TPS7A80: voltage drop during startup

Ben Kuo

Part Number: TPS7A80

Dear team,

Customer design TPS7A8001 on their system and face to start up not raising up smoothly.

Per my understood, TPS7A8001 integrated quick start circuit and RC delay for inrush current limit. So the star-up waveform should have two different slew rate.

We have disconnect load and fine-tune CNR, but cannot fix it.

Could you share your comments about this?

Regards,

Ben

over 3 years ago

0 Stephen Ziel over 3 years ago

TI__Mastermind 33315 points

Hi Ben,

How much load capacitance does the customer have?

It seems obvious that the green waveform is Vout, but can you clarify exactly what the other waveforms?

I'm wondering if there is a large Cout which causes the device to go into dropout during turn on due to the load current.

Thanks,

- Stephen

0 Ben Kuo over 3 years ago in reply to Stephen Ziel

TI__Expert 7225 points

Hi Stephen,

Update waveform information below.

CH1> Vin

CH2> Vout

CH3> EN

CH4> Iout

Output cap is only 4.7uF.

As you can find in waveform, EN signal is turn on slowly. And the voltage drop is happened in EN~ 1.2V.

I am asking to customer to apply a fast slew rate EN signal to see if we can fix this issue.

looking forward for your comments. thanks.

Regards,

Ben

0 Stephen Ziel over 3 years ago in reply to Ben Kuo

TI__Mastermind 33315 points

Hi Ben,

The issue is the very slow enable signal. I was able to replicate this on an EVM if I really slowed down the enable rise time. In my case I slowed it down so much that there appears to be a large overshoot at this rate. If you can speed up the enable signal the issue goes away. What is the rise time in your design, and can you speed up the ramp rate?

Channel 1 = Vin = 3.3V
Channel 2 = Vout = 1.8V
Channel 3 = Ven = 3.3Vpk, 100 milihertz sawtooth wave