Datasheet

LTC3542
8
3542fa
A general LTC3542 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.
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 effi ciency 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/current
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 electrical
characteristics. The choice of which style inductor to use
often depends more on the price vs size requirements
and any radiated fi eld/EMI requirements than on what the
LTC3542 requires to operate. Table 1 shows some typi-
cal surface mount inductors that work well in LTC3542
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:
II
VVV
V
RMS MAX
OUT IN OUT
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 signifi cant
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
V
IN
LTC3542
RUN
R1
3542 F01
R2
C
F
L
C
IN
C
OUT
SW
V
IN
2.7V TO 5.5V
V
OUT
V
FB
MODE/SYNC
GND
Figure 1. LTC3542 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
V
L
V
V
L
OUT
O
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
ƒ •
–1
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 specifi ed maximum, the
inductor value should be chosen according to the follow-
ing equation:
L
V
I
V
V
OUT
OL
OUT
IN MAX
=
⎛
⎝
⎜
⎞
⎠
⎟
ƒ •
–
()
Δ
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 effi ciency, 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
APPLICATIONS INFORMATION