MP3394S—4-STRING, MAX 200mA/STRING WHITE LED DRIVER
MP3394S Rev. 1.03 www.MonolithicPower.com 13
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Figure 4—Direct PWM Dimming
Table 1 shows the PWM dimming duty Range
with different PWM dimming frequency for most
of applications.
Table 1 The Range of PWM Dimming Duty
f
PWM
(Hz) D
min
D
max
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.
f
BOSC
(kHz)= 3.5 / C
BOSC
(nF)
Chose a dimming frequency in the range of
100Hz to 20kHz.
Figure 5—DC input PWM Dimming
Expanding LED Channels
The MP3394S can expand the number of LED
channels by using two or three MP3394Ss in
parallel. To connect two MP3394Ss 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’s
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.
Do not leave the I
SENSE
pin of the slave 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 MP3394S 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 refer to the signal ground. In
order to reduce the noise affection, it is
recommend to separate the power ground(PGND)
and signal ground(GND), then connect PGND
and GND together through single joint.
External MOSFET for Short Protection
When output voltage is higher than rating of
LED1-4 pins, external MOSFET is needed for
each LED channel to avoid destroying LED1-4
pins. (See Figure 8)The rating of MOSFET must
be higher than 1.2* V
OUT
t and more than 1.2* I
LED
.
