Any time the boost converter is enabled, charge is transferred from the battery to the VBST capacitor. When the boost converter is disabled, the VBST capacitor charge supplies power to any circuitry connected to VBST. This energy transfer dissipates power because the boost capacitor cannot be charged or discharged with 100% efficiency. The TPS8802 boost converter typically operates between 65% and 85% efficiency when the load current is much higher than the boost converter active current. Using a lower inductor peak current limit BST_CLIM improves the efficiency with the tradeoff of decreased maximum output current. The majority of circuitry connected to VBST consumes constant current independent of the VBST voltage. Therefore, an increase in the VBST voltage directly increases the power consumption of the connected circuitry.

The extra current from boost charging is calculated using Equation 8, where:

• f_CHARGE is the boost charge frequency
• V_BAT is the battery voltage
• C_BST is the capacitance
• V_LOW is the voltage before charging and after discharging
• V_BST is the boost converter output voltage
• η_BST is the boost converter efficiency

Equation 8 is derived by taking the difference of the energy used to charge the VBST capacitor and the energy used from discharging the VBST capacitor. The boost charging current is in addition to all other currents.

Equation 8 is particularly useful in calculating power consumption with the TPS8802 VCCLOW_BST feature enabled. VCCLOW_BST automatically enables the boost converter when the VCCLOW monitor is enabled and a low VCC voltage is detected. In this scenario, the boost charge frequency depends on the capacitance, standby current, boost Schottky diode leakage current, boost converter voltage, and VCCLOW detection voltage. The standby current here is the current continuously drawn from the VBST capacitor. The frequency and current are calculated in Equation 9 and Equation 10. Equation 10 highlights the importance of using a low-leakage Schottky diode. The Schottky diode leakage effectively adds to the standby current. Using a Schottky diode with 1 μA leakage current reduces the power drawn when VCCLOW_BST is active. Low leakage Schottky diodes generally have a higher forward voltage and reduce the boost converter efficiency. Using a Schottky diode with less than 1 µA leakage current can reduce the total system battery life because of the higher forward voltage.