LT3476
10
3476fb
through a resistor divider, or connected directly to REF to
give the full-scale threshold of 105mV. A DAC may also be
used to drive the V
ADJ
pins. The V
ADJ
pins should not be
left open. If the V
ADJ
input is connected to a voltage higher
than 1.25V, the default regulation threshold across CAP
and LED is 125mV (typ). The V
ADJ
pin can also be used
in conjunction with a PTC thermistor to provide overtem-
perature protection for the LED load as shown in Figure 2.
Dimming Control
There are two methods to control the current source for
dimming using the LT3476. The first method, popular
with LED applications, uses the PWM pin to modulate the
current source between zero and full current to achieve
a precisely programmed average current. To make this
method of current control more accurate, during the qui-
escent phase the switch demand current is stored on the
V
C
node. This feature minimizes recovery time when the
PWM signal goes high. The minimum PWM on- or off-time
will depend on the choice of operating frequency through
the RT input pin. For best current accuracy, the minimum
PWM low or high time should be at least ten switching
cycles. This guideline has two reasons: first to allow the
output to reach steady state before shutting off, and second
because the oscillator is not synchronized to the PWM
signal and there may be as much as one switching cycle
delay from PWM going high to the start of switching. This
delay, however, does not apply to the negative transition
of the PWM signal. The minimum PWM low/high time can
be reduced to five switching cycles if a disconnect switch
is used in the LED current path.
The second method of dimming control uses the V
ADJ
pin
to linearly adjust the current sense threshold during the
PWM high state. The LED current programming feature
augments the PWM dimming control, possibly increasing
total dimming range by a factor of ten.
V
REF
20k 25k
470
PTC
V
ADJ
1-4
3476 F01
Figure 2. Overtemperature Protect Circuit
ApplicAtions inForMAtion
