a8504 Allegro MicroSystems, Inc., a8504 Datasheet - Page 9

• Manufacturer Part Number: a8504
• Description: Wled/rgb Backlight Driver For Medium Size Lcds
• Manufacturer: Allegro MicroSystems, Inc.

A8504
This current is multiplied internally with a gain of 480, and then
mirrored on all enabled LEDx pins. This sets the maximum cur-
rent through the LEDs, referred to as “100% current.” The LED
current can be reduced from 100% by on/off control (PWM) with
an external PWM signal on the PWM pin
On/off Control (PWM) with an External PWM Signal on the
PWM Pin.
and all enabled LEDx pins sink 100% current. When the PWM
pin is pulled low, the IC shuts down with the LEDx pins disabled.
External PWM applied to the PWM pin should be in the range of
100 to 400 Hz for optimal accuracy.
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
Figure 3. Output Voltage Ripple During PWM Dimming. V
V
8S8P configuration, 40 mA per channel.
BAT
C2
C3
C1
= 15 V, f
When the PWM pin is pulled high, the A8504 turns on
C2
C1
SW
Figure 4. Timing of turn-on delay and turn-off delay when using the
PWM pin. V
configuration, 40 mA per channel.
= 1 MHz, f
Shutdown
(0 μA)
Symbol
C1
C2
C3
t
PWM
IN
= 5 V, V
= 100 Hz, PWM duty cycle = 50%,
Parameter Units/Division
t
V
V
Turn-on Delay
OUT(ac)
I
time
6 μs Typical
OUT
PWM
BAT
WLED/RGB Backlight Driver for Medium Size LCDs
= 15 V, f
500 mV
200 mA
2.00 V
5 ms
SW
100% Current Level
= 1 MHz, f
V
V
PWM
OUT
I
OUT
V
V
IN
PWM
OUT
I
OUT
= 5 V,
PWM
t
= 100 Hz, 8S8P
pins sink
LEDx pins reach 0.8 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 startup sequence
is shown in the Soft Start chart in the Performance Characteristics
section. The IC is shut down immediately when PWM goes low.
Device Internal Protection
Overcurrent Protection (OCP).
current limit of 2 A on the boost switch. This current limit is
independent of duty cycle.
Thermal Shutdown Protection (TSD).
junction temperature exceeds 165°C and restarts when the junc-
tion temperature falls by 40°C.
Overvoltage Protection (OVP).
protection to protect the IC against output overvoltage. The
overvoltage level can be set, from 30 to 45 V typical, with an
external resistor, ROVP, as shown in figure 5. When the current
though the OVP pin exceeds 49 μA, the OVP comparator goes
high. When the OVP pin current falls below 44 μA, OVP is reset.
Calculate the value for R
where V
in Ω. For tighter OVP limits, a low–leakage-current Zener diode,
DZ, can be used, instead of ROVP, to set OVP at up to 47 V. For
redundancy, DZ can be connected across ROVP to provide addi-
tional protection, if ROVP should open. Select a 17 V low-leak-
age Zener diode for DZ.
OVP
1
/
16
is the desired typical OVP level in V, and R
of the set 100% current until all of the enabled
R
OVP
= (V
Symbol
OVP
C1
C2
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
t
OVP
Shutdown
as follows:
(0 μA)
– 30) / 49 μA ,
The A8504 has a pulse-by-pulse
The A8504 has overvoltage
Parameter Units/Division
V
I
time
OUT
PWM
The IC shuts down when
200 mA
2.00 V
5 μs
V
I
OUT
PWM
OVP
is
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