LTC3544B_15 Datasheet, PDF (10/18 Pages) Linear Technology – Quad Synchronous Step-Down Regulator

LTC3544B
Applications Information
The basic LTC3544B application circuit is shown on the
first page of this data sheet. External component selec-
tion 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 the 300mA regulator is
ΔI_L = 120mA (40% of 300mA).
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 360mA rated inductor
should be enough for most applications (300mA + 60mA).
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 LTC3544B requires to operate. Table 1
shows typical surface mount inductors that work well in
LTC3544B applications.
Table 1. Representative Surface Mount Inductors
VALUE DCR
MAX DC
PART NUMBER (µH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
10
0.47
0.48
3.0 × 2.8 × 1.0
CDH2D09B
6.4
0.32
0.6
4.7 0.218
0.7
3.3
0.15
0.85
Wurth
TPC744029
10
0.50
6.8
0.38
4.7 0.210
3.3 0.155
0.50
2.8 × 2.8 × 1.35
0.65
0.80
0.95
TDK
VLF3010AT
10
0.67
6.8
0.39
4.7
0.28
3.3
0.17
0.49
2.8 × 2.6 × 1.0
0.61
0.70
0.87
C_IN and C_OUT Selection
In continuous mode, a worst-case estimate for the input
current ripple can be determined my 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.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C_OUT has been met, the RMS current rating
3544bfb
10