RT9272 Просмотр технического описания (PDF) - Richtek Technology

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RT9272

High Performance, Constant Current LED Driver for Dual Panel Mobile Phone

Richtek Technology 

Preliminary

RT9272

Application Information

Inductor Selection

The recommended value of inductor for RT9272 applications

is 4.7μH. Small size and better efficiency are the major

concerns for portable device, such as RT9272 used for

dual panel mobile phone. The inductor should have low

DCR for better efficency. To avoid inductor saturation,

current rating should be at least 600mA for 3LEDs

application in the input range 2.7V to 4.5V.

Capacitor Selection

2.2μF input and 1μF output ceramic capacitors are

recommended for RT9272 applications. 2.2uF input

capacitor can reduce input ripple. For better voltage stability,

to increase the input capacitor value or using LC filter is

feasible, especially in the Li-ion battery application. 1μF

output capacitor is sufficient to reduce output voltage ripple.

For better voltage filtering, ceramic capacitors with low

ESR are recommended. X5R and X7R types are suitable

because of their wider voltage and temperature ranges.

Diode Selection

Schottky diode is a good choice for RT9272 because of

its lower forward voltage drop and faster reverse recovery.

Using schottky diode can get better efficiency. The high-

speed rectification is also a good characteristic of

schottky diode for high switching frequency. Current rating

of the diode must meet the root mean square of the peak

current and output average current multiplication as

following:

IDiode (RMS) ≅ ( IOUT ×IPEAK )

The diode’sreverse breakdown voltage should be larger

than the output voltage. SS0520 is recommended schottky

diode for rectifier.

LED Current Control

Current control is to change different brightness of LEDs.

To regulate the output resistor can get different currents of

LEDs as following formula.

FB_resister (Ω) = 0.3 (V)

ILED (mA)

Dimming Control

Dimming control is the current programming method when

output resistor fixes the brightness of LEDs. Using a PWM

signal on EN pin can achieve the dimming control. RT9272

is turned on or off by the PWM signal applied to EN pin.

The brightness of LEDs for CH1 or CH2 is depended on

PWM duty cycle. By selecting the different channel, it can

achieve the dimming control function for main panel or

sub panel. The average LED current increases

proportionally as the duty cycle added. The operation

frequency of PWM signal is 100Hz to 1kHz. The average

LED current is linear in different duty cycle. It should be

notice that the magnitude of PWM signal should be higher

than the minimum EN voltage.

Channel Selection

There is one set of LC filter for RT9272 to share two

channels power requirement. Each time turning on only

one channel can save the power dissipation and easily

extend the standby time. Selection pin is internal logic

controller. SP pin is pulled low to turn on the internal channel

1 and pulling high to turn on channel 2. As the input signal

change, internal MOSFET of LX pin will turn off 1ms to

prevent the over voltage damage by the output voltage

instant change.

Layout Guide

z A full GND plane without gap break.

z The input capacitor should be placed as close as possible

to the input for good filtering.

z The switching path should be shorter to prevent EMI

problems.

z The inductor and diode must be placed closely to the

switch pin.

z The feedback path should be away from the inductor.

z The feedback resistor should be connected directly to

the GND.

DS9272-09 August 2007

www.richtek.com

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