Flexible WLED/RGB Backlight Driver
for Medium Size LCDs
A8500
13
Allegro MicroSystems, LLC
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
EN
PWM
FSET
COMP
ISET
RISET
RFSET
RA
V
A
C
C
LED1
LED3 LED5 LED7 LGND
VIN SW SW OVP
COUTROVP
V
IN
5 V ±10%
A8500
LED8 LED6 LED4
LED2
SKIP
APWM
V
IN
SEL1
SEL2
SEL3
AGND
PGND
Application Information
Figure 12. Typical application circuit for PWM dimming, using digital PWM (on the
PWM pin, with APWM high).
RISET
Q1
ISET
RISETP
Figure 13. Configuration for 1000:1 dimming.
100
95
90
85
80
75
0.1 1.0 10.0 100.0
Dimming Level (%)
Accuracy (%)
Figure 14. Typical accuracy, normalized to the 100% current level,
versus dimming level, with F
PWM
= 100 Hz.
A typical application circuit for dimming an LCD monitor
backlight with 96 LEDs is shown in figure 1. Figure 12 shows
two dimming methods: digital PWM control (PWM signal on the
PWM pin) and analog PWM control, with the analog signal, V
A
,
applied to the ISET pin through a resistor, RA.
The current flowing through RA can be calculated as:
I
A
= V
A
/ R
A
.
This current changes the reference current, I
SET
, as follows:
I
SET
= V
SET
/ R
SET
– (V
A
– V
SET
) / R
A
.
LED current can be changed by changing V
A
. I
SET
can be
changed in the range from 40 μA to 120 μA.
Application Circuit for 1000:1 Dimming Level
A wider dimming range can be achieved by changing the refer-
ence current, I
SET
, while using PWM dimming. For higher output,
current levels turn on Q1. R
ISET
and R
ISETP
set the 100% current
level. This current level can be set to 25 mA, and then it can be
dimmed by applying 100% to 0.32% duty cycle on the PWM pin.
The reference current can be reduced by turning off Q1. LED
current can be dimmed to 8 mA by reducing reference current
through ISET pin. This provides 1000:1 combined dimming level
range. Figure 14 shows the accuracy, Err
LEDX
, results using this
circuit.
