Datasheet

LT3476
8
3476fb
switch, the state of the PWM pin is sampled only during
a narrow time slot at the beginning of each switch period.
Inductor Selection
The inductors used with the LT3476 should have a satura-
tion current rating of 2.5A or greater. For best loop stability
results, the inductor value selected should provide a ripple
current of 350mA or more. For buck (step-down) or boost
(step-up) configurations, and using a 21kΩ resistor on
R
T
(T
SW
~ 1µs), inductor values from 4.7µH to 10µH are
recommended for most applications. In the buck mode,
the inductor value can be estimated using the formula:
L(µH) =
D
BUCK
• T
SW
(µS) • (V
CAP
− V
LED
)
∆I
,
D
BUCK
=
V
LED
V
CAP
V
LED
is the voltage across the LED string and V
CAP
is the
input voltage to the converter. In the boost mode, the
inductor value can be estimated using the formula:
L(µH) =
D
BOOST
• T
SW
(µS) • V
IN
∆I
,
D
BOOST
=
V
CAP
− V
IN
V
CAP
V
IN
is the input voltage and V
CAP
is the voltage across
the LED string. Table 2 below provides some suggested
components and vendors.
Table 2. Inductors
PART NUMBER
VALUE
(µH)
IRMS
(A)
DCR
(Ω)
HEIGHT
(mm)
Sumida
CDRH6D38-100 10 2.0 0.028 4.0
CDRH5D28-5R3 5.3 1.90 0.028 3.0
CDRH73-100 10 1.68 0.072 3.4
Toko
D63CB 10 1.49 0.042 3.5
D63CB 4.7 2.08 0.026 3.5
Cooper-ET
SD25-4R7 4.7 1.80 0.047 2.5
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
IN
pin of the LT3476. A
1µF, or greater, capacitor with low ESR should be used.
A ceramic capacitor is usually the best choice.
In the buck configuration, the capacitor at the input to the
power converter has large pulsed currents due to the cur-
rent returned through the Schottky diode when the switch
is off. For best reliability, this capacitor should have low
ESR and ESL and meet the ripple current requirement,
I
RMS
= I
SW
• (1− D) • D
( )
where D is the switch duty cycle. A 2.2µF ceramic type
capacitor placed close to the Schottky and the ground
plane is usually sufficient for each channel.
Output Capacitor Selection
The selection of output filter capacitor depends on the load
and the converter configuration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output filter capacitor should be
sized to attenuate the current ripple from the inductor to
35mA or less. The following equation is useful to estimate
the required capacitor value:
C
FILT
= 2 •
T
SW
R
LED
A typical filter capacitor value for R
LED
= 5Ω and T
SW
=
1µs is 0.47µF. For loop stability, consider the output pole
is at the frequency where closed loop gain should be
unity, so the dominant pole for loop compensation will
be established by the capacitor at the V
C
input.
For the LED boost applications, to achieve the same LED
ripple current the required filter capacitor value is about
five times larger than the value calculated above due to
the pulsed nature of the source current. A 2.2µF ceramic
type capacitor placed close to the Schottky and the ground
plane of the I
C
is usually sufficient for each channel.
As the output capacitor is subject to high ripple current,
ceramic capacitors are recommended due to their low
ESR and ESL at high frequency.
ApplicAtions inForMAtion