MP3388S—50V, 8-STRING, WHITE LED DRIVERS

MP3388S Rev.1.01

www.MonolithicPower.com

13

10/29/2012

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© 2012 MPS. All Rights Reserved.

Table 1: PWM Frequency Range and Dimming

Duty Cycle

100<f≤200

0.16%

100%

200<f≤500

0.40%

100%

500<f≤1k

0.80%

100%

1k<f≤2k

1.60%

100%

2k<f≤5k

4.00%

100%

5k<f≤10k

8.00%

100%

10k<f≤20k

16.00%

100%

20k<f≤22k

18.00%

100%

22k<f≤25k

20.00%

100%

25k<f≤30k

24.00%

100%

Direct PWM Dimming with Negative Logic

Similar to direct PWM dimming with positive logic.

Apply a 100Hz-to-30 kHz external square

waveform to the PWMO pin for negative-logic

PWM dimming. The minimum recommended

amplitude of the PWM signal is 1.5V (see Figure

8).

Figure 8: Direct PWM Dimming

DC-Input PWM Dimming with Negative Logic

For negative-logic DC-input PWM dimming, apply

an analog signal (from 0.2V to 1.2V) to the PWMO

pin. If applying a PWMO DC voltage <0.2V, the

PWM duty cycle will be 100%. For PWMO DC

voltages >1.2V, the output will be 0% (See Figure

9). The capacitor on the FPWM pin sets the

internal triangle waveform frequency.

Figure 9: DC-Input PWM Dimming

Selecting the Inductor

A larger inductor results in less ripple current,

lowering both the peak inductor current and stress

on the internal N-channel MOSFET. However, a

larger value inductor is larger physical size with a

higher series resistance and a lower saturation

current.

Choose an inductor that does not saturate under

worst-case load conditions. Select the minimum

inductance value to ensure that the boost

converter works in continuous conduction mode,

for high efficiency and good EMI performance.

Calculate the minimum inductance value with:

2

OUT

LOAD

SW

VD (1 D)

L

2I

f



  





IN

OUT

V

D1

V



LED load current, and η is the efficiency.

Use either a 10μH (at a 1.25MHz switching

frequency) or a 22µH (at a 625kHz switching

frequency) inductor with a DC current rating of at

least 40% higher than the maximum input current

for most applications. Select an inductor with the

smallest-possible DC resistance for greatest

efficiency.

Selecting the Input Capacitor

The input capacitor reduces both the surge

current drawn from the input supply and the

switching noise from the device. The input

capacitor impedance at the switching frequency

should be less than the input source impedance to

prevent high-frequency switching current from

passing through the input. For best results, use

ceramic capacitors with X5R or X7R dielectrics

because of their low ESR and small temperature

coefficients. For most applications, use a 4.7μF

capacitor.