RT9278

Preliminary

6

DS9278-04 August 2007

www.richtek.com

Operation

RT9278 integrates a high-efficiency step-up DC-DC converter and a linear regulator controller. The boost converter is

based on a fixed frequency, pulse-width-modulation (PWM) controller using a synchronous rectifier to obtain maximum

efficiency. Current mode control with external compensation network makes it easy to stabilize the system and keep

maximum flexibility. The linear regulator controller can use to drive the external P-Channel MOSFET switch for load

disconnection. It keeps the output voltage regulated even when the input voltage exceeds the nominal output voltage,

and keeps the output voltage completely disconnected from input voltage (battery) when the chip is in shutdown mode

Soft-start

When the chip is enabled. Soft-start is achieved by ramping up the PWM duty from very small to normal operation. The

ramping up PWM duty is achieved by sourcing 1uA from error amplifier to the compensation capacitor. When the output

voltage is regulated, the PWM duty enters the normal operation, and the error amplifier can sink and source up to 22uA.

The soft-start time is set by the following formula :

Current limit

The current of NMOS is sensed cycle by cycle to prevent over current. When over current limit, then the NMOS is off.

This state is latched and then reset automatically at next clock cycle.

Over voltage

When the chip voltage is higher than 6.5V, Switch is off. When the Over Voltage Protection is relieved, the chip operates

well again.

Thermal protection

Thermal protection function is integrated in the chip. When the chip temperature is higher than 180

°C, the controllers

are shutdown. 20

°C is the hysteresis range of temperature to prevent unstable operation when the thermal protection

happens. When the thermal protection is relieved, the chip operates well again.

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