LT3591
APPLICATIONS INFORMATION
and ground as shown in Figure 6. A Si2308 MOSFET can
beusedsinceitssourceisconnectedtoground. ThePWM
signalisappliedtotheCTRLpinoftheLT3591andthegate
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
Thecalculationsshowthatfora100Hzsignalthedimming
rangeis83to1. Inaddition, theminimumPWMdutycycle
of 1.2ꢀ ensures that the LED current has enough time
to settle to its final value. Figure 8 shows the dimming
range achievable for different frequencies with a settling
time of 120µs.
10000
I
= 200mV/R
flows through the LEDs. When the
LED
SENSE
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.
1000
PULSING MAY BE VISIBLE
100
10
PWM
5V/DIV
I
L
1
500mA/DIV
10
100
1000
10000
PWM DIMMING FREQUENCY (Hz)
3591 F08
I
LED
20mA/DIV
Figure 8. Dimming Range vs Frequency
3591 F07
V
= 3.6V
2ms/DIV
IN
10 LEDs
Inadditiontoextendingthedimmingrange,PWMdimming
improves the efficiency of the converter for LED currents
below 20mA. Figure 9 shows the efficiency for traditional
analog dimming of the front page application and PWM
dimming of the application in Figure 6.
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
3.6V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
80
PWM DIMMING
75
70
Example:
ƒ =100Hz, tSETTLE =120µs
65
ANALOG DIMMING
1
1
60
tPERIOD
=
=
= 0.01s
V
= 3.6V
ƒ 100
IN
10 LEDs
55
10
0
5
15
20
tPERIOD
tSETTLE 120µs
0.01s
Dim Range=
=
= 83:1
LED CURRENT (mA)
3591 F09
Figure 9. PWM vs Analog Dimming Efficiency
tSETTLE
tPERIOD
120µs
0.01s
Min Duty Cycle =
•100=
•100=1.2%
Duty Cycle Range =100%→1.2% at 100Hz
3591f
10
Linear Systems [ Linear Systems ]
