lt3591 Linear Technology Corporation, lt3591 Datasheet - Page 10

• Manufacturer Part Number: lt3591
• Description: White Led Driver With Integrated Schottky In 3mm X 2mm Dfn
• Manufacturer: Linear Technology Corporation

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
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.
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.
Example:
D D im Range
Mi
Duty Cycle Range
10
t
LED
PERIOD
n n Duty Cycle
100
= 200mV/R
500mA/DIV
20mA/DIV
5V/DIV
Hz t
PWM
I
LED
Figure 7. Direct PWM Dimming Waveforms
I
,
L
1
V
10 LEDs
IN
SETTLE
= 3.6V
t
t
100
PERIOD
SETTLE
SENSE
1
t
t
SETTLE
PERIOD
100
0 01
120
fl ows through the LEDs. When the
.
120
%
0 01
s
µs
.
•
2ms/DIV
100
µs
s
1 2
. %
83 1
120
0 01
:
.
at
µs
s
100
• •
100 1 2
H H z
3591 F07
. %
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 fi nal value. Figure 8 shows the dimming
range achievable for different frequencies with a settling
time of 120µs.
In addition to extending the dimming range, PWM dimming
improves the effi ciency of the converter for LED currents
below 20mA. Figure 9 shows the effi ciency for traditional
analog dimming of the front page application and PWM
dimming of the application in Figure 6.
Figure 9. PWM vs Analog Dimming Effi ciency
10000
1000
100
Figure 8. Dimming Range vs Frequency
10
80
75
70
65
60
55
1
10
0
PWM DIMMING
PWM DIMMING FREQUENCY (Hz)
ANALOG DIMMING
PULSING MAY BE VISIBLE
5
LED CURRENT (mA)
100
10
1000
15
V
10 LEDs
IN
= 3.6V
3591 F08
3591 F09
10000
20
3591f