Resistor connect to SRP pin of BQ24703

Hello,

The selection of the SRP resistor (R19) is discussed on Pg. 16 of the datasheet where it states that the function that this resistor performs is to limit the charging current of the device and fhelp filter switching noise in the case of SRP it dispates power in zero voltage operation mode which is why R19 is rated for 1W and R21 is rated for 1/10W.

"The battery charge current is limited by the filter resistor connected to SRP pin (R19). R19 must be dimensioned to withstand the worst case power dissipation when in zero volt operation mode. The zero volt operation mode is disabled when BATDEP is above the zero volt operation threshold, and the main PWM loop is turned on if charge is enabled, regulating the current to the value set by SRSET voltage. To avoid errors on the charge current both resistors on the SRP, SRN filter must have the same value. Note, however, that R21 (connected to SRN) does not dissipate any power when in zero volt operation and can be of minimum size." Pg. 16 BQ24703

Note: the resistance value of R19 and R21 must be the same value to mantain precision.

The key is to make sure that SRP/SRN filter that consists of R19, R21, C8 are sized to serve as a filter to attenuate the PWM switching noise. You then size the power of this resistor for the case of the power it will have to dissipate. Resistor R19 will be have one end with a potential of the Vout and the other end has the pontential to be connected to the input voltage so the power calaculation must take this potential conduction path into account. Pg 11 of the datasheet connected from SRP through the zero-volt chargine FET or its body diode to the VCC pin.

Ryan May

Hello

Thanks for your answer

- In my point,we can't increase value of R19 resistor to reduce power dissipation because cut-off frequency of the SRP/SRN filter (R19, R21, C8) should set higher than 1khz to avoid adding secondary poles to the PWM closed loop system.page 21 of the datasheet.

- I want to disable charging battery if  charge current is less than 440mA. How to do it?

Hello,

Are you wanting to disable the charging when it drops to this current level because of a coupled load that needs that power to operate? If that is the case I would recommend looking into programming the DPM (Dynamic Power Management) inherent in the device. This should ensure that your system will recieve the power it needs and charges the battery with what is left over.

This feature is the most straight forward method I could find in the datasheet for actively montioring and limiting the battery charging current at a user defined level.

Let me know if you have any questions.

Ryan May

Hi Ryan

Thanks for your support

We can monitor charging current directly by IBAT pin.But I don't like to use this way because ADC of MCU  has tolerant or errors.So,I want to use hardware to control battery charging (When is battery full-charging? and how to disable battery charging?And don't need to monitor IBAT pin)

Hello,

What sort of accuracy are you needed that you are concerned about utilizing the micro that you are using in your system?

Probably one of the safest ways to incorporate charging enable and disable control would be to incorporate a gas gauge to monitor the battery. This would also and add some general protection such as temperature monitoring in the cell and can be selected to help with voltage balancing between the cells.

Ryan May