RT8471 datasheet(9/15 Pages) RICHTEK

9 / 15 page

DS8471-02 December 2013

www.richtek.com

RT8471

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8471 is a simple high efficiency, continuous mode

inductive step-down converter. The device operates with

an input voltage range from 7V to 36V and delivers up to

1.2A of output current. A high side current sense resistor

sets the output current. A dedicated PWM dimming input

enables pulsed LED dimming over a wide range of

brightness levels. A high side current sensing scheme

and an onboard current setting circuitry minimize the

number of external components. A 1% sense resistor

performs a ±5% LED current accuracy for the best

performance.

Under Voltage Lockout (UVLO)

The RT8471 includes a UVLO feature with 400mV

hysteresis. The internal MOSFET turns off when VIN falls

below 4.8V (typ.).

Setting Average Output Current

The RT8471 output current which flows through the LEDs

is set by an external resistor (RS) connected between the

VIN and SENSE terminal. The relationship between output

current (IOUT) and RS is shown as below :

ADJ

OUTavg

0.4

0.1V

R0.8











OUTavg

0.1V

(A)

Analog Dimming Control

The ADJ terminal can be driven by an external voltage

(VADJ) to adjust the average output current. The average

output current is given by :

where VADJ is ranged from 0.4V to 1.2V. When VADJ is

larger than 1.2V, the output current value will only depend

on the external resistor (RS).

Digital Dimming Control

A Pulse Width Modulated (PWM) signal can drive the ADJ

terminal directly. Notice that the PWM signal logic high

level must be above 1.4V and the logic low level must be

below 0.2V at the ADJ terminal. It's recommended to

maintain the PWM dimming at low frequency (ex. 500Hz

) in order to obtain a linear dimming curve.

PWM Soft-Start Behavior

The RT8471 features an optional PWM soft-start behavior

that allows for gradual brightness transition. This is

achieved by simply connecting an external capacitor

between the ADJ pin and GND. An internal current source

will then charge this capacitor for soft-start behavior,

resulting in steady LED current increase and decrease

during power on and power off, as shown in Figure 1.

Figure 1. PWM Soft-Start Behavior Mechanism

1.2V

0.4V

Internal

VRAMP

VADJ 0V

ILED

1.2A

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