BQ25798: BQ25789 Questions

Hello Jeff, 
 

Subject: OTG Mode and Power Source Switching Issues with BQ25798

We are encountering an issue related to power source switching and OTG mode behavior with the BQ25798 charger IC. Our hardware configuration includes:

• VAC2 (USB-C input) connected directly to the USB-C port.
  
  
• VAC1 (Solar input) connected via a 5V buck converter output.
  
  
• A Li-ion battery connected to the battery terminals.
  
  

We have configured the charger control register to enable both ACDRV1 and ACDRV2 when OTG mode is required. However, different input scenarios are yielding inconsistent behavior. Below are the detailed cases and the observed issues:

Case 1: USB-C (VAC2) Connected Initially

• Setup: USB-C connected to VAC2, and battery connected.
  
  
• Initialization Sequence:
  
  

pData[0] = 0x40; // Enable OTG

BQ25798_Write(BQ25798_CHARGER_CONTROL_3, pData, 1);

pData[0] = BQ25798_CHARGER_CONTROL_4_EN_ACDRV2;

BQ25798_Write(BQ25798_CHARGER_CONTROL_4, pData, 1);

• Issue: The system operates normally while VAC2 is connected. Once VAC2 is removed and VAC1 is provided instead, enabling OTG causes the voltage at the PMID pin to drop to 0V. The battery also stops charging.
  
  

Case 2: Solar Input (VAC1) Connected Initially

• Setup: Solar buck output connected to VAC1, and battery connected.
  
  
• Initialization Sequence:
  
  

pData[0] = 0x40; // Enable OTG

BQ25798_Write(BQ25798_CHARGER_CONTROL_3, pData, 1);

pData[0] = BQ25798_CHARGER_CONTROL_4_EN_ACDRV1;

BQ25798_Write(BQ25798_CHARGER_CONTROL_4, pData, 1);

• Issue: The system functions correctly with VAC1. However, if VAC1 is removed and VAC2 (USB-C) is connected instead, enabling OTG again results in PMID dropping to 0V, and the battery stops charging.
  
  

Case 3: Both VAC1 (Solar) and VAC2 (USB-C) Connected

• Setup: Both solar and USB-C power sources are connected along with the battery.
  
  
• Initialization Sequence:
  
  

pData[0] = 0x40; // Enable OTG

BQ25798_Write(BQ25798_CHARGER_CONTROL_3, pData, 1);

pData[0] = BQ25798_CHARGER_CONTROL_4_EN_ACDRV1 | BQ25798_CHARGER_CONTROL_4_EN_ACDRV2;

BQ25798_Write(BQ25798_CHARGER_CONTROL_4, pData, 1);

• Issue: When both power sources are connected, the device works fine. However, when both VAC1 and VAC2 are disconnected and only the battery remains, OTG mode fails to provide 5V on the PMID pin. The PMID remains at 0V. If only one of ACDRV1 or ACDRV2 was previously enabled (not both), the OTG function correctly boosts the battery voltage to 5V at PMID.
  
  

Request

We would appreciate guidance on:

1. Why enabling both ACDRV1 and ACDRV2 causes PMID to drop to 0V when only the battery is present.
   
   
2. The correct way to configure ACDRV1/2 and OTG mode to ensure reliable behavior across different power source transitions.
   
   

Thank you for your support.

Hi Jeff,

Thank you for your response and support.

We would appreciate your guidance on the following:

1. How can we manage the OTG output so that it turns on from either ACDRV1 or ACDRV2 when the battery is connected? Our requirement is to allow users to connect either the USB-C input (on VAC2) or the solar input (on VAC1) — but only one at a time — to generate the 5V output on PMID and also charge the battery.

We want to ensure smooth switching between these two sources on an alternate basis while maintaining reliable 5V PMID output and charging.

Thank you again for your continued support.

Regards,
Pratham

Hi Jeff,

Thank you for your response.

I believe there may be some misunderstanding. I would like to clarify our requirement:

We have two input sources — USB-C connected to VAC2 and Solar connected to VAC1.
Our goal is simple:

• When the user connects USB-C, the charger should charge the battery from VAC2.

• If USB-C is not connected but the user connects the solar input at VAC1, the charger should charge the battery from VAC1.

So, we want the battery to be charged using either input source (one at a time) — whichever is connected. 

Could you please guide us on the best way to configure the charger to achieve this behaviour?

Thank you again for your continued support.

Regards,
Pratham

Hi Pratham,

As long as only one source is connected at a time, I see no issue.

This problem I see is the requirement for 5V at PMID.  When USB-C is connected at VAC2, V(PMID)=5V.  When solar panel connected at VAC1, V(PMID) = VOC*MPP%.  When no source is connected at VACx, using backup mode (fast turn on OTG), V(PMID) can be regulated to 5V.   I do not think this is what you want.

Regards,

Jeff

Hello Jeff,

We’re using the BQ25798 charger IC in a system with the following setup:

• VAC1: 5V from a Solar Buck Converter

• VAC2: USB-C input (5V only)

• Battery: 1-cell Li-ion (3.7V-4.2V)

• OTG mode: Used to boost battery voltage to 5V at PMID

We are facing issues related to power source switching and OTG behavior based on ACDRV configuration. Here’s what we’re observing:

 Case 1: Configured for VAC2 (ACDRV2 enabled)

• VAC2 present → System works perfectly: battery charges, PMID has 5V, OTG works.

• VAC2 unplugged, VAC1 applied → System does not work: PMID stays low, battery doesn’t charge.

 Case 2: Configured for VAC1 (ACDRV1 enabled)

• VAC1 present → Works fine.

• VAC1 unplugged, VAC2 applied → Doesn’t work: same issue as above.

 Case 3: Configured for both VAC1 and VAC2 (both ACDRVs enabled)

• Any one or both inputs connected → System works fine.

• When both VAC1 and VAC2 are unplugged (battery-only) → OTG mode fails:

  • PMID = 0V

  • Battery is present, but the boost function does not activate.

 Summary of the Problem:

• The PMIC works perfectly when only one ACDRV (VAC1 or VAC2) is enabled and the corresponding input is used.

• If we configure both ACDRVs, it supports both inputs — but in battery-only mode, OTG mode fails to generate 5V on PMID.

• It seems the PMIC may be attempting to boost through both input paths, which are disconnected, and it fails to deliver PMID output.

 What We Need Help With:

• Why does enabling both ACDRVs prevent OTG mode from working when no input source is present?

• Is there a recommended way to handle dynamic input switching so that the charger can:

  • Accept either VAC1 or VAC2 when connected,

  • And still properly support battery-only OTG operation?

We appreciate any suggestions or application notes on how to manage ACDRV control during such transitions.

Hello Jeff,

Thank you again for your continued support.

You mentioned:

Jeff F said:

This problem I see is the requirement for 5V at PMID.  When USB-C is connected at VAC2, V(PMID)=5V.  When solar panel connected at VAC1, V(PMID) = VOC*MPP%.  When no source is connected at VACx, using backup mode (fast turn on OTG), V(PMID) can be regulated to 5V.   I do not think this is what you want.

Actually, this is exactly what we want to implement.

Let me clarify our use case:

• We have two power sources: USB-C on VAC2 and a solar panel on VAC1.

• Only one source will be connected at a time, but occasionally both may be connected.

• Our goal is for the system to:

  1. Charge the battery from whichever input is available (VAC1 or VAC2).

  2. Provide 5V on PMID during charging from either source.

  3. Automatically switch to backup (OTG) mode, providing 5V on PMID from the battery when both inputs are disconnected.

We’ve observed that enabling both ACDRVs allows us to operate on either input individually and even when both are present. However, once both inputs are unplugged, the IC does not enter backup mode to maintain 5V on PMID using the battery — which is what we are trying to achieve.

Could you please guide us on how to correctly configure the BQ25798 to enter backup mode and provide 5V from the battery when VAC1 and VAC2 are not present?

Your guidance on this final piece would be greatly appreciated.

Best regards,
Pratham

Hello Jeff,

Thank you again for your detailed response — it has been very helpful.

I still have a few clarifications regarding our use case and implementation goals:

Note: For VAC1, we’ve connected a solar panel to a 5V buck converter, and the buck output is supplied to VAC1 at 5V. In this setup, we believe MPPT functionality may not be necessary.

1. How should the device operate when both inputs are connected — 5V solar (via buck) at VAC1 and 5V USB-C at VAC2?
   Based on the datasheet, VAC1 has priority over VAC2. We’d like to confirm that battery charging and system power at PMID remain stable in this scenario. Is any special configuration required to ensure reliable and predictable behavior?

2. How should the device behave when both VAC1 and VAC2 inputs are disconnected, but the battery is present?
   In this case, we would like the PMID rail to be powered from the battery — using backup or OTG mode — to maintain 5V output. Could you please confirm the recommended register settings or sequence to enable this behavior?

3. How should the device operate when only one input is connected (either USB-C at VAC2 or solar@5V at VAC1)?
   We would like the device to charge the battery and provide system power seamlessly from whichever source is connected, without requiring manual switching.

Additional Note:
We are triggering the 5V output on PMID during the device's operation — turning it ON when needed and OFF when not required. Kindly confirm if there are any additional precautions we need to take in this case.

We appreciate your continued support and look forward to your suggestions on implementing this reliably.

Best regards,
Pratham

Hi Pratham,

Regarding 1, unless the buck converter has its own MPPT, it will likely crash the panel or the charger will crash the buck which will crash the panel at some point.  You can try setting the chargers VINDPM slightly under buck converter output voltage and its IINDPM low  to prevent the crash. 

Regarding 2, To keep a voltage on PMID when using VAC1, I recommend EN_BACKUP =1 for back up mode.  Backup mode must be reset after charging restarts. But as I mention above, there will be a delay when backing up from VAC2.

Regarding 3, the charger auto switches to the source that is available but there is a 250ms start up delay each switchover.

If in forward/charge mode, PMID cannot be turned off.  If in OTG/reverse, the host can turn off OTG with EN_OTG bit. 

Regards,

Jeff

Hi Jeff,

Thank you again for your continued support.

I have one question regarding VACx threshold settings.

I would like to configure a voltage threshold for the VACx inputs so that I can monitor their status. Specifically, if the VACx input voltage drops below this threshold, I want to automatically enable OTG mode to supply 5V on the PMID pin from the battery.

I reviewed the datasheet but couldn’t find any specific register or configuration for setting a custom VACx threshold. Could you please advise if there is a way to define or monitor such a threshold, or how best to implement this type of condition?

Looking forward to your guidance.

Best regards,
Pratham