9.2.3.5 Converter Power-Up

After the input current limit is set, the converter is enabled and the HSFET and LSFET start switching. If battery charging is disabled, BATFET turns off. Otherwise, BATFET stays on to charge the battery.

The device provides soft-start when system rail is ramped up. When the system rail is below 2.2 V, the input current limit is forced to the lower of 200 mA or IINLIM register setting. After the system rises above 2.2 V, the device limits input current to the lower value of ILIM pin and IILIM register (ICO_EN = 0) or IDPM_LIM register (ICO_EN = 1).

As a battery charger, the device deploys a highly efficient 1.5 MHz step-down switching regulator. The fixed frequency oscillator keeps tight control of the switching frequency under all conditions of input voltage, battery voltage, charge current and temperature, simplifying output filter design.

A type III compensation network allows using ceramic capacitors at the output of the converter. An internal saw-tooth ramp is compared to the internal error control signal to vary the duty cycle of the converter. The ramp height is proportional to the PMID voltage to cancel out any loop gain variation due to a change in input voltage.

In order to improve light-load efficiency, the device switches to PFM control at light load when battery is below minimum system voltage setting or charging is disabled. During the PFM operation, the switching duty cycle is set by the ratio of SYS and VBUS.