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參數(shù)資料
| 型號： | A8431EEH-T |
| 廠商： | Allegro MicroSystems, Inc. |
| 元件分類： | LED驅動器 |
| 英文描述： | White LED Driver Constant Current Step-up Converter |
| 中文描述： | 白光LED驅動恒流升壓轉換器 |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 331K |
| 代理商： | A8431EEH-T |
7
26185.301
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
A8431
White LED Driver Constant Current Step-up Converter
Schematic 3. Dimming control with filtered PWM
A843
VIN
SW
EN
GND
FB
Li-ion
2.5 V to
4.2 V
C1
1.0 μF
L1
22 μH
D1
C2
0.22 μF
R1
6.3
V
C
(PMW)
R3
90 k
R2
5 k
Enable
R4
10 k
C3
100 nF
1
1
2
3
5
6
4
OVP
Schematic 4. Soft start operation
A843
VIN
SW
EN
GND
FB
Li-ion
2.5V to
4.2V
C1
1.0 μF
L1
22 μH
D1
C2
0.22 μF
R1
6.3
R3
5 k
R2
1 k
Enable
C3
2.2 nF
1
1
2
3
5
6
4
OVP
PWM Control
. The control voltage, V
C
, also can be
generated by a filtered PWM signal, as shown in schematic
3. In this case, a 0% duty cycle (PWM = 0 V) corresponds
to full brightness and a 100% duty cycle causes the LED
current, I
LOAD
, to go to 0 A.
By applying a PWM signal directly to the EN pin, the A8431
is turned on or off, and I
LOAD
is either full (as defined by
R1) or 0 A. By varying the duty cycle of the PWM signal, the
LED brightness can be controlled from off (0% duty cycle)
to full (100% duty cycle). The PWM frequency should be in
the range from 1 kHz to 10 kHz.
Several other schemes are possible, for example, digitally
switching additional resistors across R1 to increase I
LOAD
.
In this case, R1 would be selected for the minimum desired
brightness.
Start-Up
To provide fast start-up operation, no soft start is
implemented in the control circuit. At power-on, the bypass
capacitor, C1, is discharged, which means that the supply
must provide the in-rush current through the inductor, L1.
This can be reduced by modulating the feedback with a soft-start
circuit as shown in schematic 4. When power is first applied, the
capacitor C3 is discharged and pulls the FB pin high, reducing
the output drive to minimum. As C3 charges, when the bottom
drops below about 0.8 V, the feedback from the sense resistor,
R1, takes over full control of the output current.
Preliminary Data Sheet
Subject to Change Without Notice
December 7, 2004
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