LTC3541-1 Datasheet, PDF (11/20 Pages) Linear Technology – High Efficiency Buck + VLDO Regulator

LTC3541-1
APPLICATIO S I FOR ATIO
The basic LTC3541-1 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 C_IN, C_OUT, and feedback resistor values
for the buck and the selection of the output capacitor and
feedback values for the VLDO and linear regulator.
BUCK Regulator
Inductor Selection
For most applications, the appropriate inductor value will
be in the range of 1.5µH to 3.3µH with 2.2µH the most
commonly used. The exact inductor value is chosen
largely based on the desired ripple current and burst
ripple performance. Generally, large value inductors re-
duce ripple current, and conversely, small value inductors
produce higher ripple current. Higher V_IN or V_OUT may
also increase the ripple current as shown in Equation 1.
A reasonable starting point for setting ripple current is
ΔI_L = 200mA (40% of 500mA).
ΔI_L
=
(
1
f)(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 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 LTC3541-1 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3541-1 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
2.0
3.9 × 3.9 × 2.4
CDRH3D23
1.5 0.029
1.5
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
1.0
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
Sumida
1.5
0.06
1.00
3.2 × 3.2 × 1.2
CDRH2D11/HP 2.2
0.10
0.72
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. 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} ^{V}_OUT
V_IN − V_OUT
V_IN
^1/2
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is common-
ly used for design. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life. This makes it advisable to further derate the capaci-
tor or choose a capacitor rated at a higher temperature
than required. Always consult the manufacturer with any
question regarding proper capacitor choice.
The selection of C_OUT for the buck regulator is driven by
the desired buck loop transient response, required effective
series resistance (ESR) and burst ripple performance.
The LTC3541-1 minimizes the required number of external
components by providing internal loop compensation
for the buck regulator loop. Loop stability, transient re-
sponse and burst performance can be tailored by choice
of output capacitance. For many applications, desirable
stability, transient response and ripple performance can
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