A8509 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8509
Allegro MicroSystems
'A8509' PDF : 28 Pages View PDF
A8509
Wide Input Voltage Range, High Efficiency
4-Channel Fault Tolerant LED Driver
The cited values are for 1% tolerance resistors. If the calculated
VOUT
value was not present, the next lowest value of 1% resistor was
chosen.
PWM dimming
Applying an external PWM signal on the PWM pin performs
PWM dimming. When the PWM pin is pulled high, the A8509
enables the LEDx pins to sink 100% current. When PWM is
pulled low, the boost converter and LEDx sinks are turned off.
The compensation (COMP) pin is floated, and critical internal
circuits are kept active.
The typical PWM dimming frequencies fall between 100 and
1000 Hz. Figures 15 and 16 show examples of dimming at 50%
and 0.5% duty cycles.
Analog dimming
The A8509 can also be dimmed by using an external DAC or
other voltage source applied either directly to the ground side
of the RISET resistor or through an external resistor to the ISET
pin (see figure 17). The ISET current can be varied in the range
between 34 μA and 130 μA.
• For a single-resistor configuration (panel A of figure 17), the
ISET current is controlled by the following formula:
ISET
=
VISET – VDAC
RISET
(3)
where VISET is the ISET pin voltage and VDAC is the DAC output
voltage.
• For a dual-resistor configuration (panel B of figure 17), the
ISET current is controlled by the following formula:
ISET
=
VISET
RISET
–
VDAC – VISET
R1
(4)
The advantage of this circuit is that the DAC voltage can be
higher or lower, thus adjusting the LED current to a higher or
lower value of the preset LED current set by the RISET resistor:
▫ VDAC = 1.00 V: output is strictly controlled by RISET
▫ VDAC > 1.00 V: LED current is reduced
▫ VDAC < 1.00 V: LED current is increased
IOUT
C1
C2
C3
VCOMP
C4
VPWM
t
Figure 15. PWM dimming: fSW = 200 Hz, 50% duty cycle, VOUT = 30 V, VIN
= 12 V, and ILED = 240 mA per LED string; shows VOUT (ch1, 10 V/div.),
IOUT (ch2, 500 mA/div.), VCOMP (ch3, 2 V/div.), and PWM (ch4, 5 V/div.),
t = 2 ms/div.
VOUT
C1
IOUT
C2
C3 VCOMP
C4
VPWM
t
Figure 16. PWM dimming: fSW = 200 Hz, 0.5% duty cycle, VOUT = 30 V,
VIN = 15 V, ILED = 240 mA per LED string; shows VOUT (ch1, 10 V/div.),
IOUT (ch2, 500 mA/div.), VCOMP (ch3, 2 V/div.), and VPWM (ch4, 5 V/div.),
t = 10 μs/div.
DAC
VDAC
GND
RISET
A
DAC
VDAC
GND
R1
RISET
B
A8509
ISET
GND
A8509
ISET
GND
Figure 17. Typical application simplified diagram of voltage LED current
control using a DAC to control LED current.
Allegro MicroSystems, Inc.
12
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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