MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER
MP3389 Rev. 1.04 www.MonolithicPower.com 13
3/27/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
BOSC
DBRT
MP3389
PWM Dimming
100Hz~2kHz
R1
100k
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
fpwm(Hz) Dmin Dmax
100<f200 0.30% 100%
200<f500 0.75% 100%
500<f1k 1.50% 100%
1k<f2k 3.00% 100%
2k<f5k 7.50% 100%
5k<f10k 15.00% 100%
10k<f20k 30.00% 100%
2. DC Input PWM Dimming
To apply DC input PWM dimming, apply an
analog signal (range from 0.2 V to 1.2V) to the
DBRT pin to modulate the LED current directly. If
the PWM is applied with a zero DC voltage, the
PWM duty cycle will be 0%. If the DBRT pin is
applied with a DC voltage>1.2V, the output will
be 100% (See Figure 5). The capacitor on BOSC
pin set the frequency of internal triangle
waveform according to the equation:
F
UNITE
= 3.5F / C
UNITE
C1
BOSC
DBRT
MP3389
DC Signal
Figure 5—DC input PWM Dimming
Layout Considerations
Careful attention must be paid to the PCB board
layout and components placement. Proper layout
of the high frequency switching path is critical to
prevent noise and electromagnetic interference
problems. The loop of external MOSFET (M2),
output diode (D1), and output capacitor (C5) is
flowing with high frequency pulse current. it must
be as short as possible (See Figure 6).
Figure 6—Layout Consideration
The IC exposed pad is internally connected to
GND pin, and all logic signals are refer to the
GND. The PGND should be externally connected
to GND and is recommended to keep away from
the logic signals.
External MOSFET for Short Protection
When output voltage is higher than rating of
LED1-12 pins, external MOSFET is needed for
each LED channel to avoid destroying LED1 -12
pins. (See Figure 8 for 4 channels application)
The rating of MOSFET must be higher than 1.2*
V
OUT
t and more than 1.2* I
LED
.
