High Efficiency, Main Power Supply Controllers for Notebook Computers

The RT8205A/B/C dual step-down, switch-mode power-supply controller generates logic-supply voltages in battery-powered systems. The RT8205A/B/C includes two pulse-width modulation (PWM) controllers fixed at 5V/3.3V or adjustable from 2V to 5.5V. An alternative LGATE1 output LG1_CP can be used for external charge pump (RT8205B). And an optional external charge pump can be monitored through SECFB (RT8205C). This device also features 2 linear regulators providing fixed 5V and 3.3V outputs. The linear regulator each provides up to 70mA output current with automatic linear-regulator bootstrapping to the PWM outputs. The RT8205A/B/C includes on-board power-up sequencing, the power-good output, internal soft-start, and internal soft-discharge output that prevents negative voltages on shutdown.
A constant on-time PWM control scheme operates without sense resistor and provides 100ns response to load transients while maintaining a relatively constant switching frequency. The unique ultrasonic mode maintains the switching frequency above 25kHz, which eliminates noise in audio applications. Other features include diode-emulation mode (DEM), which maximizes efficiency in light-load applications, and fixed-frequency PWM mode, which reduces RF interference in sensitive application