Datasheet

7
LTC3490
3490fa
APPLICATIO S I FOR ATIO
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The LTC3490 requires only four external components to
operate: an inductor, an output capacitor, a switch and a
pull-down resistor. The inductor is nominally set at 3.3µH
and the capacitor at 4.7µF. Optional components include
an input capacitor and dimming resistors.
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3490 allows the
use of small surface mount inductors. The minimum
inductance value is proportional to the operating fre-
quency and is limited by the following constraints:
L
f
H
and
L
VV V
f Ripple V
H
IN MIN OUT MAX IN MIN
OUT MAX
≥
≥
()
3
() ( ) ()
()
•–
••
where:
f = Operating Frequency (Hz)
Ripple = Inductor Current Ripple (A)
V
IN(MIN)
= Minimum Input Voltage (V)
V
OUT(MAX)
= Maximum Output Voltage (V)
The inductor current ripple is typically set to 20% to 40%
of the inductor current.
The peak inductor current is given by:
II
VIRRI
VRI
VV V
LfV
LPK OUT
OUT OUT P N IN
IN N IN
IN OUT IN
OUT
=
+
()
+
()
•–•
–•
–
•••2
where:
V
IN
= Input Voltage (V)
V
OUT
= Output Voltage (V)
I
OUT
= LED Drive Current (A)
I
IN
= Input Current = V
OUT
/V
IN
• I
OUT
(A)
R
P
= R
DSON
of the PFET Switch (Ω)
R
N
= R
DSON
of the NFET Switch (Ω)
For high efficiency, choose an inductor with a high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I
2
R losses and must be
able to handle the peak inductor current at full load without
saturating. In single cell applications, the inductor ESR
must be below 25mΩ to keep the efficiency up and
maintain output current regulation. Dual cell applications
can tolerate significantly higher ESR (up to 75mΩ) with
minimal efficiency degradation. Molded chokes or chip
inductors usually do not have enough core to support the
peak inductor currents in the 1A to 2A region. If radiated
noise is an issue, use a toroid, pot core or shielded bobbin
inductor to minimize radiated noise. See Table 1 for a list
of suggested inductors. Look closely at the manufacturers
data sheets; they specify saturation current differently.
Table 1. Inductor Information
INDUCTOR PART NUMBER ESR (mΩ) SATURATION CURRENT (A)
TOKO A918CY-3R3M 47 1.97
TYCO DN4835-3R3M 58 2.15
TDK SLF7045T-3R3M2R5 20 2.5
Output Capacitor Selection
The output capacitor value and equivalent series resis-
tance (ESR) are the primary factors in the output ripple.
The output ripple is not a direct concern for LED drive as
the LED will operate at the average current value. However
the peak pulsed forward current rating of the LED must not
be exceeded to avoid damaging the LED.