lt3476euhf-trpbf Linear Technology Corporation, lt3476euhf-trpbf Datasheet - Page 8

• Manufacturer Part Number: lt3476euhf-trpbf
• Description: High Current Quad Output Led Driver
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3476
will give better dimming control since for turning on the
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) confi gurations, and using a 21kΩ resistor on
R
recommended for most applications. In the buck mode,
the inductor value can be estimated using the formula:
V
input voltage to the converter. In the boost mode, the
inductor value can be estimated using the formula:
V
the LED string. Table 2 below provides some suggested
components and vendors.
Table 2. Inductors
PART NUMBER
Sumida
CDRH6D38-100
CDRH5D28-5R3
CDRH73-100
Toko
D63CB
D63CB
Cooper-ET
SD25-4R7
8
T
LED
IN
L(μH) =
D
L(μH) =
D
(T
is the input voltage and V
BUCK
BOOST
SW
is the voltage across the LED string and V
~ 1μs), inductor values from 4.7μH to 10μH are
=
=
D
D
V
V
BUCK
BOOST
V
LED
CAP
CAP
V
VALUE
(μH)
CAP
5.3
4.7
4.7
10
10
10
U
• T
− V
• T
SW
ΔI
SW
IN
(μS) • (V
U
(μS) • V
IRMS
ΔI
1.90
1.68
1.49
2.08
1.80
(A)
2.0
CAP
CAP
IN
is the voltage across
W
,
0.028
0.028
0.072
0.042
0.026
0.047
− V
DCR
(Ω)
LED
)
U
,
CAP
HEIGHT
(mm)
4.0
3.0
3.4
3.5
3.5
2.5
is the
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
1μF , or greater, capacitor with low ESR should be used.
A ceramic capacitor is usually the best choice.
In the buck confi guration, 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,
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor placed close to the Schottky and the ground
plane is usually suffi cient for each channel.
Output Capacitor Selection
The selection of output fi lter capacitor depends on the load
and the converter confi guration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output fi lter 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:
A typical fi lter capacitor value for R
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
For the LED boost applications, to achieve the same LED
ripple current the required fi lter capacitor value is about
fi ve 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
As the output capacitor is subject to high ripple current,
ceramic capacitors are recommended due to their low
ESR and ESL at high frequency.
I
C
RMS
FILT
=
= 2 •
I
SW
C
R
is usually suffi cient for each channel.
•
T
LED
SW
(
(
1
−
D D
) •
)
IN
LED
pin of the LT3476. A
C
= 5Ω and T
input.
SW
3476fa
=