MP3397—4-STRING, MAX 350mA/STRING WHITE LED DRIVER

MP3397 Rev. 1.01

www.MonolithicPower.com

13

9/4/2012

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© 2012 MPS. All Rights Reserved.

Figure 4—Direct PWM Dimming

Table 1 shows the PWM dimming duty Range

with different PWM dimming frequency.

Tab 1 The Range of PWM Dimming Duty

100<f

200

0.30%

100%

200<f

500

0.75%

100%

500<f

1k

1.50%

100%

1k<f

2k

3.00%

100%

2k<f

5k

7.50%

100%

5k<f

10k

15.00%

100%

10k<f

20k

30.00%

100%

2. DC Input PWM Dimming

For DC input PWM dimming, apply an analog

signal (ranging from 0.2 V to 1.2V) to the DBRT

pin to modulate the LED current directly. If the

DBRT voltage falls below 0.2V, the PWM duty

cycle will be 0%. If the DBRT voltage goes above

1.2V, the output will be 100% (See Figure 5). The

capacitor on BOSC pin sets the frequency of the

internal triangle waveform according to the

equation.

Chose a dimming frequency in the range of

100Hz to 20kHz.

Figure 5—DC input PWM Dimming

Expanding LED Channels

The MP3397 can expand the number of LED

channels by using two or three MP3397s in

parallel. To connect two MP3397s for a total of 8

LED strings, tie the VCC pins of the master IC

and the slave IC together to power the slave IC

internal logic circuitry. Tie the COMP pins of the

slave IC and the master IC together to regulate

the voltage of all 8 strings LEDs. The slave IC

MOSFET driving signals are not used; the boost

converter can be only driven by the master IC.

floating; tie it to ground. Apply the EN and DIM

signals to both ICs. For best results, use external

PWM dimming mode for synchronized and

accurate dimming.

Layout Considerations

The circuit layout for the MP3397 requires

special attention to reduce EMI noise.

The loop from the external MOSFET (M1),

through the output diode (D1) and the output

capacitor (C2, C3) carry a high-frequency pulse

current and must be as small and short as

possible (See Figure 6).

Figure 6—Layout Consideration

All logic signals return to the signal ground. In

order to reduce the effects of noise, separate

power ground (PGND) and signal ground (GND),

and connect PGND and GND together through

single point.