A8502 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
'A8502' PDF : 35 Pages View PDF
A8502
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
Another important feature of the A8502 is the PWM signal to
LED current delay. This delay is typically less than 500 ns, which
allows greater accuracy at low PWM dimming duty cycles, as
shown in figure 12.
APWM pin
The APWM pin is used in conjunction with the ISET pin (see fig-
ure 13). This is a digital signal pin that internally adjusts the ISET
current. When this pin is not used it should be tied to ground.
The typical input signal frequency is between 20 kHz and 1 MHz.
The duty cycle of this signal is inversely proportional to the per-
centage of current that is delivered to the LEDs (figure 14).
To use this pin for a trim function, the user should set the maxi-
mum output current to a value higher than the required current by
at least 5%. The LED ISET current is then trimmed down to the
appropriate value. Another consideration that also is important
is the limitation of the user APWM signal duty cycle. In some
cases it might be preferable to set the maximum ISET current to be
25% to 50% higher, thus allowing the APWM signal to have duty
cycles that are between 25% and 50%.
10
8
6
4
2
0
0.1
Worst-case
Typical
1
10
100
PWM Duty Cycle, D (%)
Figure 12. Percentage Error of the LED current versus PWM duty cycle
(at 200 Hz PWM frequency).
150
100
50
0
0
20
40
60
80
100
PWM Duty Cycle, D (%)
Figure 14. Output current versus duty cycle; 200 kHz APWM signal.
APWM
ISET
RISET
PWM
ISET
Current
Mirror
A8502
Current
Adjust
LED
Driver
Figure 13. Simplified block diagram of the APWM and ISET circuit.
–15
–10
–5
0
0
20
40
60
80
100
PWM Duty Cycle, D (%)
Figure 15. Percentage Error of the LED current versus PWM duty cycle;
200 kHz APWM signal.
Allegro MicroSystems, Inc.
16
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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