A8502
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
As an example, a system that delivers a full LED current of
120 mA per LED would deliver 90 mA of current per LED when
an APWM signal is applied with a duty cycle of 25% (figures
16 and 17).
Although the APWM dimming function has a wide frequency
range, if this function is used strictly as an analog dimming
function it is recommended to use frequency ranges between
50 and 500 kHz for best accuracy. The frequency range must be
considered only if the user is not using this function as a closed
loop trim function. Another limitation is that the propagation
delay between this APWM signal and IOUT takes several milli-
seconds to change the actual LED current. This effect is shown in
figures 16-18.
ILED
C1
APWM
C2
ILED
C1
APWM
C2
C3
PWM/EN
t
Figure 16. Diagram showing the transition of LED current from 120 mA
to 90 mA, when a 25% duty cycle signal is applied to the APWM pin;
PWM = 1; shows ILED (ch1, 50 mA/div.), APWM (ch2, 5 V/div.), and
PWM/EN (ch3, 5 V/div.), time = 500 μs/div.
C3
PWM/EN
t
Figure 17. Diagram showing the transition of LED current from 90 mA
to 120 mA, when a 25% duty cycle signal is removed from the APWM pin.
PWM = 1; shows ILED (ch1, 50 mA/div.), APWM (ch2, 5 V/div.), and
PWM/EN (ch3, 5 V/div.), time = 500 μs/div.
C1
IOUT
APWM
C2
C3
PWM/EN
t
Figure 18. Transition of output current level when a 50% duty cycle signal
is applied to the APWM pin, in conjunction with a 50% duty cycle PWM
dimming being applied to the PWM pin; shows IOUT (ch1, 100 mA/div.),
APWM (ch2, 5 V/div.), and PWM/EN (ch3, 5 V/div.), time = 1 ms/div.
Allegro MicroSystems, Inc.
17
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com