A8504 查看數據表（PDF） - Allegro MicroSystems
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A8504 Datasheet PDF : 18 Pages
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A8504
WLED/RGB Backlight Driver for Medium Size LCDs
Functional Description
The A8504 is a multioutput WLED driver for medium display
backlighting. The A8504 works with 4.2 to 5.5 V input supply,
and it has an integrated boost converter to boost battery voltage
up to 47 V, 40 mA per LED string. An inductor can be connected
to a separate power supply, VBAT , from 5 to 25 V, with the A8504
IC powered from a 5 V source. The LED sinks can sink up to a
45 mA current.
The boost converter is a constant frequency current-mode con-
verter. The integrated boost DMOS switch is rated for 50 V at
2 A. This switch has pulse-by-pulse current limiting, with the cur-
rent limit independent of duty cycle. The switch also has output
overvoltage protection (OVP), with the OVP level adjustable,
typically from 30 to 47 V, as described in the Device Internal
Protection section.
The A8504 has individual open LED detection. If any LED
opens, the corresponding LED pin is removed from regulation
logic. This allows the remaining LED strings to function nor-
mally, without excessive power dissipation.
The switching frequency, fSW, can be set from 600 kHz to 2 MHz
by a single resistor, RFSET, connected across the FSET and
AGND pins, and with the SKIP pin set to logic low (see figure 2).
The switching frequency is set as:
FSW = 26.03 / RFSET ,
where FSW is in MHz and RFSET is in kΩ When the SKIP pin is
connected to logic low, switching frequency is as set by RFSET.
When the SKIP pin is connected to logic high, the switching
frequency is divided by 4. The SKIP pin can be used to reduce
switching frequency in order to reduce switching losses and
improve efficiency at light loads.
The IC offers a wide-bandwidth transconductance amplifier with
external COMP pin. External compensation offers optimum per-
formance for the desired application.
The A8504 has eight well-matched current sinks to provide regu-
lated current through LEDs for uniform display brightness. The
quantity of LEDx pins used is determined by the SELx pins. Refer
to table 1 for further description.
The boost converter is controlled such that the minimum voltage
on any LEDx pin is 800 mV. In a typical application, the LEDx
pin connected to the LED string with the maximum voltage drop
controls the boost loop, so the remaining pins will also have the
higher voltage drop. All LED sinks are rated for 21 V, to allow
PWM dimming control.
LED Current Setting
The maximum LED current can be set at up to 45 mA per chan-
nel, by using the ISET pin. To set the reference current, ISET ,
connect a resistor, RISET, between this pin and ground, valued
according to the following formula:
ISET = 1.23 / RISET ,
where ISET is in mA and RISET is in kΩ.
2.5
2.0
1.5
1.0
0.5
0
10
20
30
40
50
RFSET (kΩ)
Figure 2. Switching frequency setting by value of RFSET.
Table 1. LEDx Channel Enable Table
SEL1
SEL2
SEL3
LEDx Outputs
0
0
0
Only LED1 on
1
0
0
LED1 through LED2 on
0
1
0
LED1 through LED3 on
1
1
0
LED1 through LED4 on
0
0
1
LED1 through LED5 on
1
0
1
LED1 through LED6 on
0
1
1
LED1 through LED7 on
1
1
1
LED1 through LED8 on
Allegro MicroSystems, Inc.
8
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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