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BQ25306: Clarification regarding Ichg current

Duane Storey
Prodigy 60 points

Part Number: BQ25306
Other Parts Discussed in Thread: BQ25308

Hi,

Can you clarify a few things for me please.

In the PIN descriptions of the datasheet, it says ICHG can be set using a resistor based on this: "Charge current program input. Connect a 1% resistor RICHG from this pin to ground to program the
charge current as ICHG = KICHG / RICHG (KICHG = 40,000)."

So let's say we choose a 100k resistor, that would imply a ICHG of 0.4A, or 400ma.  That checks out.

Later on it talks about using a MCU with a PWM to control that pin as well:

"(1V - VPWM) / (RICHG1 + RICHG2). VPWM is the averaged DC voltage of the PWM
output and it must be lower than 1 V. The regulated voltage at the ICHG pin is 1 V"

Let's say RICHG1 is 50k and RCHG2 is 50k, which matches our 100k from before.  Let's say we set a duty cycle of 0%, so VPWM is 0 (the pin is basically grounded). Shouldn't that be functionally equivalent to the first scenario, where you have a grounded MCU pin with 100k of resistance?  But using the formula we get: ICGH = (1V - 0V) / (100000Ohm) = 1ua. 

I feel like there is some disconnect between the two methods as regardless of the units, there is a factor of 4x missing somehow between the two.  Is the KICHG missing from the formula?  That is, should it read:

ICHG = KICHG * (1V - VPWM) / (RICHG1 + RICHG2) ?

Also, can this MCU PWM method be used with other variants in the same family, such as BQ25308?

Thanks

  • 0
    TI__Guru** 106795 points

    Hello,

    We are currently out of office for the Holidays, we will respond as soon as possible. Sorry for the delay.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Mastermind 34310 points

    Hi Duane,

    Thank you for your patience. Please see my comments below. 

    Duane Storey said:
    Also, can this MCU PWM method be used with other variants in the same family, such as BQ25308?

    Yes the MCU PWM method can be used on other BQ2530x variants. The functionality of ICHG pin is the same between these parts. 

    I can also confirm your analysis is correct that KICHG needs to be included in the equation for calculating charge current using the MCU PWM method to control ICHG. 

    Duane Storey said:
    ICHG = KICHG * (1V - VPWM) / (RICHG1 + RICHG2)

    I agree with this and apologies for the lack of inclusion in section 10.2.3 of the datasheet. I have made note and our team will work to correct this in the next datasheet revision. 

    Best Regards,

    Garrett