BQ25798: BQ25798 PMID PATH

Hi Justin,

Agxin Justin xavier said:

During 15V charger plug in I am able to use the PMID pin voltage to charge the E-CAP 

From my understanding of this application, there is a 2S2P battery connected to BAT, and an E-CAP connected to PMID? The converter will not be able to regulate current from the charger plugging in to the PMID pin. If there is too much inrush current, this could damage the device.

Agxin Justin xavier said:

When charger is removed can i boost the battery voltage to 15v  and get this in PMID pin ? what is the limitation of the current when USB charger is plugged and during this backup boost mode how to configure and use this mode please help 

The charger can boost the battery voltage to 15V on the PMID pin. I do not think the PMID load current will be regulated, so the current limitation will be on the safe discharge limit for the battery, in your case 5A. Since the voltage is boosting, the current limitations on the PMID pin will be lower than 5A.

Best regards,
Michael

Hi Michael ,

Thanks for the response 

The converter will not be able to regulate current from the charger plugging in to the PMID pin. If there is too much inrush current, this could damage the device. 

For direct path from charger plugged in USB to PMID need to limit the current at the PMID pin externally 

But for the VBAT to PMID boost the current is limited to battery discharge limit circuitry got this clarified 

Now let say I have VSYS powering the system and Vbat supplied by battery PMID used for charging a cap using a charger BQ25731(which i think can limit input current)

The PMID power for Vsb path is always present but when VBUS is not present the charger can cut VBUS PATH by disabling the external VAC  input mosfet and can boost the voltage to 15v ,will this transaction from vbus SOURCE  path to vbat SOURCE BOOSTED path is seamless or will have delay my vbus can vary from 15v 3a to 5v 3a.what is the procedure from the host to do this ?or this process can be automatically from charger ?

Agxin Justin xavier said:

For direct path from charger plugged in USB to PMID need to limit the current at the PMID pin externally 

This is correct if there is a load on the PMID pin.

Agxin Justin xavier said:

will this transaction from vbus SOURCE  path to vbat SOURCE BOOSTED path is seamless or will have delay my vbus can vary from 15v 3a to 5v 3a

This can be seamless if you set EN_BACKUP=1 and add a little more capacitance to the pin. Take a look at section 9.3.7.2 Backup Power Supply Mode in the datasheet for more information.

Best regards,
Michael

Regarding the discharge current limitation of 5A need some clarity 

I have a battery 8.4V which is 1C discharge capable which is 10.5A capable now during this mode I have two power paths when Vbus is absent from VBAT 

one is Vsys path from Vbat path and other one is PMID path the discharge limitation applies only to PMID path is that correct ? for direct path there is no limitation .When the charger is unplugged what is the max Boost voltage i can get in PMID pin ? 

Can I get 22V 2A constant current at the PMID pin when its in this mode ? + VSYS OUT 1.5A 7.4 or what ever battery voltage at that point in vsys pin at the same time 

Regarding the discharge current limitation of 5A need some clarity 

I have a battery 8.4V which is 1C discharge capable which is 10.5A capable now during this mode I have two power paths when Vbus is absent from VBAT 

one is Vsys path from Vbat path and other one is PMID path the discharge limitation applies only to PMID path is that correct ? for direct path there is no limitation .When the charger is unplugged what is the max Boost voltage i can get in PMID pin ? 

Can I get 22V 2A constant current at the PMID pin when its in this mode ? + VSYS OUT 1.5A 7.4 or what ever battery voltage at that point in vsys pin at the same time 

Hi Justin,

The device will sense the battery discharge current. This will be both the direct discharge to SYS and the discharge to PMID. If the SYS is pulling too much current, the device will stop powering to PMID.

PMID will likely be able to do 2A current at 22V, but the device will be in boost mode which means the battery current will be higher than 2A (in addition to the SYS current).

Best regards,
Michael

Thanks for the response .

If i use 2s3p vs  3s2p 3s will have higher voltage and lower discharge current .. this current limitation that you mentioned does it have the limitation same across different cell voltage configuration ? 

Hi Justin,

Actually, in the backup mode, there is a bit to disable the current limit on the battery discharge:

Best regards,
Michael Bradbourne

But its mentioned that this is in OTG mode ,I am using in Backup mode ?

I am not sourcing to OTG (external USB port ) I am consuming through PMID is this still applicable .Also if let say its applicable for PMID as well as a part of boost converter then is this a constant current limiting mechanism or a fold back mechanism in case of load trying to sink more than programed limit