BQ24075 SYSOFF does not disconnect Batt from Out pins

Stephan Bitterli

Prodigy 70 points

Hi support

in my design the Vout (pins 10&11) cannot be disconnected from the battery.

Conditions:

V_sysoff = 3.57V (logic level active is >1.4V)

Vin - open, 0V

En1, En2, nCE = 0V (GND)

Vbat: = 3.76V

--> Vout still drives 3.35V / 15mA

--> Vout - Vbat = 410mV

It seems as if the internal FET Q2 would have a conducting intrisic body diode... this diode is also shown on the datasheet...

Any help??

Stephan

over 8 years ago

0 David Wiest Jr over 8 years ago

TI__Genius 11480 points

HI Stephan,

It does seem as if there are some contradictions in the datasheet - Q2 would have a body diode connecting system to battery. However, the application curve shows that system does crash when SYSOFF is asserted and no input is present, leading me to believe that there is an error in the direction of Q2.

I will try to verify this behavior on our EVM. In the mean time, what kind of system load do you have? Can you send me a waveform trying to replicate figure 40 in the datasheet? If you cannot measure IBAT, that is okay. VIN, VOUT, SYSOFF, and VBAT will be fine.

0 David Wiest Jr over 8 years ago in reply to David Wiest Jr

TI__Genius 11480 points

Stephan,

After using our EVM on the bench, I do see that the battery seems to show up through the body diode. However, this is only in a no-load condition for the output. As soon as a attach an eload or sourcemeter, the system output quickly drops to 0 V.

Using pure resistive loads at the output, the output will drop once a ~250k ohm or smaller load is attached. Otherwise, the system will maintain at roughly a diode drop from battery.

0 Stephan Bitterli over 8 years ago in reply to David Wiest Jr

Prodigy 70 points

Hi David, thanks for your replay.

Attached/below please finde a capture like figure 40.

regarding the load: we supply a micro controller and some analogue parts. the battery is still 15mA sourcing with sysoff = high (3.5V)

Regards,

Stephan

0 David Wiest Jr over 8 years ago in reply to Stephan Bitterli

TI__Genius 11480 points

Hey Stephan,

Thanks for gathering that info! I have one more question - how much capacitance do you have on the system and battery nodes?

If you don't mind sharing your schematic, I can try to make our EVM setup as close to yours as possible and re-test it. If you don't feel comfortable sharing here, you can email it to d-wiest@ti,com for me to review.