TPS84620: Power up Sequence waveform is not smooth

Vishal Batham7

Prodigy 200 points

Part Number: TPS84620
Other Parts Discussed in Thread: TPS62140

We are using TPS84620 for generating 2.5V and 3.3V and TPS62140 for generating 1.1V

Picture 1 and 2 shows the power up sequencing of 1.1V vs 2.5V and 1.1 vs 3.3V.

Issue 1: The waveform observed of 2.5V and 3.3V are not smooth as 1.1V.

Please comment on it.

over 6 years ago

0 J Arrigo over 6 years ago

TI__Mastermind 28020 points

It looks like the 2.5V and 3.3V are turning on and off multiple times during start-up? Can you include the input voltage waveforms under these conditions? Can you send me the schematic for this application? You can contact me directly at jarrigo@ti.com.

0 Vishal Batham7 over 6 years ago in reply to J Arrigo

Prodigy 200 points

Below is the input voltage start-up waveform and schematic.

0 J Arrigo over 6 years ago in reply to Vishal Batham7

TI__Mastermind 28020 points

Your input voltage waveform shows a "dip" which is probably turning the device on and off. It would be helpful to see the scope waveforms including the input voltage and the 3.3V output and the 2.5V output voltage on the same screen. This would show if the timing of the input voltage "dip" corresponds to the on/off behavior of the output voltage.

Also, looking at the 3.3V schematic, I don't know how the 3.3V rail is even powering up. It looks like the INH/UVLO pin is shorted to ground. Please review the INH/UVLO, SS/TRK, STSEL and EN input connection to make sure these connections are correct.

Contact me directly at jarrigo@ti.com to discuss offline.

0 Vishal Batham7 over 6 years ago in reply to J Arrigo

Prodigy 200 points

For your information

Pl ignore the previous 3.3V schematic , below is the actual one.

Below are the waveform of input vs 2.5V out and input vs 3.3V out.

0 Vishal Batham7 over 6 years ago in reply to Vishal Batham7

Prodigy 200 points

Pl comment on it

0 J Arrigo over 6 years ago in reply to Vishal Batham7

TI__Mastermind 28020 points

From the waveforms, the input voltage is dropping when both output voltages are turning on. The current capability of your input supply may need to be increased. You may try delaying the turn on of the output voltage until VIN is higher by using the UVLO resistor. Slowing down the rise of the output voltages with a soft-start capacitor may help as well. You can also try adding additional input capacitance to keep the input voltage more steady.

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TPS84620: Power up Sequence waveform is not smooth