RT9367C datasheet(13/18 Pages) RICHTEK

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RT9367C

DS9367C-01 March 2011

www.richtek.com

Applications Information

The RT9367C is a high efficiency charge pump white LED

driver. It provides 4 channels low dropout voltage current

source to regulated 4 white LEDs current. For high

efficiency, the RT9367C implements a smart mode

transition for charge pump operation. The RT9367C

provides I2C dimming function for LED brightness control.

Input UVLO

The input operating voltage range of the RT9367C is 2.8V

to 5.5V. An input capacitor at the VIN pin could reduce

ripple voltage. It is recommended to use a ceramic 1uF or

larger capacitance as the input capacitor. This IC 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 0.1V.

Soft Start

The RT9367C includes a soft start circuit to limit the inrush

current at power on and mode switching. The soft start

circuit limits the input current before output voltage

reaching a desired voltage level.

Mode Decision

The RT9367C uses a smart mode decision method to

select the working mode for maximum efficiency. The

charg pump can operation at x1, x1.5 or x2 mode. The

mode decision circuit senses the output voltage and LED

voltage for up/down selection.

Selecting Capacitors

To get better performance of RT9367C, 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. The 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's impedance. The flying

capacitor CFLY1 and CFLY2 determine the supply current

capability of the charge pump that will influence the overall

efficiency of the system. The lower value will improve

efficiency, but it will limit the LED's current at low input

voltage. For 4 X 25mA load over the entire input voltage

range of 2.8V to 5.5V, it is recommended to use 1

μF

ceramic capacitor on the flying capacitor CFLY1 & CFLY2.

Power Sequence

In order to assure the RT9367C's operation in normal

condition, the Input voltage and ENA should be ready

before the RT9367C get the I2C signal showed in Figure 3

and the RT9367C can be shut down by pulling ENA low.

When ENA is reset, the I2C signal also needs to be resent

again for operating at normal condition.

Dual LDO

Like any low-dropout regulator, the external capacitors

used with the RT9367C must be carefully selected for

regulator stability and performance. Using a capacitor

whose value is > 1

μF on the RT9367C input and the amount

of capacitance can be increased without limit. The input

capacitor must be located at a distance of not more than

0.5 inch from the input pin of the IC and returned to a

clean analog ground. Any good quality ceramic or tantalum

can be used for this capacitor. The capacitor with larger

value and lower ESR (equivalent series resistance) provides

better PSRR and line-transient response. The output

capacitor must meet both requirements for minimum

amount of capacitance and ESR in all LDOs application.

The RT9367C is designed specifically to work with low

ESR ceramic output capacitor in space-saving and

performance consideration. Using a ceramic capacitor

whose value is at least 1

μF with ESR is > 20mΩ on the

RT9367C output ensures stability. The RT9367C still

works well with output capacitor of other types due to the

wide stable ESR range. Figure 4. shows the curves of

allowable ESR range as a function of load current for various

Figure 3. The power sequence

SCL

……………

ENA

SDA

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