BQ24172: Battery current has no limit

1. What is the VREF voltage you measured? ISET and ACSET is analog input thus the voltage on the pin comes from VREF through the resistor divider.

From your schematic, you are not using sense resistor for the input, ACN and ACP is floating. Then ACSET will not make any effect.

2. "I have set CELL/FB at 10V" --> Do you mean you have set the battery regulation voltage at 10V? If so, then using a resistor load will not work. Please refer to figure 14 for a typical Li-ion charging profile. You will need to use a battery simulator. 

When battery is absent, battery detection is on.  The output waveform will look like figure 16.

3. STAT is blinking on resistve load is because of battery is absent.

4. All of your current sense feedbacks are floating. ACP, ACN, SRP, SRN, thus there will not be any current limit feature.

5. Not sure the reason for this. Need to look at scope on the input V/I and output V/I

Hello Jing

Thanks for replying.

1. Vref is 3.3V

2.CELL/FB at 10V : What i meant here was that for simulation sake , i had set the voltage as 10V and replaced he battery with a resistive load.You are right that it does not work.

3.STAT blinks on resistive load . This too seems right since battery is absent.

4. Current sense feedbacks do go to respective pins on the IC. Pl look closely , as I have named those nets near sense resistors and the pins on the IC as same.

5. Post all this i had installed an actual Li-Ion 3-cell pack at the output (CHG_OUT). Now I set the CELL/FB as 12.6V with R28 & R29

6. I gave 16Vdc as input to the board ,with 10A as capacity of the input source (which unfortunately doesn't have a settable current limit)

7. I expected to see a current limited to 4A since I have ISET & ACSET adjusted as 4A. On the contrary , there was no limit to the current and all the current went through to the battery. This was for a second or so before which i cut off the input source.

Please read my original post again to understand voltages at various pins on the IC.

Kindly advise further

There is a 0.1uf cap from SRP to gnd.

I think you missed that.

And I sincerely request you to take a deeper look into the schematic.

What happens if the BATFET turns on?

I believe it should discharge the battery into system in case when system demands more current than input current.

Will the system flow into battery?

Pl comment

BATFET will turnon for the battery to supplement the system load when input source is overloaded. 

This control logic is done internally. 

If BATFET is falsely turned externally, current will float from system to battery.

When I have limited input current to 4A, shouldn't the input FET turn off through ACP and ACN sense, if current is more than 4A?

Which means that even if BATFET turns on there's no current flow into the battery.

Or am I wrong ?

Pl advise

Hi 

I removed R26 & R75. Now BATFET doesn't turn on and there is no unlimited current flow in the battery. So you were right that the BATFET is getting turned on.

Now there are new set of problems.

All below conditions are met

• ISET pin above 120 mV.
• Device is not in UVLO mode (that is, VAVCC > VUVLO).
• Device is not in SLEEP mode (that is, VAVCC > VSRN).
• OVPSET voltage is from 0.5 V to 1.6 V to qualify the adapter.
• 1.5-s delay is complete after initial power up.
• REGN LDO and VREF LDO voltages are at correct levels.
• Thermal Shutdown (TSHUT) is not valid.
• TS fault is not detected.
• ACFET turns on 

But there is no Charge current flow in the Battery now!

STAT is blinking.

No switching is observed at SW pins.

What is wrong here?

Awaiting your response

Hi 

There was a problem in my design which probably you missed.

At the TS pin i had setup a voltage of 1V but it was a mistake since it will require a voltage between 1.53V to 2.39V for healthy temperature.

I corrected this by adjusting 1.75V at the TS pin to simulate a healthy temperature condition.

I connected the battery at the output of the board.

I gave input of 15Vdc current limited at 1A from a DC power supply to the input of the BQ24172. (Pl Check my schematic)

BUT THE BQ24172 FRIED AS SOON AS I CONNECTED 15Vdc TO IT!!!!!!!!!!!!!!!!!!!!!!!!

I am unable to understand why this has happened.

Kindly guide

Hello Abhijit,

Can you please let us know which pins are damaged? Does this happen on one board or multiple boards?

We first need to locate the damaged area for further debugging. 

Please use the BQ24172EVM schematic for reference. I have looked into your schematic, besides the BATFET connection and TS connection, for ACVV, we suggest to only add a 10ohm resistor for the AVCC rising slew rate control. 

Abhijit,

We typically need to run an FA on the IC to allocate where the first failure comes from on the IC. 

There are several possibilities.

1. As previously all the 10A current passes through from input to the battery. When it is turned off, due to the parasitic inductance on the path, it is highly likely the voltage of the can spike up to exceed the voltage rating for AVCC and PVCC.

2. Can you please check if the 10ohm resistor connect in series with AVCC is damaged or not?

3.  Have you tried on a fresh board that has not been tested before?

Hi Jing

I got 4 more boards assembled.

There was a severe bug on our boards which somehow got overlooked. In one of the layers, the output inductor L1 from SW pin was acting as a direct short link to the shunt resistor R24. We removed this short circuit physically by cutting the track and then set current as 0.78A , and 0.78A started flowing!!

Although on the same board , when we tried to increase the ISET current by adjusting Viset voltage by increasing R17, the ISET pin somehow got damaged. It now presents a 0v and shows a resistance of 124R. Naturally there is no current flow. I am trying to understand why this pin got damaged, since we did not do nothing more than change a resistor there! Is i so sensitive to ESD? if we assume that ESD damaged it?

On another board , we again assembled and set the current to 1.5A and FB voltage of 12.6V . Current flows, tapers when voltage nears 12.5V , never reaches 12.6 since Iref of 10% of Iset reaches sooner and the current is cut off.

So it behaves perfectly and as expected!

Please comment on the cause of the damage , after which i will mark this thread as resolved!!

Thanks for the support