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LM3478Q-Q1: unstable output @high temp

Dongbao Zhou
Expert 3960 points
Part Number: LM3478Q-Q1
Other Parts Discussed in Thread: LM3478

Hello team,

One of my customer use LM3478 to generate +-40V from 12V battery, the output voltage of P_40V_NEG_ISO became unstable ( fluctuate between -37.5V and  -39.8V, no rule) and P_40V_POS_ISO is stable. This issue happens when the temp of LM3478 rise, the load is a 1.5Kohm resistor in the output.

Could you help check below schematics and give some feedback?

For the R340, could you let me know the function of this resistor? Can we remove it? Thanks.

Dongbao

  • 0
    TI__Mastermind 46905 points
    Hi Donghao

    The engineer is out of office now. the post had been assigned to him. He will reply after back to office.
  • 0 in reply to Jasper Li
    TI__Expert 3960 points
    Hi Jasper,
    I just mis-click the button of "this resolve my issue", please keep this thread.
    One more question is, since the negative output is open loop. Can we gurantee the output ripple with in +-1V with no more than 30mA load?
    Thanks.
    Dongbao
  • 0 in reply to Dongbao Zhou
    TI__Mastermind 21720 points
    Hi Dongbao,

    What switching frequency is the application circuit at?

    To help stabilize the loop please try increasing C268 and decrease R355. This will lower the crossover frequency of the loop and should help stabilize the output voltage.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    TI__Expert 3960 points

    Hi Garrett,

    You can see the RT is set as 39K, so the switching frequency should be around 400kHz. 

    1. The customer tried to change R355=100k and C268=10nF, we can have a stable output of -40V rail. We would like to know for previous COMP value, only some of their board have the unstable issue and this issue happens in high temp, Could you kindly help explain the possible reasons?
    2. If we short the R340, we could solve the unstable problem? Could you give some comments on the R340? Do we need it?
    3. I asked the customer to place 10kohm resistor as the load for +-40V rail, however, the output voltage drops to 38V. Is there any problem with their schematic?

    Thanks.

    Dongbao

  • 0 in reply to Dongbao Zhou
    TI__Mastermind 21720 points
    Hi Dongbao,

    Please see my responses below.

    1. The customer tried to change R355=100k and C268=10nF, we can have a stable output of -40V rail. We would like to know for previous COMP value, only some of their board have the unstable issue and this issue happens in high temp, Could you kindly help explain the possible reasons?
    - This could just be due to tolerances of the external components. It is possible that the compensation was marginally stable before and with the temperature shift became unstable.

    2. If we short the R340, we could solve the unstable problem? Could you give some comments on the R340? Do we need it?
    - The resistor might just be too large. I would recommend keeping a place holder but it can be populated with a 0Ohm resistor.

    3. I asked the customer to place 10kohm resistor as the load for +-40V rail, however, the output voltage drops to 38V. Is there any problem with their schematic?
    - What happens if the load is increased? If the load is decreased?

    Thanks,

    Garrett