LT1937 LINER [Linear Technology], LT1937 Datasheet - Page 12

• Manufacturer Part Number: LT1937
• Description: 20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN
• Manufacturer: LINER [Linear Technology]

LT3498
Direct PWM Dimming
Changing the forward current fl owing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously on. Additionally, by modulating the duty cycle
(amount of “on-time”), the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 5 shows a Li-Ion powered driver for four white
LEDs. Direct PWM dimming method requires an external
NMOS tied between the cathode of the lowest LED in the
string and ground as shown in Figure 5. A simple logic
level Si2304 MOSFET can be used since its source is
connected to ground. The PWM signal is applied to the
CTRL1 pin of the LT3498 and the gate of the MOSFET.
APPLICATIONS INFORMATION—LED DRIVER
12
C
OUT1
1 F
Si2304BDS
R
SENSE1
10
Q1
Figure 5. Li-Ion to Four White LEDs
with Direct PWM Dimming
100k
LED1
CTRL1
CAP1 SW1
PWM
FREQ
5V
0V
GND1
15 H
L1
GND2
LT3498
V
V
IN
IN
SW2
3V TO 5V
CTRL2
CAP2 V
FB2
C
1 H
OUT2
IN
3498 F05
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
R
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 6 shows the PWM dimming
waveforms for the circuit in Figure 5.
SENSE1
200mA/DIV
20mA/DIV
fl ows through the LEDs. When the PWM signal
5V/DIV
Figure 6. Direct PWM Dimming Waveforms
PWM
I
LED
80
75
70
65
60
55
50
I
Figure 7. PWM Dimming Effi ciency
L
0
V
4 LEDs
IN
V
4 LEDs
100Hz = PWM
IN
= 3V
2
= 3.4V
4
LED CURRENT (mA)
6
8
2ms/DIV
10
12
14
16
18
3498 F07
3498 F06
LED
20
= 200mV /
3498f