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# TPS628501-Q1: inrush current during startup

Part Number: TPS628501-Q1

Hello team,

To consider the current rating of the buck converter in front of the TPS628501-Q1, we would like to know the inrush current during  start up.

Do you have any good idea to estimate this?

Do you have a waveform of the inductor current during startup?

The schematic is not decided yet.

Vin=3.3V, Vout=1.1V, Cout = TBD

Thank you.

Regards,

Hirata

• Hi Hirata,

The device has an internal soft start time of 1ms.

Therefore, you should be able to compute the theoretical current at startup as I = Cout*1.1V/1ms.

To give you a better idea, you can take a look at the PSpice/Simplis simulations (you find the model in the product folder TPS628501-Q1 data sheet, product information and support | TI.com).

Thanks,

Stefano

• Stefano-san,

I did some simulation with PSPICE for TI.

Under my simulation, if the inductor current becomes less than 800mA, inductor current always reached to 800mA and frequency is modulated.

Question.

Q1. During the startup, if the peak inductor current does not exceed the 800mA with forced PWM mode, does this IC use PFM mode during startup irrespective of the MODE setting? and peak inductor current always reached to approx. 800mA in the PFM mode?

Q2. I see one big peak current initially which exceeds the 800mA. What is this? Does this actually happen? If so how to estimate this?

Vout = 1.1V

Cout = 100uF

Red wave form: Inductor current

Blue wave form: Current through load resistor

Thank you.

Regards,

Hirata

• Hi Hirata-san,

The first phase of the startup will be in PFM independent from the mode selection, with inductor current peaks around 800mA.

Regarding to the second question, it should be only a simulation error.

CH1 VOUT, CH2 I_inductor

Thanks,

Stefano

• Hi Stefano-san,