LT3477 LINER [Linear Technology], LT3477 Datasheet - Page 11

• Manufacturer Part Number: LT3477
• Description: 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
PWM Dimming
For LED applications where a wide dimming range is
required, two competing methods are available: analog
dimming and PWM dimming. The easiest method is to
simply vary the DC current through the LED—analog
dimming—but changing LED current also changes its
chromaticity, undesirable in many applications. The better
method is PWM dimming, which switches the LED on and
off, using the duty cycle to control the average current.
PWM dimming offers several advantages over analog
dimming and is the method preferred by LED manufactur-
ers. By modulating the duty cycle of the PWM signal, the
average LED current changes proportionally as illustrated
in Figure 5. The chromaticity of the LEDs remains un-
changed in this scheme since the LED current is either zero
or at programmed current. Another advantage of PWM
dimming over analog dimming is that a wider dimming
range is possible.
The LT3477 is a DC/DC converter that is ideally suited for
LED applications. For the LT3477, analog dimming offers
a dimming ratio of about 10:1; whereas, PWM dimming
with the addition of a few external components results in
a wider dimming range of 500:1. The technique requires a
PWM logic signal applied to the gate of both NMOS (refer
to Figure 7). When the PWM signal is taken high the part
runs in normal operation and I
0.01
100
0.1
10
Figure 5. LED Current vs PWM Duty Cycle
Wide Dimming Range (500:1)
1
0.1
R
V
BOOST
4 LEDs
PWM FREQUENCY = 100Hz
T
IN
= 6.81k
= 5V
U
PWM DUTY CYCLE (%)
1
U
LED
10
= 100mV/R
W
3477 F05
100
U
SENSE
runs
through the LEDs. When the PWM input is taken low, the
LEDs are disconnected and turn off. This unique external
circuitry produces a fast rise time for the LED current,
resulting in a wide dimming range of 500:1 at a PWM
frequency of 100Hz.
The LED current can be controlled by feeding a PWM
signal with a broad range of frequencies. Dimming below
80Hz is possible, but not desirable, due to perceptible
flashing of LEDs at lower PWM frequencies. The LED
current can be controlled at higher frequencies, but the
dimming range decreases with increasing PWM frequency,
as seen in Figure 6.
PWM dimming can be used in Boost (shown in Figure 7),
Buck mode (shown in Figure 8) and Buck-Boost mode
(shown in Figure 9). For the typical boost topology,
efficiency exceeds 80%. Buck mode can be used to in-
crease the power handling capability for higher current
LED applications. A Buck-Boost LED driver works best in
applications where the input voltage fluctuates to higher or
lower than the total LED voltage drop.
In high temperature applications, the leakage of the Schot-
tky diode D1 increases, which in turn, discharges the
output capacitor during the PWM “off” time. This results
in a smaller effective LED dimming ratio. Consequently,
the dimming range decreases to about 200:1 at 85°C.
Figure 6. Dimming Range vs PWM Frequency
1000
100
10
1
0.1
R
T
= 6.81k
PWM FREQUENCY (kHz)
1
10
3477 F06
100
LT3477
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