LT3476 LINER [Linear Technology], LT3476 Datasheet - Page 9

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

APPLICATIO S I FOR ATIO
Ceramic type capacitors using X7R dielectric are best for
temperature and DC bias stability of the capacitor value.
All ceramic capacitors exhibit loss of capacitance value
with increasing DC voltage bias, so it may be necessary to
choose a higher value capacitor or larger case size to get
the required capacitance at the operating voltage. Always
check that the voltage rating of the capacitor is suffi cient.
Table 3 shows some recommended capacitor vendors.
Table 3. Low-ESR Surface Mount Capacitors
VENDOR
Taiyo-Yuden
AVX
Murata
Compensation Design
The LT3476 uses an internal transconductance error
amplifi er whose V
The external inductor, output capacitor, and compensa-
tion resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at V
stability. The component values shown in the typical ap-
plications circuits yield stable operation over the given
range of input-to-output voltages and load currents. For
most buck applications, a small fi lter capacitor (1μF or
less) across the load is desirable. In this case, a 10nF
compensation capacitor at V
A compensation resistor of 5kΩ placed between the V
output and the compensation capacitor minimizes chan-
nel-to-channel interaction by reducing transient recovery
time. The boost confi guration will have a larger output
capacitor, 2.2μF to 10μF .
The following circuit techniques involving the compensa-
tion pin may be helpful where there is a large variation in
programmed LED current, or a large input supply range is
expected. At low duty cycles (T
average inductor current (less than 500mA), the LT3476
may start to skip switching pulses to maintain output
regulation. Pulse-skipping mode is usually less desirable
because it leads to increased ripple current in the LED.
To improve the onset of pulse-skipping behavior, place a
capacitor between the SW node and the compensation
C
C
U
output compensates the control loop.
are selected to optimize control loop
Ceramic
Ceramic
Ceramic
U
TYPE
C
ON
is usually quite adequate.
less than 350ns) and low
W
X5R, X7R
X5R, X7R
X5R, X7R
SERIES
U
C
capacitor that is 1:1000 the value of the compensation
capacitor. In the buck confi guration, an additional tech-
nique is available. The fi lter capacitor between the CAP
node and the LED bottom (see the Typical Application on
the fi rst page) can be moved to between the LED top and
the LED bottom. This circuit change places the inductor
ripple current through the sense resistor, which improves
pulse-skipping behavior. There is usually less than 1%
impact to the current regulation point.
Diode Selection
The Schottky rectifi er conducts current during the interval
when the switch is turned off. Select a diode with V
for the maximum SW voltage. For boost circuits that may
use the output disconnect feature, the diode should be
rated for at least 40V. It is not necessary that the forward
current rating of the diode equal the switch current limit.
The average current I
of the switch duty cycle, so select a diode with forward
current rating of I
ture for dimming, it may also be important to consider
diode leakage from the output (especially at hot) during
the PWM low interval. Table 4 has some recommended
component vendors.
Table 4. Schottky Diodes
PART NUMBER
On Semiconductor
MBRM140
Diodes Inc.
DFLS140L
B140 HB
Philips Semiconductor
PMEG4010EJ
F
= 1.5A • (1-D). If using the PWM fea-
F
(V)
V
40
40
40
40
through the diode is a function
R
I
(A)
AVE
1
1
1
1
LT3476
V
F
(mV)
550
550
530
540
AT 1A
R
rated
3476fa
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