A8504EC View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8504EC Datasheet PDF : 18 Pages
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Typical Application Circuits
A typical application circuit for dimming an LCD monitor back-
light with multiple LED strings is shown in figure 1. Figure 8
shows two dimming methods: digital PWM control (PWM signal
on the PWM pin) and analog PWM control, with the analog sig-
nal, VA , applied to the ISET pin through a resistor, RA.
The current flowing through RA can be calculated as:
ISET
RISETP RISET
Q1
IA = VA/ RA .
This current changes the reference current, ISET, as follows:
ISET = VSET / RSET – (VA – VSET) / RA .
LED current can be changed by changing VA. ISET can be
changed in the range from 40 μA to 100 μA.
Application Circuit for 1000:1 Dimming Level
Figure 9. Configuration for 1000:1 dimming.
100
95
90
A wider dimming range can be achieved by changing the refer-
ence current, ISET, while using PWM dimming. For higher output,
current levels turn on Q1. RISET and RISETP set the 100% current
level. This current level can be set to 45 mA, and then it can be
dimmed by applying 100% to 0.25% duty cycle on the PWM pin.
The reference current can be reduced by turning off Q1. LED
current can be dimmed to 18 mA by reducing reference current
through ISET pin. This provides 1000:1 combined dimming level
range. Figure 10 shows the accuracy, ErrLEDX , results using this
circuit.
85
80
75
0.1
1.0
10.0
Dimming Level (%)
100.0
Figure 10. Typical accuracy, normalized to the 100% current level,
versus dimming level, with FPWM = 100 Hz.
VIN
5 V ±10%
CIN
VBAT
24 V
VOUT
ROVP COUT
VIN
SW SW OVP
CC
RFSET
RA
VA
RISET
PWM
GND
SKIP
COMP
FSET
A8504
AGND
PGND
SEL3 VIN
SEL2
ISET
SEL1
LED1
LED2
LED3 LED5 LED7 LGND LED8 LED6 LED4
Figure 8. Typical application circuit for PWM dimming, using digital PWM (on the PWM pin).
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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