TPS65132S: Vneg drop under about 150mA output

Jiayu Ye

Part Number: TPS65132S

Dear,

Here my customer use TPS65132s for the bias supply of tablet LCD panel. We use the high-current application.

Vneg is set -5.7V, SYNC is high. We found the Vneg drop is about 425mV when there is a high current transient supply which is about 140mA~210mA considering ripple. So is this normal, do you have any suggestion to decline the Vneg drop to about 100mV? Thanks. And would you please help to verify on 65132sEVM what the actual Vneg drop value is when there is a 150mA current supply need. Thanks.

over 3 years ago

0 Liaqat Khan over 3 years ago

TI__Mastermind 18550 points

Hello Jiayu,

Yes, it is normal to have the VNEG drop with increasing current beyond 150mA. You can try adding another 10uF on the VNEG output to see if it can reduce the drop. For typical EVM performance, please see datasheet figure 93 that shows VNEG drop for a load transient 10mA-150mA-10mA.

Kind Regards,

Liaqat

0 Jiayu Ye over 3 years ago in reply to Liaqat Khan

TI__Intellectual 1790 points

Hi Liaqat，

Thanks for reply. Here we have two more questions:

1) Will there be bad side effect if we add another 10uF(totally 20uF) in VNEG pin ?

2) Does 150mA refer to the average current or peak current? I mean there will be some current ripple in customer's application.

0 Jiayu Ye over 3 years ago in reply to Jiayu Ye

TI__Intellectual 1790 points

Hi Liaqat,

One more question,

3) It seems only 14mV is decreased in VNEG drop when customer change 10uF to 20uF. The average Ineg is 150mA, do we have some better suggestions? Thanks.

0 Liaqat Khan over 3 years ago in reply to Jiayu Ye

TI__Mastermind 18550 points

Hi Jiayu,

Current out of TPS65132S VNEG output is specified at 150mA. I measured on an EVM the the output voltage difference between load of 0mA and load of 150mA and it is only 33mV so the device regulates pretty well up to specification of 150mA but as the current is increased beyond 155mA, output voltage of VNEG starts to sag significantly and I measured drop of ~500mV at load current of 170mA. If the peak current goes beyond 150mA, there will be a significant voltage drop like the one you are seeing.

Kind Regards,

Liaqat

0 Jiayu Ye over 3 years ago in reply to Liaqat Khan

TI__Intellectual 1790 points

Hi Liaqat,

Did you test the EVM load transient with a e-load? What is the slope of load current changing from 0 to 150mA you set? We use the EVM and LCD panel to find voltage drop much larger than 33mV. I wonder whether it is related about the slope difference? Thanks.

0 Liaqat Khan over 3 years ago in reply to Jiayu Ye

TI__Mastermind 18550 points

Hi Jiayu,

I measured with steady state load to get an idea of the output current capability and not load transient. As I mentioned earlier, load transient performance is shown in figure 93 of the datasheet for a 10mA-150mA-10mA load step and output voltage drop is similar to what I measured with steady load.

Kind Regards,

Liaqat

0 Jiayu Ye over 3 years ago in reply to Liaqat Khan

TI__Intellectual 1790 points

Hi iLiaqat,

Thanks for reply. One more question, is it okay if we change Cfly from 4.7uF to 10uF? Customer tested and found it's okay with 10uF Cfly, which is aimed at achieving BOM compatibility with another P2P device, you know TPS65132s is out of stock.

We are looking forward to your comments. Thanks.

0 Liaqat Khan over 3 years ago in reply to Jiayu Ye

TI__Mastermind 18550 points

Hello,

Per datasheet, flying capacitor value for charge pump must be lower than boost output capacitance at REG pin. Since boost output capacitance at REG pin is 10uF, flying capacitor can not be 10uF. Please see highlighted portion in the data sheet excerpt provided below.

Kind Regards,

Liaqat

0 Jiayu Ye over 3 years ago in reply to Liaqat Khan

TI__Intellectual 1790 points

Hi Liaqat,

Thanks for reminder, so I guess Cfly can be 6.8uF or 8.2uF as long as smaller than 10uF, if customer insist right?

0 Liaqat Khan over 3 years ago in reply to Jiayu Ye

TI__Mastermind 18550 points

Hi Jiayu,

That is correct but since the whole point of changing the capacitor was to keep BOM combability with a different competitor device, which 6.8uF or 8.2uF will not achieve, why change it from 4.7uF.

Kind Regards,

Liaqat

0 Jiayu Ye over 3 years ago in reply to Liaqat Khan

TI__Intellectual 1790 points

Hi Liaqat,

Because it has a positive effect on VNEG spike on competitor device when changing Cfy from 4.7uF to 10uF.

0 Liaqat Khan over 3 years ago in reply to Jiayu Ye

TI__Mastermind 18550 points

Hi Jiayu,

Changing Cfly to 6.8uF or 8.2uF should be ok if the capacitor at boost output (REG pin) is 10uF.

Kind Regards,

Liaqat

Power management

Power management forum

TPS65132S: Vneg drop under about 150mA output