LM27762: Related to Response

Hi Shivakant,

With a load, Vin might not be enough for the device to regulate both outputs.

I would suggest to increase the input voltage and try to repeat the measurements with and without load. 

Also, while calculating the dropout for the negative output, as it does not contain the required equation. You must compute both the voltage drop from the charge pump’s output impedance (figure 4) and the LDO drop. Please read section 8.1 from the datasheet for a better understanding of headroom calculation.

Please let me know if this helps.

Regards,

Febin

Hi Febin,

Thanks for response.

In datasheet,

it is mentioned that 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.When selecting input (VIN) and output (OUT+, OUT-) voltages ranges.

But how headroom is calculated?

Is there any formula for it?

In datasheet , further information regarding headroom is not mentioned.

Also LM27762 can take max +5.5V input & i run simulation for +5.5V too but still same problem is coming for +/-5V output.

Hi Shivakant,

I understand your concern. Unfortunately, this is an old part and we cannot update the datasheet at the moment. Thank you for the feedback and we will take this into consideration.

There is no such formula, but you can make an estimate as shown below:

1) +ve output voltage will be +VIN – LDO+(worst case) @ Iout+ 

2) –ve output voltage will be +VIN – charge pump drop – LDO-(worst case) @ Iout-

Please refer to Fig 3 and Fig 4 for estimating LDO Dropout Voltage & Charge Pump Output Resistance. But again, these values can be used only for a reference.

Please use the same conditions for with and without a load. You must be able to see a difference.

I hope this helps. Please let me know otherwise.

Regards,

Febin

Hi Febin,

LM27762 is designed in WEBENCH® Tools & response captured are as follow:

i am getting proper response for +/-70 mA load which is as follow:

But while doing simulation for same circuit using TINA with considering same load, i am getting  very low value like 8mV for -5 v output.

but for +5V out put getting proper response of +5.05V( when input is +5.5V).

Please verify whether it's Unencrypted PSpice Transient Model (Rev. B) is rightly coded or not as i imported this file into tina to make my design.

If possible please run it's simulation for +/-5V output to power any op amp(which can take current under limit).

you can use my design for verification:

Didn't hear back yet regarding my problem.

If you want TINA file then please find attached.

Please verified once from your end regarding negative output from LM27762Chain_SIM_LM27762 - autosave 20-05-25 16_32.TSC

Hi Shivakant,

I am working on your request.

I am checking with the corresponding teams to verify the models. Please give me some time and I will get back to you in 2 days.

Regards,

Febin

Hi Shivakant,

I have changed a few settings in the TINA-TI model and please find attached the updated file. 

It looks like the pspice model is fine but after importing into TINA-TI you need to have proper initial conditions for CN, CP, CPOUT, OUT_P and OUT_N capacitors.

Please note the changes made:

1. EN_P and EN_N input signal is a Step source. This model requires a step input source to calculate appropriate operating point. Please note PSpice model on TI website includes both Startup and Steady state setup. Please refer to the same.
2. Transient analysis with “Calculate Operating Point” ticked.
3. Time step is increased

Please try the transient simulation and let me know if it helps.

LM27762_updated.tsc

Regards,

Febin

Hi Febin,

Thanks for your response.

Without Load condition,  i am getting proper response ie =/-5V  but when i am providing load, in that case -5V is not coming.

Please find the file attached which i am using with Load condition.2020_May_29_Chain_SIM_LM27762__autosave_200525_16_32.TSC

Please also find response of LM27762 with Load condition to power AD8337 & LMH6553:

How ever without load condition

, LM27762 is responding proper:

In both cases, i am using the modified file of LM27762 , given by you.

please see my attached file(2020_May_29_Chain_SIM_LM27762) with load condition & prescribe the solution for LM27762 for load condition.

Hi Shivakant,

I tried to simulate with a resistive load and the output is as expected. Could be the error in the Opamp modelling. I shall check again.

Please note that Monday is a holiday for us in Germany. I hope to get back to you by mid of next week.

Thank you for your understanding!

Regards,

Febin

No issue Febin.

Hi Shivakant,

Please be informed that our TINA-TI team is still working on your request and I have not heard from them yet. Please give me some more time and I will get back to you early next week with some updates. It is taking longer than I expected. I apologize for the delay.

Thank you so much for your patience and understanding!

Regards,

Febin

Hi Shivakant,

Here is the updated version of the simulation file. A few changes were done and now the output of LM27762 is as expected.

- Transient analysis with “Calculate Operating Point” checked

- It looks like VEE(-5V) output of LM27762 is getting loaded by AD8337 and LMH6553. Hence added a buffer at the output.

LM27762_updated_V2_updated.TSC

Please check and let me know if you need further support.

Regards,

Febin

Hi Febin,

thank u so much for responding & your dedication towards us.

I will check it & inform you.

Just for now, want to know that in my Orcad schematic design, should i include buffer @ -5V output of LM27762 to power AD8337 & LMH6553?

I feel than same problem will occur with LM27761 also which is co part of LM27762.

Hi Shivakant,

This change was done for the simulation to run properly. Usually in the schematic you don't need to indicate this change unless you have some real-time issues. This would be my opinion. You can test it on the bench and confirm.

Regards,

Febin

Hi Shivakant,

Could you simulate the model and verify?

Do you need any further support? If not, can we close this thread?

Regards,

Febin

Hi Shivakant,

Thank you for the quick confirmation.

Please reach out to us for any further support.

Regards,

Febin