LM2776: not start up

Hirotsugu Kobayashi

Mastermind 9630 points

Part Number: LM2776

Hi, team.

I tested LM2776 with increasing load gradually, but it did not start at 165 ohms.

Load is resistance only. Why?

Can the LM2776 be started up with load?

Is EN control necessary?

over 7 years ago

0 Chris Glaser over 7 years ago

TI__Guru**** 151530 points

Hi Kobayashi-san,

Is your load just resistors or the actual amplifier that will be used in the system? Usually, the output is pulled positive like this through the amplifier. You can turn on the negative rail before the positive one to overcome this, in most cases.

0 Hirotsugu Kobayashi over 7 years ago in reply to Chris Glaser

Mastermind 9630 points

Hi, Chris.

The load is resistor only, there is no amplifier.

Can the LM2776 start up even with a resistive load?

0 Chris Glaser over 7 years ago in reply to Hirotsugu Kobayashi

TI__Guru**** 151530 points

Yes, it can startup. Figure 13 in the D/S shows this.

Here is what the EVM gives when EN is tied to Vin and Vin applied. The shape of Vin is just related to my input power supply and not to the device.

Can you test the EVM? Or can you send your BOM of caps used around the LM2776?

0 Hirotsugu Kobayashi over 7 years ago in reply to Chris Glaser

Mastermind 9630 points

Hi, Chris.
The test is not EVM, it is a breadboard, but the capacitor is placed closest.
C1,C2 0603X225K100CT (Walsin)
C3 C1608JB1E105KT (TDK)
Does this happen if the capacity is small?

0 Chris Glaser over 7 years ago in reply to Hirotsugu Kobayashi

TI__Guru**** 151530 points

Thanks for explaining. Can you send a photo of the breadboard?

As well, with the long wires of the input supply connection to the breadboard, there may be too much input impedance. You might try adding some large capacitance on the input to the breadboard to overcome it.

Was the waveform measured on the pins of the IC on the breadboard?

0 Hirotsugu Kobayashi over 7 years ago in reply to Chris Glaser

Mastermind 9630 points

Hi, Chris.

Please check the photo.
Thank you for the advice. I will add an input capacitor and try it.

0 Chris Glaser over 7 years ago in reply to Hirotsugu Kobayashi

TI__Guru**** 151530 points

Thanks for sending. Yes, you could order and test the EVM to verify that it works the same. It worked fine for me, as I showed on the 18th.

I also recommend verifying that the capacitors are the ones that you think they are. Just replace them with new ones, for example, and measure the capacitance.

0 Hirotsugu Kobayashi over 7 years ago in reply to Chris Glaser

Mastermind 9630 points

Hi, CHris.

Current evaluation is done with resistive load, but it is used for negative power supply of amplifier.

Advice is given to turn on the negative power supply after turning on the positive power as shown below.

"Usually, the output is pulled positive like this through the amplifier. You can turn on the negative rail before the positive one to overcome this, in most cases."

However, since there are only amplifier and LM 2776 in the circuit, we can not combine the sequence. Is there a workaround?

0 Chris Glaser over 7 years ago in reply to Hirotsugu Kobayashi

TI__Guru**** 151530 points

Hi Kobayashi-san,

Were you able to get the circuit working with the resistor load? Let's debug this first.

I initially thought that you were connected to an amplifier for your testing. Some (but not all) amplifiers can pull the output positive like you showed. Let's test with your actual amplifier after we debug the resistive circuit.

0 Hirotsugu Kobayashi over 7 years ago in reply to Chris Glaser

Mastermind 9630 points

Hi, Chris.
I think that it is a defect that LM2776 can not be used depending on the type of operational amplifier.
Please note in the datasheet.

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LM2776: not start up