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LT3497
APPLICATIONS INFORMATION
converter turns off. The MOSFETs ensure that the LEDs
quickly turn off without discharging the output capacitors
which in turn allows the LEDs to turn on faster. Figures 7
and 8 show the PWM dimming waveforms and ef?ciency
for the Figure 6 circuit.
The time it takes for the LEDs current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LEDs current in Figure 7 is approximately 40μs for a
3V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
? = 100Hz, t SETTLE = 40μs
t PERIOD = 1/? = 1/100 = 0.01s
Dim Range = t PERIOD /t SETTLE = 0.01s/40μs = 250:1
Min Duty Cycle = t SETTLE /t PERIOD ? 100
= 40μs/0.01s = 0.4%
Duty Cycle Range = 100% → 0.4% at 100Hz
The calculations show that for a 100Hz signal the dimming
range is 250 to 1. In addition, the minimum PWM duty
cycle of 0.4% ensures that the LEDs current has enough
3V TO 5V
1 μ F
L1
15 μ H
L2
15 μ H
SW1
CAP1
V IN
SW2
CAP2
1 μ F
R SENSE1
10 ?
LT3497
R SENSE2
10 ?
1 μ F
LED1 LED2
CTRL1 GND CTRL2
Q1
Si2318DS
Q2
Si2318DS
100k
5V
5V
100k
0V
PWM
PWM
0V
3497 F06
FREQ
FREQ
Figure 6. Li-Ion to 4/4 White LEDs with Direct PWM Dimming
I LED
20mA/DIV
I L
200mA/DIV
PWM
5V/DIV
80
78
76
74
V IN = 3.6V
4/4 LEDs
V IN = 3.6V
2ms/DIV
3497 F07
72
4 LEDs
Figure 7. Direct PWM Dimming Waveforms
70
0
5
10
15
20
LED CURRENT (mA)
3497 F08
Figure 8. Ef?ciency
3497f
12
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