MP3308 – STEP-UP WHITE LED DRIVER WITH DUAL PWM DIMMING INPUT CONTROL

MP3308 Rev. 1.02

www.MonolithicPower.com

10

7/19/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

VIN

EN

PWMB

C1 C2

L1

B0530

D1

C3

160k

R3

10k

R4

NS

C6

LED+

LED-

NC

C5

100k

R11

0.1

C4

100

R5

JP1

1

R1

NS

R2

68nF

C8

68nF

C9

10nF

C7

100k

R10

100k

R9

PWMA

Bias

10k

R6

10k

R7

10k

R8

PWMOB

8

PGND

2

LED

12

FSET

7

PWMIA

4

SW

1

GND

10

ISET

13

IN

3

EN

6

GND

14

OVP

11

PWMOA

9

PWMIB

5

U1 MP3308

33nF

C10

Figure 2—Typical Application Driving 9 Strings of 3 WLEDs

Figure 2 shows a typical application circuit that

can drive up to nine strings of three white LEDs

with an output of 9V/180mA. A 4.7µF output

capacitor is sufficient for most applications. A

3.3µH inductor with low DC inductor resistance

(DCR) will improve efficiency. Two 4.7μF ceramic

capacitors provide sufficient input decoupling.

Use Schottky diodes with fast recovery for high

frequencies, and rated for a 500mA peak

The ramped voltage added to the current sense

amplifier reduces the maximum output current as

the duty cycle increases. As more LEDs are

added, the output voltage rises but the maximum

current delivered to the load falls as well.

Setting the LED Current

The resistor R1 sets the LED current, where the

current value is given by:

LED

200mV

I

R1

=

Table 1 pairs resistor values with an LED current.

R1 (Ω)

1

200

5

40

10

20

20

10

60

3.33

180

1.11

200

1

PWM Dimming

PWM dimming requires the application of square

waves to both the PWMIA and the PWMIB pins.

A capacitor on the FSET pin sets the PWM

dimming frequency according to the equation:

The minimum recommended amplitude of the

PWM signal is 2V.

External capacitors on both PWMOA and

PWMOB, together with internal 100kΩ resistors

filter the square waveform to provide a DC value.

The time constant should be ten times greater

than a PWM waveform cycle:

PWM

RC 10 T

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