RT8489 datasheet(13/16 Pages) RICHTEK

13 / 16 page

RT8489

DS8489-00 November 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Programmable Switching Frequency

The RSET frequency adjust pin allows the user to program

the switching frequency from 100kHz to 1MHz in order to

optimize efficiency and performance or minimize external

component size. Higher frequency operation yields

smaller component size but increases switching losses

and gate driving current, and may not allow sufficiently

high or low duty cycle operation. Lower frequency

operation gives better performance, but is more costly

with larger external component size. An external resistor

from the RSET pin to GND is required, and do not leave

this pin open. For an appropriate RSET value, refer to Figure

Figure 4. LED Current vs. PWM Dimming Duty

Cycle

Figure 5. Switching Frequence vs RSET

LED Current vs. PWM Duty Cycle

100

120

140

160

2040

6080

100

Duty Cycle (%)

200Hz

1kHz

3kHz

Frequency vs. RSET Resistance

100

200

300

400

500

600

700

800

900

1000

100 110

RSET(kΩ)

VIN = 12V

LED Pin External Resistor Connection

The RT8489 equips 4 channel LEDdrivers and each channel

supports numerous LEDs. The 4 LED strings are

connected from VOUT to pin LEDx (x = 1 to 4) respectively.

If one of the LED channel is not used, the LEDx (x = 1 to

4) pins should be connected to ground directly.

In this case, there should be a current limiting resistor

between external MOSFET Drain node and LEDx pin to

limit the LEDx pin input current below 100

μA.

The formula for this resistor is

Rx = (VOUT

− |VLEDx(MAX)|) / 100μA

Figure 6. External Resistor Connection for LEDx Pin

Input Over Current Protection

The resistor, RSENSE, between the Source of the external

N-MOSFET (MSW) and GND should be selected to provide

adequate switch current.

The RT8489 senses the inductor current through ISENSE

pin in the switch on-period. The duty cycle depends on

the current sense signal summed with the internal slope

compensation and compared to the VC signal. The

external N-MOSFET will be turned off when the current

signal is larger than the VC signal. In the off-period, the

inductor current will descend. The external N-MOSFET is

turned on by the oscillator in the next beginning cycle.To

drive the application without exceeding the 120mV (typical)

current limit threshold on the ISENSE pin of the RT8489.

Select a resistor that gives a switch current of at least

20% greater than the required LED current according to :

SENSE

OUT

V0.1V

( )

⎛⎞

⎜⎟

⎝⎠

LEDx

GATEx

SENSEx

RT8489

RSx

VOUT

GND

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