RT9385 datasheet(8/10 Pages) RICHTEK

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RT9385

DS9385-01 April 2011

www.richtek.com

Applications Information

The RT9385 uses a fractional switched capacitor charge

pump to power up to five white LEDs with a programmable

current for uniform intensity. The part integrates current

sources and automatic mode selection charge pump. It

maintains the high efficiency by utilizing an x1/x1.5/x2

fractional charge pump and current sources. The small

equivalent x1 mode open loop resistance and ultra-low

dropout voltage of current source extend the operating

time of x1 mode and optimize the efficiency in white LED

applications.

Input UVLO

The input operating voltage range of the LED driver is from

2.8V to 4.5V. An input capacitor at the VIN pin could reduce

ripple voltage. It is recommended to use a ceramic 1

μF or

larger capacitance as the input capacitor. The RT9385

provides an under voltage lockout (UVLO) function to

prevent it from unstable issue when startup. The UVLO

threshold of input rising voltage is set at 2V typically with

a hysteresis of 100mV.

Soft Start

The charge pump employs a soft start feature to limit the

inrush current. The soft-start circuit prevents the excessive

inrush current and input voltage drop. The soft-start clamps

the input current in a typical period of 50

μs.

Mode Decision

The RT9385 uses a smart mode selection method to decide

the working mode for optimizing the efficiency. Mode

decision circuit senses the output and LED voltage for

up/down selection. The RT9385 automatically switches

to x1.5 or x2 mode whenever the dropout condition is

detected from the current source and returns to x1 mode

whenever the dropout condition releases.

LED connection

The RT9385 supports up to 5 white LEDs. The 5 LEDs

are connected from VIN to pin1, 2, 3, 15 and 16

respectively. If the LED is not used, the LED pin should

be connected to VIN directly.

Capacitors Selection

To get the better performance of the RT9385, the selection

of peripherally appropriate capacitor and value is very

important. These capacitors determine some parameters

such as input/output ripple voltage, power efficiency and

maximum supply current by charge pump. To reduce the

input and output ripple effectively, the low ESR ceramic

capacitors are recommended. For LED driver applications,

the input voltage ripple is more important than output

ripple. Input ripple is controlled by input capacitor CIN,

increasing the value of input capacitance can further reduce

the ripple. Practically, the input voltage ripple depends on

the power supply impedance. The flying capacitor CFLY1

and CFLY2 determine the supply current capability of the

charge pump to influence the overall efficiency of the

system. The lower value will improve efficiency. However,

it will limit the LED's current at low input voltage. For

5x25mA load over the entire input range of 2.8V to 4.5V, it

is recommended to use a 1

μF ceramic capacitor on the

flying capacitor CFLY1 and CFLY2.

Brightness Control

The RT9385 implements a PWM dimming method to

control the brightness of white LEDs. When an external

PWM signal is connected to the EN pin, brightness of

white LED is adjusted by the duty cycle. The suggested

PWM dimming frequency range is from 1kHz to 200kHz.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. The maximum

power dissipation depends on the thermal resistance of

IC package, PCB layout, the rate of surroundings airflow

and temperature difference between junction to ambient.

The maximum power dissipation can be calculated by

following formula :

PD(MAX) = ( TJ(MAX)

− TA ) / θJA

Where TJ(MAX) is the maximum operation junction

temperature, TA is the ambient temperature and the

θJAis

the junction to ambient thermal resistance.

For recommended operating conditions specification of

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