LT3476 LINER [Linear Technology], LT3476 Datasheet - Page 10

• Manufacturer Part Number: LT3476
• Description: High Current Quad Output LED Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT3476
Programming the LED Current
The LED Current is programmed using an external sense
resistor in series with the load. This method allows fl ex-
ibility in driving the load (i.e., sensing one of several
parallel strings) while maintaining good accuracy. The
V
the external sense resistor between 10mV and 120mV.
A 1.05V reference output (REF) is provided to drive the
V
directly to REF to give the full-scale threshold of 105mV.
A DAC may also be used to drive the V
pins should not be left open. If the V
to a voltage higher than 1.25V, the default regulation
threshold across CAP and LED is 125mV (typ). The V
pin can also be used in conjunction with a PTC thermistor
to provide overtemperature protection for the LED load
as shown in Figure 2.
10
ADJ
ADJ
pins either through a resistor divider, or connected
input sets the voltage regulation threshold across
Figure 2. Overtemperature Protect Circuit
V
REF
U
470
PTC
20k
U
1.05V
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W
25k
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input is connected
V
pins. The V
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1-4
U
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Dimming Control
There are two methods to control the current source for
dimming using the LT3476. The fi rst 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
PWM signal goes high. The minimum PWM on- or off-time
will depend on the choice of operating frequency through
the R
PWM low or high time should be at least ten switching
cycles. This guideline has two reasons: fi rst 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 fi ve switching cycles if a disconnect switch
is used in the LED current path.
The second method of dimming control uses the V
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.
C
node. This feature minimizes recovery time when the
T
input pin. For best current accuracy, the minimum
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pin