TPS51200: Overshoot observed in VO and REFOUT

VO_Overshoot

REFOUT_OVERSHOOT

Kalpesh,

What capacitance do you have on the REFOUT pin? Do you have a schematic that I can view? People have seen this issue in the past by not using the recommend capacitance on the REFOUT pin.

Hi Kadeem,

We are using 100nF on REFOUT pin.

We have tried with 1UF and 100UF.

Overshoot reduced to 80mV with 1uF and reduced to 30mV with 10uF on REFOUT Pin. (Rise time increased to around 250uS with 10UF cap on REFOUT pin)

We have used AM3352 soc with this project and which need 3.3V later to 1.5V.

So, TPS51200 also have 1.5V first and then 3.3V on VIN and EN pin. 

Please suggest if this delay in 3.3V on VIN and EN will create this issue or not.

Below are few detail on risetime and overshoot observed.

Does this 216mV overshoot and fast rise time of 11uS will create issue to Ref voltage of DDR? Please advise.

Kalpesh,

To answer your questions:

1. We have used AM3352 soc with this project and which need 3.3V later to 1.5V. So, TPS51200 also have 1.5V first and then 3.3V on VIN and EN pin. Please suggest if this delay in 3.3V on VIN and EN will create this issue or not.

> I have seen customers delay the 3.3V for EN without issue, but cannot say that I have seen them do this for VIN as well. Based on the functional block diagram, I do not believe that it will cause an issue.

2. Does this 216mV overshoot and fast rise time of 11uS will create issue to Ref voltage of DDR?

> I do not know, but rather than take the risk, I think that there may be a few things that we can try to reduce the overshoot.

What voltage are you seeing on the REFIN/VLDOIN pins during the VO startup? If overshoot is present on the VLDOIN, then there will also be overshoot on VO. During power-up, a larger current is present at VLDOIN to charge the output capacitors. It is possible for this current to be as much as the current limit (>3A), resulting in the overshoot.

What may help is increasing the capacitor C57 from 1nF to 10uF or 22nF.

Are you able to take a waveform of VLDOIN, REFIN, REFOUT, & VO before and after this change?

Hi Kadeem,

We have tried with delay on 1.5V and make the 3.3V earlier just to validate the behavior. (However, We may not able to do this as we have used AM3352 in this project which need 1.5V first and then 3.3V). 

Refer the images, Overshoot removed and glitch lower down.

We have validated REFIN pin and no any overshoot observed on it. Refer two more image with white color waveform. (3.3V direct means, 3.3V before to 1.5V). (3.3V enable with 1.5 means, 3.3V after 1.5V).

What could be the maximum capacitor value can be used on REFOUT? Increasing REFOUT capacitor is helping to reduce overshoot.

With 10uF on VREFOUT, Overshoot reduced to 32mV from 216mV. 

Image-1 : 3.3V after 1.5V

Image-2 : 3.3V before 1.5V

Image-3 : No overshoot on VIN and REFIN(3.3V after 1.5V)

Image-4 : No overshoot on VIN and REFIN(3.3V before 1.5V)

Hi,

Is this ok to delay the 3.3V supply to this IC after the 1.5V supply? Or We should put some enable control specially for TPS51200 and make it delay further than 3.3V?