LTC3542-1_15 Datasheet, PDF (9/16 Pages) Linear Technology – 500mA, 2.25MHz 2.8V Output Synchronous Step-Down DC/DC Converter

LTC3542-1
APPLICATIONS INFORMATION
A general LTC3542-1 application circuit is shown in Figure1.
External component selection is driven by the load require-
ment and begins with the selection of the inductor L. Once
the inductor is chosen, C_IN and C_OUT can be selected.
L
V_IN
V_IN
SW
V_OUT
C_IN
LTC3542-1
RUN V_OUT
MODE/SYNC
C_OUT
35421 F01
GND
Figure 1. LTC3542-1 General Schematic
Inductor Selection
The inductor value has a direct effect on ripple current ΔI_L,
which decreases with higher inductance and increases with
higher V_IN or V_OUT, as shown in following equation:
ΔI_L
=
V_OUT
ƒ_O • L
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
where f_O is the switching frequency. A reasonable starting
point for setting ripple current is ΔI_L = 0.4 • I_OUT(MAX),
where I_OUT(MAX) is 500mA. The largest ripple current ΔI_L
occurs at the maximum input voltage. To guarantee that
the ripple current stays below a specified maximum, the
inductor value should be chosen according to the follow-
ing equation:
L
=
V_OUT
ƒ_O • ΔI_L
⎛
⎜
⎝
1–
V_OUT
V_IN(MAX)
⎞
⎟
⎠
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, chose a low DC-resistance
inductor.
The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor’s peak current falls below a level set by
the burst clamp. Lower inductor values result in higher
ripple current which causes the transition to occur at lower
load currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values cause the burst frequency to
increase.
Inductor Core Selection
Different core materials and shapes change the size/cur-
rent and price/current relationships of an inductor. 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 elec-
trical characteristics. The choice of which style inductor
to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3542-1 requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3542-1 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V_OUT/V_IN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
( ) _{IRMS ≈IMAX}
V_OUT V_IN – V_OUT
V_IN
where the maximum average output current I_MAX equals
the peak current minus half the peak-to-peak ripple cur-
rent, I_MAX = I_LIM – ΔI_L/2. This formula has a maximum at
V_IN = 2V_OUT, where I_RMS = I_OUT/2. This simple worst-case
is commonly used to design because even significant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
only 2000 hours life time. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet the size or height requirements of the
35421f
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