LM27762: LM27762DSST

krishna prasad

Part Number: LM27762

Hello TI Support Team

We are using LM27762DSST(U7) to provide +2.5V & -2.5 Supply to a Analog Mux ADG708CRUZ (U1 to U6)which is the Load, 6 Mux are used in the Circuit.

Observations : Using 5V Lab Bench PS when we power on the U7 & Mux is not enabled, we see the following

1)+2.5V GOES TO 2.3V instead of 2.44V- Please refer to Circuit Diagram below

2)-2.5V goes to 1.33V.

3)Upon reducing External 5V to4V or below we see the outputs are +2.4V & -2.4V.

4)It remains stable till Extenal 5V drops till 2.5V.

5)Directly Power on with External Supply at 3.3V the outputs are +2.33 & -1.33V.

Portions of the Circuit are shown below U7 & U1- 1 Mux Connection & Capacitors used are X5R/X7R Type.

Can you please let us know what is wrong with Our Circuit or U7?? -2 PCBS have been tested & behave the same way.

Thanks n regards

Krishna P..N.

over 5 years ago

0 Febin over 5 years ago

TI__Mastermind 25430 points

Hi Krishna,

I am not able to see the images attached. Could you please resend the schematic?

Regards,

Febin

0 Febin over 5 years ago in reply to Febin

TI__Mastermind 25430 points

Hi Krishna,

I have not heard back from you? Could you solve your problem?

Regards,

Febin

0 krishna prasad over 5 years ago

Intellectual 415 points

SCHEMATICS_TI_Jul21-2020.docx

0 krishna prasad over 5 years ago

Intellectual 415 points

5265.SCHEMATICS_TI_Jul21-2020.docx

0 krishna prasad over 5 years ago in reply to Febin

Intellectual 415 points

Febin

Have sent you schematics twice as attachment word and pdf is unable to see pasted below from word doc

SCHEMATICS U7-LM27762 BELOW

U1 ADG708 ONLY SHOWN THERE ARE 5 MORE MUX CONNECTED TO POWER THRO U7

0 Febin over 5 years ago in reply to krishna prasad

TI__Mastermind 25430 points

Hi Krishna,

The schematic for LM27762 looks fine.

1. What is the maximum load for this application? When calculating the dropout for the negative output, headroom required by the charge pump and LDOs must to be considered. Charge-pump minimum headroom can be estimated based on the maximum load current and charge pump output resistance.

2. Can you please share the Vin, +Vout, -Vout, Ccp, Cfly waveforms ?

3. Do you have the chance to swap the IC with one of our EVMs?

Regards,

Febin

0 Febin over 5 years ago in reply to Febin

TI__Mastermind 25430 points

Hi Krishna,

Do you have some updates to share?

Regards,

Febin

0 krishna prasad over 5 years ago in reply to Febin

Intellectual 415 points

Febin

1)The load to dual regulator is 6 Mux whose current consumption is 12-15uA

2)We dont have EVAL Board of TI to check if the same behaviour of these 2 Parts on our board

I hope u will get back to us with a solution as we power up at 5V output voltages are lower than regulation limits but when Vin <4V it goes to Correct Value of 2.45V

0 Febin over 5 years ago in reply to krishna prasad

TI__Mastermind 25430 points

Hi Krishna,

I reviewed the schematic again and noticed that you have not used any input and output capacitors. This is not recommended. I am sorry that I did not notice this before. Could you please add 2.2uF :COUT+, COUT– and 4.7uF CVIN. Please try again and let me know if it solves the problem.

Regards,

Febin

0 krishna prasad over 5 years ago in reply to Febin

Intellectual 415 points

Fabin

Capacitors for Vin and Vout portion was not shown in earlier schematics and is as below

0 Febin over 5 years ago in reply to krishna prasad

TI__Mastermind 25430 points

Hi Krishna,

I would like to see the complete schematic and layout.

If you cannot share it here, please send me an email: febin.hameed@ti.com

Regards,

Febin

0 Febin over 5 years ago in reply to Febin

TI__Mastermind 25430 points

Hi Krishna,

I will go ahead and close this post due to inactivity. Please send me an email if you would like to get more support on this issue.

Regards,

Febin

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LM27762: LM27762DSST