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LM2576: LM2576T-12 (fixed) output overshoots when feeding synchronous regulator to generate -7V

luck luck
Intellectual 450 points
Part Number: LM2576
Other Parts Discussed in Thread: LM337

  refer the LM2576 schematic above, it generates perfect 12V DC. But the output is observed to overshoot (>24V DC) when its output is connected as input to another synchronous regulator from Maxim-integrated as depicted below.

Objective is to augment an existing system that has +28VDC for generating five different tune-able negative voltage range -2.5V to -5V DC. LM2576 would step-down 28 to 12V DC and Max17502 in turn would convert 12V into -7V DC. Five LM337 parts would provide tune-able -2.5V to -5V DC from this -7VDC.

The second schematic works perfectly with 12VDC from external SMPS, but when cascaded to LM2576 setup above, causesLM2576 output to overshoot beyond +24VDC instead of stable 12VDC.

Please advice reason and possible solution such that LM2576 output is not impacted by input to Max17502H

  • 0
    TI__Guru**** 191445 points
    What condition causes the overshoot? Are you hot plugging the second converter? Do you have any waveforms that show this? can you include the SW (output pin 2)?
  • 0 in reply to JohnTucker
    Intellectual 450 points
    NO hot-plugging at all. DSO out of order, working with multi-meter at the moment.
    I have two PCB setups:

    PCB 1: has both circuits populated. Connecting 28VDC, the output at junction of C38 and L7 for LM2576-12 starts ramping rapidly beyond 24V whereby I disconnect 28V(to avoid blowing of LM2576, one already sacrificed already). Removing LM2576 from this setup, and sourcing the junction of C38 and L7 with 12V from another SMPS gives perfect -7V from Max17502

    PCB 2: has just the LM2576 circuit. The junction of C38 and L7 for LM2576-12 remains at 11.9V DC. No under/over voltages.
  • 0 in reply to luck luck
    TI__Guru**** 191445 points
    What happens if you hold the EN of the 17502 low, allow the LM2576 to completely reach regulation , the release the EN of the 17502?
  • 0 in reply to JohnTucker
    Intellectual 450 points
    Monitoring LM2576 output and controlling 17502-EN dynamically could amount to adding more complexity.

    By the way, re-look at this cascaded arrangement of LM2576 and 17502, it seems that the moment 17502-EN is above 8V, the two inductors L7 (2576) and L1(17502) get connected in kind of series connection. Could it be root cause for De-stabilization of LM2576 output. In that case this cascaded arrangement approach is best avoidable !
  • 0 in reply to luck luck
    TI__Expert 4365 points

    I think John is on the right track.

    I suggest two things:

    Increase the SS cap on the MAXIM inverting Buck-Boost by a factor of 10 from 0.01 up to 0.1uF. This will slow down the ramp-up time of -7Vout.

    Make sure the voltage divider on R2/R4 on the UVLO input of the MAXIM device is set for 10.5 or 11V so the first converter is nearly stable before the second converter attempts to start.