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LMR36506RREVM: EVM starts to oscillate at Vin <3.37V

Franz Schellhase
Expert 3605 points
Part Number: LMR36506RREVM

Hi Team,

A customer is looking for a DCDC to supply a motor at 3.3V. A drop to 3V is okay.

Requirements:

  • Vin = 3,3V..27V
  • Vout = 3,3V (3,0V)
  • Iout = 50mA
  • Iq < 20µA  @ Vin < 3.7V (supply can be both from line at up to 27V or Li-Ion battery)
  • 98% duty cycle
  • Small board space
  • Available next year
  • Good EMI resilience

They have tested LMR36506RREVM so far, which showed the following behaviour:

  • Iq of EVM 35uA at 3.7V (seems high compared to typical Iq of the IC)
  • Circuit starts to oscillate at VIN <3.37V, current at 4mA
  • Circuit stabilizes at >3.65V

Since they want to be able to operate from battery they would like to avoid the oscillation and would prefer the IC to go into sleep. Thus following requirements are added.

  • UVLO at Vin=3.2V (Vout can drop to 3V)
    • Restart at 3.4V

Do you have a suggestion how to avoid the oscillation and relatively high Iq or do you have an alternative DCDC suggestion?

Thank you,
Franz

  • 0
    TI__Mastermind 34790 points

    Hello Franz,

    The boostrap capacitor is charged during the conduction of the low-side FET (off time) in a synchronous buck converter.

    With a high duty cycle such as this application, their is minimal off-time. In addition, with a low load current, the "true" off-time is further reduced, as the average inductor current is low, the inductor current will go discontinuous at which point the low-side FET will turn off and both FETs will tristate.

    The device can support high duty cycles, close to 100%. The vin to vout differential can be approximated to be: Vin - Iout*RDS,HS; Where RDS,HS is the RDS of the high side.

    The converter will need to operated in FPWM to ensure boot UVLO is not hit, which I presume to be the case here.

    The higher iQ from the typical is due to the fact the converter is operating in dropout, with a very reduced switching frequency and a long conduction time in the high-side FET. This is expected .

    Could you share the waveforms of Vin, Vsw, Vbst, and Vout