Flexible WLED/RGB Backlight Driver
for Medium Size LCDs
A8500
11
Allegro MicroSystems, LLC
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
At startup, the output capacitor is discharged and the IC enters
soft start. The boost current is limited to 1 A, and all active LEDx
pins sink
1
/
16
of the set 100% current until all of the enabled
LEDx pins reach 0.5 V. After the IC comes out of soft start, the
boost current and the LEDx pin currents are set to 100% current.
The output capacitor charges to the voltage level required to sup-
ply full LEDx current within a few cycles. The IC is shut down
immediately when PWM goes low.
Analog Dimming with an External PWM Signal on the APWM
Pin.
When the APWM pin is pulled high, with the EN and PWM
pins low, the A8500 turns on in this mode. The first pulse after
shutdown should be greater than t
HI(init)
. The logic level PWM
signal applied to the APWM pin multiplies I
SET
by the duty cycle
to set the reference current level for the LED pins. The typical
range for the APWM signal frequency is 20 kHz to 2 MHz. The
output current ripple at 20 kHz, 50% duty cycle, is less than 5%
of the set value. The LED current accuaracy at 2 MHz, 50% duty
cycle, is less than 3%. In this mode, the A8500 goes through a
soft start routine similar to serial dimming.
Device Internal Protection
Overcurrent Protection (OCP). The A8500 has a pulse-by-pulse
current limit of 2 A on the boost switch. This current limit is
independent of duty cycle.
Thermal Shutdown Protection (TSD). The IC shuts down when
junction temperature exceeds 165°C and restarts when the junc-
tion temperature falls by 40°C.
Overvoltage Protection (OVP). The A8500 has overvoltage
protection to protect the IC against output overvoltage. The over-
voltage level can be set, from 30 to 45 V typical, with an external
resistor, ROVP, as shown in figure 10. When the current though
the OVP pin exceeds 54.9 μA, the OVP comparator goes high.
When the OVP pin current falls below 47.8 μA, OVP is reset.
180
160
140
120
100
80
60
40
20
0
0 20406080
f
APWM
(kHz)
100
Duty Cycle (%)
I
OUT
(mA)
20
100
500
Figure 9. APWM pin dimming linearity.
Figure 10. Overvoltage protection circuit. Three alternative
configurations at (A) are available, as follows:
External Component OVP Rating
ROVP only up to 45 V
DZ only up to 47 V
both ROVP and DZ redundancy
1.23 V
28.8 V
22 kΩ
SW SW
OVP
ROVP
DZ
D1
V
OUT
V
IN
COUT
OVP
Disable
+
–
4.4 kΩ
A
