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LT3591
APPLICATIONS INFORMATION
and ground as shown in Figure 6. A Si2308 MOSFET can
be used since its source is connected to ground. The PWM
signal is applied to the CTRL pin of the LT3591 and the gate
of the MOSFET. The PWM signal should traverse between
0V to 5V, to ensure proper turn on and off of the driver
and the NMOS transistor Q1. When the PWM signal goes
high, the LEDs are connected to ground and a current of
I LED = 200mV/R SENSE ?ows through the LEDs. When the
PWM signal goes low, the LEDs are disconnected and
turn off. The MOSFET ensures that the LEDs quickly turn
off without discharging the output capacitor which in turn
allows the LEDs to turn on faster. Figure 7 shows the PWM
dimming waveforms for the circuit in Figure 6.
PWM
5V/DIV
The calculations show that for a 100Hz signal the dimming
range is 83 to 1. In addition, the minimum PWM duty cycle
of 1.2% ensures that the LED current has enough time
to settle to its ?nal value. Figure 8 shows the dimming
range achievable for different frequencies with a settling
time of 120μs.
10000
1000
PULSING MAY BE VISIBLE
100
10
I L
500mA/DIV
1
10
100 1000
10000
PWM DIMMING FREQUENCY (Hz)
3591 F08
I LED
20mA/DIV
Figure 8. Dimming Range vs Frequency
V IN = 3.6V
2ms/DIV
3591 F07
10 LEDs
Figure 7. Direct PWM Dimming Waveforms
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 120μs for a
In addition to extending the dimming range, PWM dimming
improves the ef?ciency of the converter for LED currents
below 20mA. Figure 9 shows the ef?ciency for traditional
analog dimming of the front page application and PWM
dimming of the application in Figure 6.
t PERIOD = = = 0 . 01 s
3.6Vinputvoltage.Theachievabledimmingrangeforthis
application and 100Hz PWM frequency can be determined
using the following method.
Example:
? = 100 Hz , t SETTLE = 120 μs
1 1
? 100
80
75
70
65
60
PWM DIMMING
ANALOG DIMMING
V IN = 3.6V
10 LEDs
D im Range = = = 83 : 1
t SETTLE
t PERIOD 0 . 01 s
120 μs
55
0
5
10
LED CURRENT (mA)
15
20
3591 F09
Mi n Duty Cycle =
t SETTLE
t PERIOD
? 100 =
120 μs
0 . 01 s
? 100 = 1 . 2 %
Figure 9. PWM vs Analog Dimming Ef?ciency
Duty Cycle Range = 100 % → 1 . 2 % at 100 H z
10
3591f
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