To keep the discharge current below battery OCP rating during hybrid boost mode or battery only boost mode, the BQ24810 device supports discharge current regulation. After device powers up, the REG0x37[15] is 0 to disable discharge current regulation. To enable discharge current regulation, host writes 1 to REG0x37[15]. REG0x37[15]=1 enables battery discharge current regulation during the hybrid power boost mode and battery only boost mode if the conditions to start either boost mode are valid.

Once the battery discharge current is limited, the input current goes up to meet the system current requirement. The user can assert PROCHOT to detect input current increase (ICRIT or INOM), and request CPU throttling to lower the system power.

To set the discharging current limit, write a 16-bit DischargeCurrent() command (REG0x39) using the data format listed in Table 6-17. When using a 10-mΩ sense resistor, the BQ24810 device provides a discharge current limit range of 512 mA to 32.256 A, with 512-mA resolution. Upon POR, default discharge current limit is 6.144 A on 10-mΩ current sensing resistor (R_SR).

To provide secondary protection during battery discharge, the BQ24810 has an ILIM pin with which the user can program the maximum discharge current. Typically, the user sets the limit below battery pack over current protection (OCP) threshold for maximum battery discharge capacity. Refer to battery specification for OCP information. Internal discharge current limit is the lower one between the voltage set by DischargeCurrent(), and the voltage on ILIM pin. To disable this function, the user can pull ILIM pin above 1.6V, which is the maximum discharge current regulation limit. Setting REG0x38[7] to 0 will also disable the ILIM pin discharge current limiting so that only the REG0x39 value is used. When ILIM is below 60mV, hybrid boost and battery only boost are disabled. The set discharge current limit can be derived from Equation 5.

Equation 5.

The SRP and SRN pins are used to sense R_SR with default value of 10 mΩ. However, resistors of other values can also be used. The actual current is scaled by the ratio of 10 mΩ and R_SR. For example, the discharge current setting code of 4096mA on 10 mΩ becomes 2048mA if sense resistor is 20 mΩ. For a larger sense resistor, a larger sense voltage is given, and a higher regulation accuracy; but, at the expense of higher conduction loss. If current sensing resistor value is too high, it may trigger an overcurrent protection threshold because the current ripple voltage is too high. In such a case, either a higher inductance value or a lower current sensing resistor value should be used to limit the current ripple voltage level. A current sensing resistor value no more than 20 mΩ is suggested.

When operating in battery only boost mode, the battery is the only power source in the system. Limiting the battery discharge current will cause system voltage to drop if the system load is greater than the amount that may be supplied by the battery with the discharge limit set. When boosting the battery to a higher voltage, the battery discharge current will be larger than the system current according to the ratio of system to battery voltage. It is common for the battery current to be double that of the system current. The discharge limit should be set to the largest current that the battery and switching components can support without damage and should be used only as a protection against high-current damage as might occur from a short circuit. Limiting current in battery boost mode will cause the system voltage to fall below the regulation set point.