RT8494

5

DS8494-01 November 2015

www.richtek.com

©

Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Operation

The startup voltage of the RT8494 is around 4.5V. When

VCC voltage is greater than 4.5V, the RT8494 starts

operation and a regulated GBIAS supply voltage is

generated by an internal LDO circuit. With VCC greater

than 10V, the GBIAS supply will be regulated around 8.5V

to supply the power for the internal GATE pin driver circuit.

As the system starts, the capacitor at the soft-start pin is

slowly charged by an internal current source around 6

μA.

During soft-start period, the VC pin voltage follows the

soft-start pin voltage up by one VBE and gradually ramps

up. The slowly rising VC pin voltage allows the PWM duty

to increase gradually to achieve soft-start function.

In normal operation, the GATE turns high when the

oscillator (OSC) turns high. The ISW pin voltage is the

triangular feedback signal of the sensed switch current

(which equals inductor current ramp).

The PWM comparator compares the ISW pin voltage to

the VC pin voltage. When the ISW pin voltage exceeds

the VC pin voltage, the PWM comparator resets the latch

and turns off GATE. If the ISW pin voltage does not exceed

the VC pin voltage by the end of the switching cycle, the

GATE will be turned off by the OSC circuit for a minimum

off time. The cycle repeats when the GATE is turned on

at the beginning of the next OSC cycle.

The RT8494 features high voltage LED driver control. The

common mode operation voltage of the ISP and ISN pins

−

charged higher by the internal OP AMP in the PWM

control loop and vice versa. By the PWM closed loop

The actual LED output current can be adjusted by the

sense resistor between the ISP and ISN pins.

The dimming can be done by varying the ACTL/DCTL pin

voltage signal. The internal sense threshold reference for

achieve dimming control.

The fault protection features of the RT8494 include (1)

VCC Under-Voltage Lockout (UVLO) (2) VOUT Over -

Voltage Protection (OVP) (3) switch Over-Current

Protection (OCP) (4) Over-Temperature Protection (OTP).