, I am happy enough about the communications between the two devices but am getting a bit stuck with the power supplies.

For my my Atmel chip I am getting a 5V supply from a standard L7805CV voltage regulator, this is working fine.

The GPRS chip requires 3.3 - 4.5V with approx 420mA max average/nominal current that can peak to 1600mA depending on baud rates and Rx/Tx power.

I was hoping to be able to find a similar voltage regulator that will meet these characteristics but am not comming across much on Farnell or RS.

I see an old Nokia charger has 3.7V/355mA DC output but upon testing it with a meter it is reading closer to 9 DC, does this switch to a lower value when load is applied, im a bit wary of applying it to my modem incase I damage it.

Kind of 3 questions I know:

1 Is it possible to get a Volatge regulator for this output, ideally with 5 Volt input (for testing at this stage) and ripple < 2.5mV.

2 If I could configure the device to consume about 250mA nominal, would the Nokia charger be a good solution? (again only for testing at this stage).

3 For peace of mind/future knowledge, what is the deal with the charger, why does it give 9V off load when rated at 3.7V?

Thanks

Justin

 

[Diver300]

I don't think you have any idea how evil GPRS modems are in the way that they load their power supply. This thread gives some idea:-

https://www.electro-tech-online.com/threads/interfacing-tc35i-problem.115959/

The current peak is so big, so long and starts so quickly that you would need some serious capacitors on the supply to run the GPRS module with a power supply that is only rated to 420 mA. Your Nokia charger will not work, and its no-load voltage is large enough to destroy the GPRS module, which will take no current at all when first connected.

Mobile phones will not work without a battery, because it is the battery that supplies the current peaks. The 355 mA is there to recharge the battery in a few hours.

When I have designed power supplies for GPRS modules, I have built a SMPS that can supply 2 A. The thermal design can be based on 2 A for about 1/4 of the time, but the basic design has to supply the 2 A.

 

[justin_t]

Ah Ok thanks, so it needs to cover the worst case scenario from the data sheet.

I have emailed the supplier to see if they have an app note on a recommended power supply and configuration.

Is it too simple an idea to use a 5V regulator capable of supplying 3 A and put a resistor in series with the supply to the modem to provide the appropriate supply voltage with the required current as it is needed?

 

[Diver300]

Is it too simple an idea to use a 5V regulator capable of supplying 3 A and put a resistor in series with the supply to the modem to provide the appropriate supply voltage with the required current as it is needed?

Of course it is too simple. The resistor that would supply 3.5 V or so at 2 A would supply almost the full 5 V at all other times.

https://www.telit.com/module/infopool/download.php?id=2554 around page 26 has some suggestions. I realise that it is from another manufacturer, but all modules have to meet the same power output specifications, so they all have virtually identical power requirements.

Where is the power ultimately coming from? Can you power the Atmel chip at a lower voltage?

 

[DirtyLude]

An LD1086 would be fine. Max is 1.5A, but bursts of 1.6 would not hurt it.

Otherwise, TPS75233 is a 2A linear.

Otherwise you'll need to use a switcher.

 

[justin_t]

Thanks for the help, in the mean time the modem supplier has emailed me the attached schematic if it is of interest to anyone.
I'm going to give it a try and will let you know how I get on.

Justin