BQ25713: Input capacitor

1) It looks that the EVM has 60uF of CERMs and 33uF of TA balk on the VIN line. If there is more capacitance(e.g. total 200uF), should it be acceptable case or is there any concern for larger input capacitor case. Some application may have a larger bulk cap on the VBUS input.

Adding additional input capacitance will not directly cause an issue with the BQ25713, but there are two things to consider. Firstly, be sure that the extra capacitors don't add extra parasitic resistance and inductance. If you are putting multiple ceramics in parallel, then the parasitics should be low, but that is one thing to be careful of. Secondly, from a system perspective, having a large amount of capacitance on the input may cause issues. For instance, if you are hot-plugging the device, there will be larger inrush current upon initial plugin if you have more capacitance. The soft-start circuit (Q5/Q6 and associated drive circuitry back to CHRG_OK) may need to be adjusted accordingly, both to ensure that the larger inrush currents are properly handled and to ensure that the additional current doesn't overstress the switches.


2) Can the 33uF of TA alter to CERM or E-caps as well?
The tantalum electrolytic has a lower derating over DC voltage than ceramic capacitors, so if you are going to replace it with ceramic, you would need more than 33 uF of ceramics to handle the voltage derating. This would be especially important if you need to handle a full 20V range on VIN as the EVM is designed for. If you know that you only will have 5V on VIN, then this voltage derating may be less of a factor.

If replacing with an Aluminum electrolytic, both the ESL the ESR would likely be higher. The resistance will cause more ripple on input voltage and the inductance would increasing ringing/overshoot when the high-side switch opens and closes. So I would probably recommend staying away from aluminum electrolytic if possible.

In summary, I'm not saying that it can't be done, but you really need to pay attention to the parasitics and derating if you are planning to change out the caps versus what we have put on the EVM/reference design.

Regards,
Steve