LT3474/LT3474-1
APPLICATIONS INFORMATION
the C-RC string (tied to the V pin) shown in Figure 7 for
To minimize the error from variations in V pin current,
C
ADJ
properoperationduringstart-up. WhenthePWMpingoes
high again, the LED current returns rapidly to its previous
onstatesincethecompensationandoutputcapacitorsare
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
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 pin by tying a low on-resistance FET to
ADJ
dimming ratio (PWM
) can be calculated from the
RATIO
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
maximum PWM period (t
) and minimum PWM pulse
MAX
width (t ) as follows:
MIN
dimming ratio (I
) can be calculated from the maxi-
MAX
RATIO
mum LED current (I
tMAX
tMIN
= PWMRATIO
) and the minimum LED current
(I ) as follows:
MIN
Total dimming ratio (DIM
dimming ratio and the current dimming ratio.
) is the product of the PWM
RATIO
IMAX
IMIN
= IRATIO
Example: I
= 1A, I = 0.1A, t
= 12ms, t = 40μs
MAX
MIN
MAX
MIN
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
1A
0.1A
IRATIO
=
=10:1
12ms
PWMRATIO
=
=300:1
40μs
V pin, storing the voltage in the capacitor tied there. Use
C
DIMRATIO =10•300=3000:1
REF
R1
R2
LT3474
GND
PWM
60Hz TO
10kHz
PWM
LED
V
ADJ
LT3474
GND
3474 F05
DIM
3.3nF 10k
0.1μF
3474 F06
Figure 6. Dimming with an NFET and Resistor Divider
Figure 7. Dimming Using PWM Signal
3474fd
16
Linear [ Linear ]
