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TPS61165EVM-283: Duty cycle limitation with 3.3V Vin and 21V/65mA 1p7s WLED

Part Number: TPS61165EVM-283
Other Parts Discussed in Thread: TPS61165


We're using TPS61165's EVM for an evaluation under the following conditions:

  • Vin,typ = 3.3V
  • Vout,max = 21V (for 1 string of 7 series-connected WLEDs)
  • Iload,max = 65mA (i.e. Rset ~ 3 ohm)
  • PWM dimming via CTRL pin

From our test result, it seems the 65mA load current (or VFB = 200mV) can't be reached as only as Vin is below 4V.

It seems the maximum duty cycle is reached so that the output voltage can not be further boosted.

Datasheet says the maximum duty cycle is around 90%~93% with the condition of VFB=100mV

From the calculation based on 3.3V Vin and 21V Vout, the theoretical duty cycle is about 85%.

Could you help me understand whether the problem we met is really due to the maximum duty cycle limitation of TPS61165?

If so, may I know the maximum duty cycle specification with the condition of VFB=200mV?

Thank you,