10
LT3491
3491fa
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 7 is approximately 30µs for a 3V
input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
ƒ
ƒ
==
== =
100 30
11
100
001
Hz t µs
ts
D
SETTLE
PERIOD
,
.
iim Range
t
t
s
µs
Min
PERIOD
SETTLE
.
:===
001
30
300 1
•
.
•Duty Cycle
t
t
µs
s
SETTLE
PERIOD
==100
30
001
1000 0 3
100 0 3 100
=
= →
.%
%.%Duty Cycle Range at Hz
The calculations show that for a 100Hz signal the dimming
range is 300 to 1. In addition, the minimum PWM duty
cycle of 0.3% ensures that the LED current has enough
time to settle to its final value. Figure 8 shows the dimming
range achievable for three different frequencies with a
settling time of 30µs.
The dimming range can be further extended by changing
the amplitude of the PWM signal. The height of the PWM
signal sets the commanded sense voltage across the
sense resistor through the CTRL pin. In this manner both
analog dimming and direct 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 height of the CTRL
signal. For the four LED application described above, the
LEDs can be dimmed first, modulating the duty cycle of the
PWM signal. Once the minimum duty cycle is reached, the
height of the PWM signal can be decreased below 1.5V
APPLICATIO S I FOR ATIO
WUUU
down to 100mV. The use of both techniques together
allows the average LED current for the four LED applica-
tion to be varied from 20mA down to less than 20µA.
Figure 9 shows the application for dimming using both
analog dimming and PWM dimming. A potentiometer
must be added to ensure that the gate of the NMOS
receives a logic-level signal, while the CTRL signal can be
adjusted to lower amplitudes.
PWM DIMMING RANGE
1 10 100 1000
3491 F08
100Hz
10kHz
1kHz
Figure 8. Dimming Range Comparison
of Three PWM Frequencies
CTRL
V
IN
L1
10µH
V
IN
3V TO 5V
R
SENSE
10Ω
3491 F09
LT3491
SW
CAP
LED
100k
GND
C2
1µF
Q1
Si2302
C1
1µF
PWM
FREQ
0V
2.5V
Figure 9. Li-Ion to Four White LEDs with Both
PWM Dimming and Analog Dimming
