RT9285APQW datasheet(9/12 Pages) RICHTEK

9 / 12 page

RT9285A/B

Preliminary

DS9285A/B-02 August 2007

www.richtek.com

Application Information

LED Current Control

The RT9285A/B regulates the LED current by setting the

current sense resistor (RSET) connecting to feedback and

ground. The RT9284A/B feedback voltage (VFB) is 0.25V.

The LED current (ILED) can be set by a resistor RSET.

ILED = 0.25/RSET

In order to have an accurate LED current, a precision resistor

is preferred (1% is recommended).

Inductor Selection

The recommended value of inductor for 4 to 5WLEDs

applications are 10

µH to 22µH. For 3WLEDs, the

recommended value of inductor is 4.7

µH to 22µH. Small

size and better efficiency are the major concerns for portable

device, such as RT9285A/B used for mobile phone. The

inductor should have low core loss at 1MHz and low DCR

for better efficiency.

GND

VDD

VOUT

RT9285A/B

1uF

SET

Chip Enable

10uH to 22uH

2.7V to 5.5V

0.22uF to 1uF

12.5

GND

VDD

VOUT

RT9285A/B

1uF

SET

Chip Enable

10uH to 22uH

2.7V to 5.5V

0.22uF to 1uF

12.5

Figure 3. Application for Driving 4 Series WLEDs

Figure 4. Application for Driving 5 Series WLEDs

The inductor saturation current rating should be considered

to cover the inductor peak current.

Capacitor Selection

Input and output ceramic capacitors of 1

µF are recommend-

ed for RT9285A/B applications. 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.

Output Voltage Control

The output voltage of R9285 can be adjusted by the divider

circuit on FB pin. Figure 6 shows a 2-level voltage control

circuit for OLED application. The output voltage can be

calculated by the following equations in Figure 6.

GND

VDD

VOUT

RT9285A/B

1uF

Chip Enable

10uH to 22uH

2.7V to 5.5V

0.22uF to 1uF

10k

590k

OUT

15V

Figure 5. Application for Constant Output Voltage

10k

;

0.25

VOUT

Figure 6. Application Circuit for Output Voltage Control

and Related Equations

VDD

VOUT

GND

RT9285A/B

GPIO

OLED

RT9285APQW Datasheet(PDF) 9 Page - Richtek Technology Corporation

RT9285APQW Datasheet(HTML) 9 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12

Datasheet Download

Link URL