LM2776 not working??

Nebojsa Zdravkovic

Other Parts Discussed in Thread: LM2776, LM324

Hi all, I have trouble getting my LM2776 samples to work at all with basic application circuit from datasheet. I have set one 2u2 X7R at the input and one 2u2 X7R at the output, with 1uF X7R as flying capacitor. Vin is 5V from usb, output has nothing connected. Vout in this setup is barely -2V. I tried three different samples and they all misbehave. I have no idea what I am doing wrong here??

over 9 years ago

0 Anston Lobo over 9 years ago

TI__Genius 16025 points

Hi Nebojsa,

I will redirect your question to the appropriate forum.

Thanks,
Anston

0 Greg Lubarsky over 9 years ago

TI__Intellectual 1940 points

Hello,

I have a few questions regarding your setup

Have you tried using a power supply instead of a USB rail? Is there anything in series between the USB rail and the LM2776 such as a resistor? Can you provide a schematic?

Greg

0 Zhang jian over 8 years ago in reply to Greg Lubarsky

Prodigy 90 points

I have the same problem ,my LM2776 input +5V however I just get +0.68V.My SCH is as follow.

0 Greg Lubarsky over 8 years ago in reply to Zhang jian

TI__Intellectual 1940 points

Hello,
Is the LM2776 starting up into a load when this happens? Additionally, what is creating the +5V power supply and what is the current rating? If the supply does not allow enough current or if there is a large series resistance between the supply and the device, the part might not start up. Can you attach a start up waveform showing what happens to the input output and C1+ pin when power is first applied?

Greg

0 John Evans65 over 8 years ago in reply to Zhang jian

Prodigy 10 points

I'm having the exact problem, seeing the exact voltage. My voltage supply is a bench top supply with 3500 low esr electrolytic between the rails on the supply.

0 Greg Lubarsky over 8 years ago in reply to John Evans65

TI__Intellectual 1940 points

Hello,
Are you observing this on the evaluation board or in your own design? Before enabling the device, what voltage is the output at? Under normal conditions it should be 0V. If it is higher than that with the part disabled (greater than 0V or GND), it would imply that the output is being driven by the load. As I mentioned above, seeing scope photos of this behavior will help in debugging this issue.

Greg

0 Moha Deeb1 over 8 years ago in reply to Greg Lubarsky

Intellectual 270 points

Hi,

I have the same problem on a customized board, where the output is connected to the negative supply of LM324 OP Amp

when the En pin is disabled, the output is almost +0.6V.

I'll try to get a scope waveform for the startup.

Thanks.

0 Greg Lubarsky over 8 years ago in reply to Moha Deeb1

TI__Intellectual 1940 points

Hello,

As I mentioned in a previous reply, this sounds like the output is being pulled high as there is nothing internal to the LM2776 that should cause this behavior. Is the positive rail of the LM324 active when the LM2776 is disabled?

Greg

0 Moha Deeb1 over 8 years ago in reply to Greg Lubarsky

Intellectual 270 points

Hi,

Yes, its connected to +12v

0 Greg Lubarsky over 8 years ago in reply to Moha Deeb1

TI__Intellectual 1940 points

If you remove the +12V from the LM324, what does the output of the LM2776 go to? Does it still go positive?
When the EN pin goes high, does the device start up properly? Or does it stay clamped to the +0.6V?

0 Moha Deeb1 over 8 years ago in reply to Greg Lubarsky

Intellectual 270 points

Hi Greg,

It worked!

I made a layout mistake and connected In/Out Caps GND through the ground plane away from the common ground of the LM2776 which caused higher inductance.

Here is top layer:

I just rotated the caps, and its OK now.

Thanks.

0 Greg Lubarsky over 8 years ago in reply to Moha Deeb1

TI__Intellectual 1940 points

Glad to hear things are working.

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LM2776 not working??