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LT3477

3477fc

APPLICATIONS INFORMATION

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 bet-

ter 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

manufacturers. 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 unchanged 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 ILED = 100mV/RSENSE 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.

PWMdimmingcanbeusedinboost(showninFigure7),buck

mode (shown in Figure 8) and buck-boost mode (shown

in Figure 9). For the typical boost topology, efﬁciency ex-

ceeds 80%. Buck mode can be used to increase 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

Schottky 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 5. LED Current vs PWM Duty Cycle

Wide Dimming Range (500:1)

Figure 6. Dimming Range vs PWM Frequency

PWM DUTY CYCLE (%)

0.1

100

3477 F05

0.01

0.1

VIN = 5V

BOOST

4 LEDs

PWM FREQUENCY = 100Hz

RT = 6.81k

PWM FREQUENCY (kHz)

0.1

100

1000

100

3477 F06

RT = 6.81k

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