LT3475/LT3475-1
14
3475fb
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load current generally goes to zero
once the circuit has started. The typical performance char-
acteristics section shows a plot of minimum load to start
and to run as a function of input voltage. Even without an
output load current, in many cases the discharged output
capacitor will present a load to the switcher that will allow
it to start. The plots show the worst case, where V
IN
is
ramping very slowly.
Programming LED Current
The LED current can be set by adjusting the voltage on the
V
ADJ
pin. For a 1.5A LED current, either tie V
ADJ
to REF or
to a 1.25V source. For lower output currents, program the
V
ADJ
using the following formula: I
LED
= 1.5A • V
ADJ
/1.25V.
Voltages less than 1.25V can be generated with a voltage
divider from the REF pin, as shown in Figure 6. In order
to have accurate LED current, precision resistors are
preferred (1% or better is recommended). Note that the
V
ADJ
pin sources a small amount of bias current, so use
the following formula to choose resistors:
R2 =
V
ADJ
1.25V – V
ADJ
R1
+ 50nA
To minimize the error from variations in V
ADJ
pin current,
use resistors with a parallel resistance of less than 4k. Use
resistor strings with a high enough series resistance so as not
to exceed the 500μA current compliance of the REF pin.
Dimming Control
There are several different types of dimming control
circuits. One dimming control circuit (Figure 7) changes
the voltage on the V
ADJ
pin by tying a low on resistance
FET to the resistor divider string. This allows the se-
lection of two different LED currents. For reliable op-
eration program an LED current of no less than 50mA.
The maximum current dimming ratio (I
RATIO
) can be
calculated from the maximum LED current (I
MAX
) and the
minimum LED current (I
MIN
) as follows:
I
MAX
/I
MIN
= I
RATIO
Another dimming control circuit (Figure 8) uses the PWM
pin and an external NFET tied to the cathode of the LED.
An external PWM signal is applied to the PWM pin and the
gate of the NFET (For PWM dimming ratios of 20 to 1 or
less, the NFET can be omitted). The average LED current is
proportional to the duty cycle of the PWM signal. When the
PWM signal goes low, the NFET turns off, turning 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 shown in Figure 8 and Figure 9 tied to the V
C
pin for
proper operation during startup. 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
PWM
LED
GND
LT3475
3475 F08
PWM
100Hz TO
10kHz
V
C
10k
0.1μF
3.3nF
Figure 8. Dimming Using PWM Signal
REF
V
ADJ
GND
LT3475
3475 F07
R1
R2
DIM
Figure 7. Dimming with a MOSFET and Resistor Divider
Figure 6. Setting V
ADJ
with a Resistor Divider
REF
V
ADJ
GND
LT3475
3475 F06
R1
R2
APPLICATIONS INFORMATION
