LTC3545_15 Datasheet, PDF (12/20 Pages) Linear Technology – Triple 800mA Synchronous Step-Down Regulator–2.25MHz

LTC3545/LTC3545-1
APPLICATIONS INFORMATION
The basic LTC3545/LTC3545-1 application circuit is shown
on the first page of this data sheet. External component
selection is driven by the load requirement and begins
with the selection of L followed by C_IN and C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1μH to 10μH. Its value is chosen based on the
desired ripple current. Large inductor values lower ripple
current and small inductor values result in higher ripple
currents. Higher V_IN or V_OUT also increases the ripple
current as shown in Equation 1. A reasonable starting
point for setting ripple current for an 800mA regulator is
ΔI_L = 320mA (40% of 800mA).
ΔI_L
=
1
(ƒ)(L)
V_OUT
⎛
⎝⎜ ^1–
V_OUT
V_IN
⎞
⎠⎟
(1)
The DC current rating of the inductor should be at least equal
to the maximum load current plus half the ripple current
to prevent core saturation. Thus, a 960mA rated inductor
should be enough for most applications (800mA + 160mA).
For better efficiency, choose a low DCR inductor.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI requirements
than on what the LTC3545/LTC3545-1 require to operate.
Table 1 shows typical surface mount inductors that work
well in LTC3545/LTC3545-1 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
(μH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Wurth WE-
1.5
TPC 744031 2.5
3.6
0.035
0.045
0.065
1.75
3.8 × 3.8 × 1.65
1.45
1.38
CoilCraft
LPS4012
1
0.06
1.5
0.07
2.2
0.1
3.3
0.1
2.5
4.0 × 4.0 × 1.1
2.5
2.1
1.5
Sumida
1.4 0.055
1.8
4.0 × 4.0 × 1.2
CDH38D11/ 2.4 0.094
1.3
SLD
3.6
0.13
1.1
Sumida
1.5 0.043
1.55
3.8 × 3.8 × 1.8
CDRH3D16 2.2 0.075
1.2
3.3
0.11
1.1
C_IN and C_OUT Selection
In continuous mode, a worst-case estimate for the input
current ripple can be determined by assuming that the
source current of the top MOSFET is a square wave of
duty cycle V_OUT/V_IN, and amplitude I_OUT(MAX). To prevent
large voltage transients, a low ESR input capacitor sized for
the maximum RMS current must be used. The maximum
RMS capacitor current is given by:
( ) _{IRMS ≅ IOUT(MAX)}
V_OUT V_IN – V_OUT
V_IN
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life (non-ceramic capacitors). This makes it advisable to
further de-rate the capacitor, or choose a capacitor rated
at a higher temperature than required. Always consult the
manufacturer if there is any question.
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