Datasheet

LT3497
12
3497f
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 effi 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:
ƒ = 100Hz, t
SETTLE
= 40μs
t
PERIOD
= 1/ƒ = 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
Figure 6. Li-Ion to 4/4 White LEDs with Direct PWM Dimming
Figure 7. Direct PWM Dimming Waveforms
APPLICATIONS INFORMATION
SW1 V
IN
LT3497
GND
SW2
L2
15µH
R
SENSE1
10
Q1
Si2318DS
Q2
Si2318DS
R
SENSE2
10
1µF
1µF
3497 F06
1µF
3V TO 5V
L1
15µH
CAP1 CAP2
LED1 LED2
CTRL1 CTRL2
100k 100k
0V
PWM
FREQ
5V
PWM
FREQ
0V
5V
I
LED
20mA/DIV
I
L
200mA/DIV
PWM
5V/DIV
V
IN
= 3.6V
4 LEDs
2ms/DIV
3497 F07
LED CURRENT (mA)
0
EFFICIENCY (%)
74
76
20
3497 F08
72
70
5
10
15
80
V
IN
= 3.6V
4/4 LEDs
78
Figure 8. Effi ciency