LT3498
APPLICATIONS INFORMATION—LED DRIVER
Direct PWM Dimming
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
Changing the forward current flowing in the LEDs not only
changestheintensityoftheLEDs,italsochangesthecolor.
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.
are connected to ground and a current of I
SENSE1
= 200mV /
LED
R
flows 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 6 shows the PWM dimming
waveforms for the circuit in Figure 5.
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.
I
LED
20mA/DIV
I
L
200mA/DIV
PWM
5V/DIV
3498 F06
2ms/DIV
V
= 3V
IN
4 LEDs
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.
Figure 6. Direct PWM Dimming Waveforms
80
V
= 3.4V
IN
4 LEDs
75
100Hz = PWM
V
IN
3V TO 5V
C
IN
R
70
65
SENSE1
10Ω
1µH
L1
15µH
CAP1 SW1
V
SW2 CAP2 V
IN OUT2
60
55
50
LT3498
C
OUT1
1µF
LED1
CTRL1
CTRL2
FB2
GND1
GND2
Q1
Si2304BDS
0
2
4
6
8
10 12 14 16 18 20
LED CURRENT (mA)
5V
0V
100k
3498 F07
3498 F05
Figure 7. PWM Dimming Efficiency
PWM
FREQ
Figure 5. Li-Ion to Four White LEDs
with Direct PWM Dimming
3498f
12
Linear [ Linear ]
