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LM26420-Q1: Full-Load Startup VOUT1 falls then rises after VOUT2 is ramped

Part Number: LM26420-Q1
Other Parts Discussed in Thread: LM26420


I was curious as to why VOUT1 would fall then rise roughly 5ms after VOUT2 is ramped during a full-load startup.

Enables for both bucks are tied to the same power good signal.

Here is the scopeshot showing the startup behavior. The blue trace is VOUT1 (1.8V) and pink is VOUT2 (1.2V). Green trace is output of supply whose PG signal enables the LM26420-Q1 bucks. Yellow trace is the input voltage.

I have also attached the schematic for your reference.

I look forward to hearing your feedback.

TIDA-01492E1([No Variations])_Sch.PDF

  • Can you provide a zoomed in waveform of 1.8 V and its switching node?
  • Hi John,

    The EN of the LM26420 is controlled by the open-drain Power Good of another supply. The PG pull-up voltage is the 3.3-V, main system supply which turns on first. Once this other supply gains control of PG, it pulls it low, then high when the output has settled. This explains the on-off-on behavior of the LM26420. Switch node activity of the LM26420 follows PG.

    On no-load startup, there is a very slow discharge of output. When the LM26420 is disabled by the PG of the other supply (once it gains control of PG, it pulls it low), there is no current sunk anywhere, just a slow discharge of current, and retention of ~1.8-1.7 V on the output.

    On full-load startup, the output is discharged quickly due to the full load.

    Do you agree with the explanation for the output voltage given load conditions? I have attached screenshots below for both full load and no load (in that order) switch nodes with PG signal for your reference.

  • Let me consider this.  Please remind me Sunday morning ( or Monday if you don't work on Sunday)

  • That seems reasonable to me.
  • Hi John,

    After going back over this, I am still not entirely sure why switching does not start again when the power good signal is high during no load start up. The full load start up behavior is expected, where the switching starts again once power good is high. Here is the screenshot.

    Let me know if you need more information.