LM2736: LM2736, voltage drop with high load

Juan Cerrudo

Part Number: LM2736
Other Parts Discussed in Thread: LM2734

I used Texas Webench to design a power supply with an LM2736 (see attached). The regulator is supplying an MCU and some sensors. The board also has an LCD with a tipical consumption of 360mA and a maximum of 575mA, in both cases that's for controller and backlight.

Without the LCD the board starts without any issue.
If I conect the LCD and then power up the board, the LM2736 and other parts (diode, inductor) gets really hot, the voltage drop from 3.3V to 2.5V and the MCU won't start.
If I power up the board AND THEN plug in the LCD the LM2736 works whitout any problem, I have the 3.3V and it doesn't get too hot.

It seems like the LM2736 won't start if I try to source an initial high current. I'm not particularly comfortable with switching supplies, (or how to debug it) any points to solve this issue?

Thanks!

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

Soft-start time for LM2736 is quite fast at 200 usec. You are probably reaching current limit during start up. Can you show waveforms for the SW node and Iout during start up?

0 Juan Cerrudo over 7 years ago in reply to JohnTucker

Prodigy 180 points

This is the SW node rigth?

If that's correct, here are a couple of waverforms, first with the LCD connected:

and whitout:

The Iout waveform migth take me a while as I will need to modify the board, whit a series resitor or something.

0 Juan Cerrudo over 7 years ago in reply to Juan Cerrudo

Prodigy 180 points

Hi there!

I put a 1ohm resistor in series whit the load and measured the current across it, whitout the LCD:

and whit the LCD plugged in:

Whit the LCD the peak current it's close to the 750mA limit of the LM2736, could that be the issue? The LM2734 has a 1Amp limit, do you think it would be possible to change the LM2736 for the LM2734?

Thanks!

0 Brani Dubocanin over 7 years ago in reply to Juan Cerrudo

TI__Expert 4100 points

Hi Juan,

it does look like you might be hitting current limit for LM2736.

As you know this is 750mA max load rated device and from your screenshot peak current is very close to this limit.

Since your application needs slightly more current/operation headroom you could use LM2734, which is capable of delivering 1A output load.

For correct operation you will need to adjust resistive divider, since this device internal reference is 0.8V.

See details in device datasheet, page 17.

Please let us know if you have further questions.

Best regards

Brani

0 Juan Cerrudo over 7 years ago in reply to Brani Dubocanin

Prodigy 180 points

Hi Brani,

It turned out to be an error in the PCB, every time I plugged the LCD module I was shorting the 3.3V rail. Luckily it wasn't bad enough to kill the LCD module or the LM2736, probably for all the safety features it has, quite a nice device!

Thank you all for the help!

Juan I.

0 Brani Dubocanin over 7 years ago in reply to Juan Cerrudo

TI__Expert 4100 points

Hi Juan,

Thank you for the update.

Glad to hear that everything worked out.

Please designate the question as answered.

Thank you

Brani

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LM2736: LM2736, voltage drop with high load