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LT3476
APPLICATIONS INFORMATION
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 sufficient.
Table 3 shows some recommended capacitor vendors.
Table 3. Low-ESR Surface Mount Capacitors
capacitor that is 1:1000 the value of the compensation
capacitor. In the buck configuration, an additional tech-
nique is available. The filter capacitor between the CAP
node and the LED bottom (see the Typical Application on
the first 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.
VENDOR
TYPE
SERIES
Diode Selection
Taiyo-Yuden Ceramic X5R, X7R
AVX Ceramic X5R, X7R
Murata Ceramic X5R, X7R
Compensation Design
The LT3476 uses an internal transconductance error
amplifier whose V C output compensates the control loop.
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 C are selected to optimize control loop
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 filter capacitor (1μF or
less) across the load is desirable. In this case, a 10nF
compensation capacitor at V C is usually quite adequate.
A compensation resistor of 5kΩ placed between the V C
output and the compensation capacitor minimizes channel-
to-channel interaction by reducing transient recovery time.
The boost configuration 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 ON less than 350ns) and low
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
The Schottky rectifier conducts current during the interval
when the switch is turned off. Select a diode with V R rated
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 F through the diode is a function
of the switch duty cycle, so select a diode with forward
current rating of I F = 1.5A ? (1-D). If using the PWM fea -
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
V R I AVE V F AT 1A
PART NUMBER (V) (A) (mV)
On Semiconductor
MBRM140 40 1 550
Diodes Inc.
DFLS140L 40 1 550
B140 HB 40 1 530
NXP Semiconductor
PMEG4010EJ 40 1 540
Programming the LED Current
The LED Current is programmed using an external sense
resistor in series with the load. This method allows flex-
ibility in driving the load (i.e., sensing one of several parallel
strings) while maintaining good accuracy. The V ADJ input
sets the voltage regulation threshold across the external
sense resistor between 10mV and 120mV. A 1.05V refer-
ence output (REF) is provided to drive the V ADJ pins either
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LT3476IUHF#TRPBF
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LT3477
制造商:LINER 制造商全称:Linear Technology 功能描述:Full-Featured LED Driver with 1.5A Switch Current