A8500 Datasheet, PDF (13/16 Pages) Allegro MicroSystems – Flexible WLED/RGB Backlight Driver for Medium Size LCDs

A8500
Flexible WLED/RGB Backlight Driver
for Medium Size LCDs
Application Information
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:
ISET
RISETP RISET
Q1
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 150 µA.
Figure 13. Configuration for 1000:1 dimming.
100
95
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 30 mA, and then it can be
dimmed by applying 100% to 0.35% 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.
90
85
80
75
0.1
1.0
10.0
Dimming Level (%)
100.0
Figure 14. Typical accuracy, normalized to the 100% current level,
versus dimming level, with F_PWM = 100 Hz.
V_S
5 V ±10%
ROVP COUT
EN
VIN
SW SW OVP
C_C
RFSET
RA
V_A
RISET
PWM
APWM
SKIP
COMP
FSET
A8500
AGND
PGND
SEL3
V_IN
SEL2
ISET
LED1
SEL1
LED2
LED3 LED5 LED7 LGND LED8 LED6 LED4
Figure 12. Typical application circuit for PWM dimming, using digital PWM (on the
PWM pin, with APWM high).
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com