LT3587
18
3587fc
Figure 16. Dimming Range Comparison
of Three PWM Frequencies
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. Figure 15 shows the average current
variation over duty cycle for a 100Hz PWM frequency with
the circuit in Figure 13.
Notice that at lower end of the duty cycle, the linear rela-
tion between the average LED current and the PWM duty
cycle is no longer preserved. This indicates that the loop
requires a fi xed amount of time to reach its fi nal current.
When the duty cycle is reduced such that the amount of
on time is in the order of or less than this settling time, the
loop no longer has the time to regulate to its fi nal current
before it is turned off again and the initial current before
settling is a larger proportion of the average current.
Depending on how much linearity on the average LED
current is required, the minimum LED on time is chosen
based on the graphs in Figure 15. For example, for ap-
proximately 10% deviation from linearity at the lower
duty cycle, the minimum on time of the LED current is
approximately 320µs for a 3.6V input voltage.
The achievable dimming range for this application with
a 100Hz PWM frequency can be determined using the
following method.
APPLICATIONS INFORMATION
Example:
f = 100Hz → t
PERIOD
= 1/f = 0.01s, t
MIN-ON
= 320µs
Dim Range = t
PERIOD
/t
MIN-ON
= 0.01s/320µs ≈ 30:1
Min Duty Cycle = (t
MIN-ON
/t
PERIOD
) • 100 = 3.2%
Duty Cycle Range = 100% → 3.2% at 100Hz
The calculations show that for a 100Hz signal the dimming
range is 30 to 1. In addition, the minimum PWM duty cycle
of 3.2% ensures that the LED current varies linearly with
duty cycle to within 10%. Figure 16 shows the dimming
range achievable for three different frequencies with a
minimum on time of 320µs.
The dimming range can be further extended by combin-
ing this PWM method with the DAC and resistor method
discussed previously. In this manner both analog dimming
and PWM dimming extend the dimming range for a given
application. The color of the LEDs no longer remains
constant because the forward current of the LED changes
with the output voltage of the DAC. For the six LED ap-
plication described above, the LEDs can be dimmed fi rst
by modulating the duty cycle of the PWM signal with the
DAC output at 0V. Once the minimum duty cycle is reached,
the value of the DAC output voltage can be increased to
further dim the LEDs. The use of both techniques together
allows the average LED current for the six LED application
to be varied from 20mA down to less than 1µA.
PWM DIMMING RANGE
3587 F16
1 10 100
1kHz
300Hz
100Hz
Figure 15. Average LED Current Variation with
PWM Duty Cycle at 100Hz PWM Frequency
DUTY CYCLE (%)
AVERAGE CURRENT (mA)
3587 F15
100
0.1
1
10
0.01
1 10 100
IDEAL
MEASURED
