BQ76925: Zero Current Output Range

Hi Keith,

The voltage on Viout is always positive and for zero current is set to ¾ of the output range.  There is no max or min value for the zero current Viout, as the only min/max spec listed is limited to the Viout output range for which there are min and max values listed. 

The Viout zero current voltage exists to allow the MCU to determine whether the current across the sense resistor is charging or discharging because the Viout voltage is always positive, so positive/ negative voltage drops across the sense resistor that indicates current direction will not be kept as positive/negative voltages when they are outputted on the Viout pin, thus "zero current" cannot be denoted as 0V. 

For REF_SEL = 1, the Viout range is from 0.5 - 2.5V.  Viout = 0.5 - 2V indicates a discharge current, and Viout = 2 - 2.5V indicates a charge current.  As you can see, the resolution for discharge current calculation will be higher because discharge current can be calculated from a voltage range of 1.5V, whereas charge current will be calculated from a range of 0.5V.  Essentially, Rd = 3Rc, and this ratio appears to be fixed and non-configurable.  To reiterate, the Viout zero current seems to be fixed at ~0.75* Viout output range, and it does not appear to be configurable from the datasheet documentation. 

Best,

Andria

Hi Keith,

In response to your questions:

• From looking at the part datasheet and design spec, only typical values are provided in the documentation, so I cannot directly provide a range of zero current Viout voltages.
• I did not see any mention of tolerances in the 0.75 ratio.  I would assume this value is non-varying.
• For the current amplifier accuracy, I believe that the +/-1% tolerance would affect the Viout output range as you stated above.

Just a quick aside--The wording is a bit odd.  Did the customer observe a Viout zero current value of ~200-300mV over the typical Viout zero current, or were they expecting that value, and the actual zero current Viout was much closer to 2V? Can you send a schematic of the sense resistor connections so I can ensure that everything looks fine on that end? 

• Viout zero current is determined by the Viout gain amplifier, which is offset to either 1 or 2V.  Your customer has selected 2V, and that is the extent of the Viout zero current customizability. If there is an offset error, then Viout zero current could be abnormally high or low.
• Calibration is not available to the customer to adjust the offset.  Please note that in a zero curernt situation, the Viout voltage will be cancelled out when calculating Vsense (as shown in the equation below), so it is okay if Viout offset is not exactly at 2V.

Finally, I saw a previous E2E that discussed excess noise on Viout.  You may want to take a look: (+) BQ76925: BQ76925 inaccurate and noisy current measurement - Power management forum - Power management - TI E2E support forums.

Best,

Andria