LTC3544_15 Datasheet, PDF (10/16 Pages) Linear Technology – Quad Synchronous Step-Down Regulator

LTC3544
APPLICATIONS INFORMATION
The basic LTC3544 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 selec-
tion 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 LTC3544 requires to operate. Table 1
shows typical surface mount inductors that work well in
LTC3544 applications.
10
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
10
0.50
TPC744029
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
10
0.67
VLF3010AT
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 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.
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
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple ΔV_OUT is determined by:
ΔV_OUT
≅
ΔI_L
⎛
⎝⎜ ^ESR
+
8
•
ƒ
1
• C_OUT
⎞
⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
3544fa