BQ25504: Inconsistent VSTOR voltage

Kerem Asaf Tecirlioglu

Part Number: BQ25504

Hello,

Here is the schematic of the circuit that I'm working on.

/cfs-file/__key/communityserver-discussions-components-files/196/bq25504_2D00_sch.pdf

I've tied VOC_SAMPLE pin to VSTOR (to disable the MPPT) via an external cable (turns out it shouldn't be floating). And from the excel file TI provided to calculate resistance values for BAT_UV and BAT_OV I believe i set BAT_UV=2.7V , BAT_OV=3.5V. Yet , even though VSTOR charged way beyond 1.8V threshold batter is not connected. Also VSTOR boosts upto 7V and even 10V (yes that fried my chip :'| ) is this normal ? Another question would be is it convenient to use kohm resistances rather than Mohm resistances to set parameters as mentioned in the datasheet ?

Note: I think I have misunderstood the mechanics of this chip. I thought as long as I set the BAT_OV to some desired value VSTOR would not go above that, guess I'm wrong.

over 7 years ago

0 Jeff F over 7 years ago

TI__Guru**** 168436 points

Kerem,

Regarding the resistor sizes, larger resistors consume less current and therefore don't waste power if input source is weak. If you source can provide the power and your application is not concerned with efficiency, you can use lower sized resistors.

Regarding VBAT_OV, you are correct in that the IC is supposed to stop boosting when VSTOR reaches the programmed VBAT_OV threshold. Based on your description, it appears that the boost converter is running open loop, i.e. the VBAT_OV resistors are not connected. Can replace the IC and put a load on VSTOR that prevents it from over-voltage? Then use an oscope to monitor the RDIV pin like d/s figure 19.

0 Kerem Asaf Tecirlioglu over 7 years ago in reply to Jeff F

Prodigy 70 points

Hello,

Load: 1kohm

VBAT=4.1V

VSTOR=4.7V

RDIV:

I double checked the resistors.

0 Jeff F over 7 years ago in reply to Kerem Asaf Tecirlioglu

TI__Guru**** 168436 points

Kerem,

The first pulse should be at the VSTOR voltage and the 2nd at 2/3 of the VBAT_OV voltage. I have never tested this part with such small resistors. It is possible that they are loading RDIV. Can you increase by a factor of 10 or slightly more and retry?

0 Kerem Asaf Tecirlioglu over 7 years ago in reply to Jeff F

Prodigy 70 points

Can I let VOC_SAMP float ? Would it interfere with the device's control loop ?

--Will try with bigger res ASAP

0 Jeff F over 7 years ago in reply to Kerem Asaf Tecirlioglu

TI__Guru**** 168436 points

Unfortunately no. It is very high impedance and will float to a voltage that may not work for your application. If you want to disable the MPPT function you can connect to VSTOR or set to a voltage close to your low impedance (I assume) power source that doesn't have a MPP.

0 Kerem Asaf Tecirlioglu over 7 years ago in reply to Jeff F

Prodigy 70 points

OK, I've tried various things. When VOC_SAMP is tied to VSTOR.
VBAT=4.1V
VBAT_OV =3,538V (with bigger resistors)

I did not get any pulse neither from RDIV nor VSTOR. When the boost is disabled does that mean VSTOR is disabled too ?
Yet IC was draining power but did not get hot.

0 Jeff F over 7 years ago in reply to Kerem Asaf Tecirlioglu

TI__Guru**** 168436 points

If VBAT_OV regulated VSTOR to 3.5V then RDIV had to pulse at some point. But, how can VSTOR be 3.5V and VBAT be 4.1V. There is a FET between the two thought should be on.

0 Kerem Asaf Tecirlioglu over 7 years ago in reply to Jeff F

Prodigy 70 points

Hello, I've managed to get meaningful results at last,

VBAT: 4.02V

VSTOR: 4.08V

And VRDIV Pulse:

VBAT_OV is set to be 3.538V

VBAT_OV seems to be correctly understood by chip. Yet I wonder why VSTOR goes above VBAT_OV. Is it caused by the battery's higher voltage than VBAT_OV ?

0 Jeff F over 7 years ago in reply to Kerem Asaf Tecirlioglu

TI__Guru**** 168436 points

Great! Yes, if the battery attached as VBAT is higher than the VBAT_OV and the internal PFET between VSTOR and VBAT is on then the voltage at VSTOR will be at the battery voltage.

Power management

Power management forum

BQ25504: Inconsistent VSTOR voltage