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TPS54331 100% duty cycle

Dusan Stan
Prodigy 40 points
Other Parts Discussed in Thread: TPS54331

I need to design a switching power supply for a project that runs on batteries so Vin is 5-8.5V. The Vout is 5V. This needs a 100% duty cycle. From datasheet: "To improve
drop out, the TPS54331 is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is
greater than 2.1 V typically." It is not clear in which conditions I get 100% duty cycle. Help would be appreciated.

Dusan

  • 0
    TI__Guru**** 191445 points

    It will operate at 100% duty cycle for some number of cycles until the BOOT charge runs out then the device will turn of the high side until the BOOT cap is recharged.  So the net effect is operating at 99% + duty cycle. If you are operating at low input voltage headroom so that Vin - Vout is < 2.1 V and teh load current is low such that the inductor current goes to zero or are in eco-mode operation, then the output voltage will decay to Vin-2.1 V (PH is at Vout not clamped to one diode drop below ground) before the BOOT cap is charged above the BOOT UVLO threshold.  The net effect is 2.1 V p-p ripple on the output.

    More important, there are losses involved, so if you have only 5 V input, then the output will likely be partially regulated to 4.8 V or so even at near 100 % duty cycle.  If you need a tightly regulated output, you should probably look at a buck-bost topology to mauntain your 5 V regulation.