LT3474/LT3474-1
16
3474fd
APPLICATIONS INFORMATION
To minimize the error from variations in V
ADJ
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 V
ADJ
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 (I
RATIO
) can be calculated from the maxi-
mum LED current (I
MAX
) and the minimum LED current
(I
MIN
) as follows:
I
I
I
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
V
C
pin, storing the voltage in the capacitor tied there. Use
the C-RC string (tied to the V
C
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 (PWM
RATIO
) can be calculated from the
maximum PWM period (t
MAX
) and minimum PWM pulse
width (t
MIN
) as follows:
t
t
PWM
MAX
MIN
RATIO
=
Total dimming ratio (DIM
RATIO
) is the product of the PWM
dimming ratio and the current dimming ratio.
Example: I
MAX
= 1A, I
MIN
= 0.1A, t
MAX
= 12ms, t
MIN
= 40μs
I
RATIO
=
1A
0.1A
=10:1
PWM
RATIO
=
12ms
40μs
= 300:1
DIM
RATIO
=10 •300 = 3000:1
REF
V
ADJ
GND
LT3474
3474 F05
R1
R2
DIM
Figure 6. Dimming with an NFET and Resistor Divider
PWM
LED
10k3.3nF
GND
LT3474
3474 F06
PWM
60Hz TO
10kHz
0.1μF
Figure 7. Dimming Using PWM Signal
