For the boost converter, it provides output current for both V_(MAIN) and V_(SUB) rails. Its maximum output capability is determined by the input to output ratio and the current limit of the boost converter and can be estimated by Equation 3.

Equation 3.

where

• η is the boost converter power efficiency estimation
• 50 mA is half of the inductor current ripple value

Minimum input voltage, maximum boost output voltage and minimum current limit I_{LIM_BST} should be used as the worst case condition for the estimation.

Internal current limit is also implemented for the integrated LDO/load switch. So the maximum output current of VSUB rail should be lower than I_{LIM_SUB}, which has 200 mA minimum value. For LDO version, the maximum output current is also limited by its input to output headroom, that is V_(MAIN) - V_(SUB). Make sure the headroom voltage is enough to support the load current. Please refer to Section 7.5 for the dropout voltage information.

In this example, assume the power efficiency is 80% (lower than typical value for the worst case estimation), so the calculated maximum output current of the boost converter is 50.9 mA, which satisfies the application requirements (15 mA + 10 mA). The load of VSUB rail is 10 mA, which is well below the V_(SUB) rail current limit and the dropout voltage is also within the headroom.