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LT3477E

3A, DC/DC Converter with Dual Rail-to-Rail Current Sense

Linear Technology 

LT3477

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 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.

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.

100

RT = 6.81k

10

1

0.1 VIN = 5V

BOOST

4 LEDs

PWM FREQUENCY = 100Hz

0.01

0.1

1

10

100

PWM DUTY CYCLE (%)

3477 F05

Figure 5. LED Current vs PWM Duty Cycle

Wide Dimming Range (500:1)

1000 RT = 6.81k

100

10

1

0.1

1

10

100

PWM FREQUENCY (kHz)

3477 F06

Figure 6. Dimming Range vs PWM Frequency

3477fb

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