problem Using LM338T with SIM508

[alzomor]

Using LM338T with SIM508

Hi

I am using the LM338 to supply the SIM508.
The supply voltage for sim508 is coming from 12V car battery.
I followed the datasheet for LM338 using 2 electrolytic capacitors of 22uF @ input and output.
R1 is 120 ohm and R2 is 220 ohm, so the output voltage is about 3.54 Volt.

The problem is that the sim508 module resets from time to time.
I put 10uF electrolytic cap. parallel to R2 to reduce the harmonics without any improvement.
attached you can find the schematic .

I have checked several posts on EDAboard i found two proposed solutions.
1- Putting 2 * 1000 uF cap on output of regulator
2- increasing the output voltage to about 4 V

the problem is that my casing is very small to accommodate 2*1000 uF caps

the 2nd problem is that I am supplying the micro from the same supply ,I am afraid the Tx pin form micro when operated at 4 volt may destroy the Rx pin of SIM508

My Question is why the LM338T is not capable of supplying the 2Amps and maintaining the voltage level according to its datasheets

Best Regards
Hossam Alzomor[/img]

 

[Audioguru]

Using LM338T with SIM508

Your LM338 is dissipating 20W of heat if its load current is 2A. Then if its heatsink is not huge it will shutdown due to over-temperature.

 

[alzomor]

Re: Using LM338T with SIM508

Hi Audioguru,

these 2Amps are not continues , they are pulsating with very short duty cycle during Tx burst which is not continuous also.
the 2Amps are consumed for 566us with 4.615us off .
on the other hand I soldered metal back of the LM338T to the VCC plan on the PCB for better heat dissipation.

Do you think this is enough?

Best Regards
Hossam Alzomor

 

[Audioguru]

Using LM338T with SIM508

You probably destroyed your LM338T by soldering to it. Look at its datasheet to see its max allowed temperature that is much less than molten solder.

Its tab has a hole that is used for a bolt.

 

[alzomor]

Re: Using LM338T with SIM508

I don't think so.

I have measured the Output of the regulator, it's OK.
and the regulator can power up my circuit. and iit works for some time.
the problem appears when the SIM508 module starts to send, this the point at which the module consumes bursts of 2A as I told before.
I think the problem is Voltage stability during the burst.
My Question is why following up the datasheet is not sufficient to maintain the supply stable during 2A current bursts ?

How can I overcome this problem?
if you suggest putting heat sink , do you have any recommendations for size or design?

Salam
Hossam Alzomor
www()i-g()org

 

[GSM Man]

Re: Using LM338T with SIM508

My Question is why following up the datasheet is not sufficient to maintain the supply stable during 2A current bursts ?
How can I overcome this problem?
if you suggest putting heat sink , do you have any recommendations for size or design?

You probably do not have a stable supply - this is the most common problem I see with GSM modules. If you look the BAT input of the SIM508 with an oscilliscope you'll probably see short spikes on the line that drop below 3.4V. Besides the need for good filtering you also need to make sure that all of your power and ground traces are low impedance and that you follow good layout practices.

 

[alzomor]

Thanks GSM man
I think you are focusing to the point

Regarding low impeadancetraces I amusing power plans top layer for supply and bottom layer for ground and the PCB is FR4.

What do you recommend for fitering?
currently I am using 100uF aluminium Electrolytic capacitor paraller with 0.1uF ceramic for both input and output of regulator and still have problem.

the power supply passes network attach step but fails during the 1st data transmitt, do you have any suggesions?

Salam
Hossam Alzomor
www()i-g()org

 

[GSM Man]

currently I am using 100uF aluminium Electrolytic capacitor paraller with 0.1uF ceramic for both input and output of regulator and still have problem.

I know this is what SIMCom recommends, but I've never seen a design with that small of a Electrolytic that works reliably. As a matter of fact, today I just visited a customer that had a design with a 100uF Electrolytic that was resetting about every 5 minutes.

Your best bet is to get a DSO and scope the power pin (make sure the scopes ground lead is attached at the radio). You'll probably see some spikes that go below 3.4V. Even very short spikes can reset the radio. You need to put a large enough cap to suppress those dips. On my circuits I usually put a 2200uF and a 0.01uF at the power pin on the module. You may be able to get away with a smaller value, but I'd verify this with a DSO.

 

[erikl]

Re: Using LM338T with SIM508

the problem is that my casing is very small to accommodate 2*1000

Did you ever consider a gold cap with a capacitance of 0.1 .. 1F, which comes in about the same size as a 100 .. 1000µF Al cap and can stand up to 5.5V? Ok, it probably can't deliver 2A in a short time, but the LM 338T can, can't it?

Or even a small LiPo accumulator?

 

[yassin.kraouch]

hi,
I think you shoulod choose a regulator with a very low dropout voltage as used in the SIM508 EVB, you can choose a regulator manufactured by ST micro. and it will work well.
regards,

 

[namlooc]

GSM module in SIM508/548C needs good power supply. GPS module needs only for example lm317.

I used lm2673 5V 3A step-down regulator with programmable peak current limit to 5A and it didn't manage. Modem was still reseting in bad GSM signal conditions.

Programmable 7A peak current is sufficiently efficient to allow without risking power sim548c/508.
So LM2679 will do the job without any problems

 

[alzomor]

Thanks namlooc
LM338 is capable of doing the JOB with good cpacitors on the output
2*1000uF low ESR +1 0.1uF

Salam
Hossam alzomor

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