LTC3541EDD-2(RevA) View Datasheet(PDF) - Linear Technology
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Description
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LTC3541EDD-2 Datasheet PDF : 20 Pages
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LTC3541-2
APPLICATIO S I FOR ATIO
The basic LTC3541-2 application circuit is shown on the
first page of this data sheet. External component selection
is driven by the load requirement and requires the selection
of L, followed by CIN, COUT and the selection of the output
capacitor for the VLDO and linear regulator.
BUCK REGULATOR
Inductor Selection
For most applications, the appropriate inductor value will be
2.2µH. Its value is chosen largely based on the desired ripple
current and burst ripple performance. Generally, large value
inductors reduce ripple current, and conversely, small value
inductors produce higher ripple current. Higher VIN or VOUT
may also increase the ripple current as shown in Equation
1. A reasonable starting point for setting ripple current is
ΔIL = 200mA (40% of 500mA).
ΔIL
=
(
1
f)(L
)
VOUT
1−
VOUT
VIN
(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 600mA rated
inductor should be enough for most applications (500mA
+ 100mA). For better efficiency, choose a low DC resis-
tance 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 requirement and any radiated field/EMI requirements
rather than what the LTC3541-2 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3541-2 applications.
Table 2. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm3)
Sumida
1.0
0.025
2
3.9 × 3.9 × 2.4
CDRH3D23
1.5
0.029
1.65
2.2
0.038
1.3
3.3
0.048
1.1
Sumida
CMD4D06
2.2
0.116
3.3
0.174
0.950
0.770
3.5 × 4.3 × 0.8
Coilcraft
ME3220
1.0
0.058
2.7
2.5 × 3.2 × 2.0
1.5
0.068
2.2
2.2
0.104
1.8
3.3
0.138
1.3
Murata
LQH3C
1.0
0.060
1.00
2.5 × 3.2 × 2.0
2.2
0.097
0.79
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. 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:
( ) cIN required IRMS ≅ IOMAX VOUT
VIN − VOUT
VIN
1/2
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple, worst-case condition is com-
monly used for design. Note that the capacitor manu-
facturer’s ripple current ratings are often based on 2000
hours of life. This makes it advisable to further derate the
capacitor or choose a capacitor rated at a higher tempera-
ture than required. Always consult the manufacturer with
any question regarding proper capacitor choice.
The selection of COUT for the buck regulator is driven by
the desired buck loop transient response, required effective
series resistance (ESR) and burst ripple performance.
The LTC3541-2 minimizes the required number of external
components by providing internal loop compensation for
the buck regulator loop. Loop stability, transient response
and burst ripple performance can be tailored by choice
of output capacitance. For many applications, desirable
35412fb
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