A8504 查看數據表（PDF） - Allegro MicroSystems

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A8504

WLED/RGB Backlight Driver for Medium Size LCDs

Allegro MicroSystems 

A8504

WLED/RGB Backlight Driver for Medium Size LCDs

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. When the PWM pin is pulled high, the A8504 turns on

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

pins sink 1/16 of the set 100% current until all of the enabled

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). The A8504 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.

VPWM

Overvoltage Protection (OVP). The A8504 has overvoltage

C1

protection to protect the IC against output overvoltage. The

C2

VOUT

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

IOUT

high. When the OVP pin current falls below 44 μA, OVP is reset.

C3

Calculate the value for ROVP as follows:

t

Symbol

C1

C2

C3

t

Parameter

VPWM

VOUT(ac)

IOUT

time

Units/Division

2.00 V

500 mV

200 mA

5 ms

Figure 3. Output Voltage Ripple During PWM Dimming. VIN= 5 V,

VBAT= 15 V, fSW= 1 MHz, fPWM= 100 Hz, PWM duty cycle = 50%,

8S8P configuration, 40 mA per channel.

ROVP = (VOVP – 30) / 49 μA ,

where VOVP is the desired typical OVP level in V, and ROVP is

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.

C1

Turn-on Delay

6 μs Typical

Shutdown

(0 μA)

C2

VPWM

VOUT

100% Current Level

IOUT

t

Figure 4. Timing of turn-on delay and turn-off delay when using the

PWM pin. VIN= 5 V, VBAT= 15 V, fSW= 1 MHz, fPWM= 100 Hz, 8S8P

configuration, 40 mA per channel.

VPWM

Shutdown

(0 μA)

IOUT

Symbol

C1

C2

t

Parameter

VPWM

IOUT

time

Units/Division

2.00 V

200 mA

5 μs

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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