TPS25750: Soft Start Settings

Zeming,

The TPS25750 is preprogrammed to select the PPHV soft start and is not configurable by the end customer.

Are you having specific issues in your system?

Regards

Chuck

Hi Chuck,

Thanks for your reply.

The system will have a 60us 2A load transient in device side, and this will cause a 900mA pulse in USB 2.0 host side (default 500mA). Input current limit has been set to 500mA in battery charger IC, but it cannot response to this short pulse, we are considering adding more bulk capacitance to reduce the peak current. 

Thus we are afraid of the inrush current if the load capacitance is too large, so we would like to confirm the soft start function.

What will happen if the PPHV capacitance is beyond recommended max. 100uF?

Also, could you please teach us if there is other way to solve this problem?

Best,
Zeming

Zeming,

I can increase the capacitance on my EVM to test what happens, but all specifications are only guaranteed with the datasheet limits.

We may be able to help you with some configuration changes on the charger if it is a TI charger.  What charger are you using?

Regards,

Chuck

Hi Chuck,

Understood. The charger is BQ24192. I have also confirmed with charger team, and was told that the response time of load transient can not be configured faster. Thus this pulse cannot be limited in charger.

And this pulse is necessary for the load so we cannot set a limit in the load side (EE is something like a camera, when the shutter operates the load current will sharply increase)

According to USB PD specifications, source bulk should be 100uF max., I guess that is why TPS25750 PPHV capacitance is recommended as 100uF max.

The concern is if the negotiation can be passed when the bulk capacitance is beyond the max. value. I am thinking it should be no problem if TPS25750 is able to limit the inrush current with soft start function.

It will be very helpful if you can test on you EVM to see if any problem may happen.

By the way, soft start cannot be configured by user, then what is the initial setting? We can see 4 settings from the datasheet.

Best,
Zeming

Additionally, we calculated the necessary bulk capacitance to handle the pulse current, it may be 330uF at least, could you please help check if this will cause any problem? 

Best,
Zeming

Zeming,

The TPS25750 does not support FRS, so the cSnkBulk value does not come into play.

The VBUS capacitance is specified by the USB type C PD specification, so adding more that 100uF is not allowed.  Most customers use 1uF.

Is there any reason that you system cannot have the bulk capacitance on the PPHV pin instead?  

Hi Chuck,

As I mentioned, the load of this system will have a 60us 2A transient, and this will cause a 900mA pulse in the host side. If the host is USB 2.0 default, the 900mA pulse will violate the max. 500mA output current, so we are considering adding the PPHV bulk capacitance to reduce the peak current.

Extra bulk will cause inrush current, so we wants to know if the soft start can sufficiently limit the inrush current, and also if extra capacitance will bring other problems?

Best,
zeming

Hi Chuck,

Understood. Thank you very much.

Best,
zeming

Hi Chuck,

Sorry to open this thread again.

Could you please teach me the initial soft start setting?

The customer wants to know it even if it is not configurable for user.

Best,
Zeming

Zeing,

I am inquiring about the exact setting that was selected.  I will get back to you tomorrow.

Regards,

Chuck

Hi Chuck,

Please let me know if any update for this.

Thank you and regards,
Zeming

Thank you so much Chuck, this really helps!

Best,
Zeming