RT8494

11

DS8494-01 November 2015

www.richtek.com

©

Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Applications Information

The RT8494 is a current mode PWM controller designed

to drive an external MOSFET for high current LED

applications. The LED current can be programmed by an

external resistor. The input voltage range of the RT8494

can be up to 36V and the output voltage can be up to 90V.

The RT8494 provides analog and PWM dimming to achieve

LED current control.



SS

SS

2.4V

tC

6μA

Where,

When the voltage of ACTL is higher than 1.2V, the LED

current is regulated to :



LED(MAX)

SENSE

190mV

I

R

GBIAS Regulator and Bypass Capacitor

The GBIAS pin requires a capacitor for stable operation

and to store the charge for the large GATE switching

currents. Choose a 25V rated low ESR, X7R or X5R

ceramic capacitor for best performance. The value of a

1

μF capacitor will be adequate for many applications.

Place the capacitor close to the IC to minimize the trace

length to the GBIAS pin and also to the IC ground. An

internal current limit on the GBIAS output protects the

RT8494 from excessive on-chip power dissipation.

The GBIAS pin has its own under voltage disable (UVLO)

set to 4.3V(typical) to protect the external FETs from

excessive power dissipation caused by not being fully

enhanced. If the input voltage, VIN, will not exceed 8V,

then the GBIAS pin should be connected to the input

supply. Be aware if GBIAS supply is used to drive extra

circuits besides RT8494, typically the extra GBIAS load

should be limited to less than 10mA.

Loop Compensation

The RT8494 uses an internal error amplifier whose

compensation pin (VC) allowing the loop response

optimized for specific application. The external inductor,

output capacitor and the compensation resistor and

capacitor determine the loop stability. The inductor and

output capacitor are chosen based on performance, size

and cost. The compensation resistor and capacitor at VC

are selected to optimize control loop response and

stability. For typical LED applications, a 3.3nF

compensation capacitor at VC is adequate, and a series

resistor should always be used to increase the slew rate

on the VC pin to maintain tighter regulation of LED current

during fast transients on the input supply to the converter

an external resistor in series with a capacitor is connected

soft-start capacitor is discharged when EN/UVLO falls

below its threshold, during an over temperature event or

during an GBIAS under voltage event.

LED Current Setting

The LED current is programmed by placing an appropriate

value current sense resistor between the ISP and ISN pins.

Typically, sensing of the current should be done at the

top of the LED string. The ACTL pin should be tied to a

voltage higher than 1.2V to get the full-scale 190mV

(typical) threshold across the sense resistor. The ACTL

pin can also be used to dim the LED current to zero,

although relative accuracy decreases with the decreasing

voltage sense threshold. When the ACTL pin voltage is

less than 1.2V, the LED current is :





ACTL

LED

SENSE

(V

0.2) 0.19

I

R

from the VC pin to GND to provide a pole and a zero for

proper loop compensation. The typical compensation for

the RT8494 is 10k

Ω and 3.3nF.

Soft-Start

The soft-start of the RT8494 can be achieved by connecting

a capacitor from SS pin to GND. The built-in soft-start

circuit reduces the start-up current spike and output

voltage overshoot. The soft-start time is determined by

the external capacitor charged by an internal 6

μAconstant

charging current. The SS pin directly limits the rate of

voltage rise on the VC pin, which in turn limits the peak

switch current.

The soft-start interval is set by the soft-start capacitor

selection according to the equation :