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LT3474/LT3474-1

3474fd

APPLICATIONS INFORMATION

To minimize the error from variations in VADJ pin current,

use resistors with a parallel resistance of less than 4k. Use

resistors with a series resistance of 5.11k or greater so as

not to exceed the 250μA current limit on the REF pin.

Dimming Control

There are several different types of dimming control cir-

cuits. One dimming control circuit (Figure 6) changes the

voltage on the VADJ pin by tying a low on-resistance FET to

the resistor divider string. This allows the selection of two

different LED currents. For reliable operation, program an

LED current of no less than 35mA. The maximum current

dimming ratio (IRATIO) can be calculated from the maxi-

mum LED current (IMAX) and the minimum LED current

(IMIN) as follows:

MAX

MIN

RATIO

Another dimming control circuit (Figure 7) uses the PWM

pin and an external NFET tied to the cathode of the LED.

When the PWM signal goes low, the NFET turns off, turn-

ing off the LED and leaving the output capacitor charged.

The PWM pin is pulled low as well, which disconnects the

VC pin, storing the voltage in the capacitor tied there. Use

the C-RC string (tied to the VC pin) shown in Figure 7 for

proper operation during start-up. When the PWM pin goes

high again, the LED current returns rapidly to its previous

on state since the compensation and output capacitors are

at the correct voltage. This fast settling time allows The

LT3474 to maintain diode current regulation with PWM

pulse widths as short as 40μs. If the NFET is omitted and

the cathode of the LED is instead tied to GND, use PWM

pulse widths of 1ms or greater. The maximmum PWM

dimming ratio (PWMRATIO) can be calculated from the

maximum PWM period (tMAX) and minimum PWM pulse

width (tMIN) as follows:

PWM

MAX

MIN

RATIO

Total dimming ratio (DIMRATIO) is the product of the PWM

dimming ratio and the current dimming ratio.

Example: IMAX = 1A, IMIN = 0.1A, tMAX = 12ms, tMIN = 40μs

RATIO =

0.1A

=10:1

PWM

RATIO =

12ms

40μs

= 300:1

DIM

RATIO = 10 • 300 = 3000 :1

REF

VADJ

GND

LT3474

3474 F05

DIM

Figure 6. Dimming with an NFET and Resistor Divider

PWM

LED

10k

3.3nF

GND

LT3474

3474 F06

PWM

60Hz TO

10kHz

0.1μF

Figure 7. Dimming Using PWM Signal

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