LTC7138
12
7138f
For more information www.linear.com/LTC7138
applicaTions inForMaTion
teristics. The choice of which style inductor to use mainly
depends on the price versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
TDK, Toko, and Sumida.
Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts current
only during the switch off time. Average forward current
in normal operation can be calculated from:
I
D(AVG)
=I
OUT
IN
OUT
V
where I
OUT
is the output load current. The maximum av-
erage diode current occurs with a shorted output at the
high line. For this worst-case condition, the diode current
will approach 75% of the programmed peak current. The
diode reverse voltage rating should be greater than the
maximum operating input voltage. When the OVLO pin is
used to limit the maximum operating input voltage, the
diode reverse voltage should be greater than the OVLO
pin setting, but may be lower than the maximum input
voltage during overvoltage lockout.
For high efficiency at full load, it is important to select a
catch
diode with a low reverse recovery time and low for
-
ward voltage drop. As a result, Schottky diodes are often
used
as
catch diodes. However, Schottky diodes generally
exhibit much higher leakage than silicon diodes. In sleep,
the catch diode leakage current will appear as load current,
and may significantly reduce light load efficiency. Diodes
with low leakage often have larger forward voltage drops
at a given current, so a trade-off can exist between light
load and full load efficiency.
The selection of Schottky diodes with high reverse voltage
ratings is limited relative to that of silicon diodes. There
-
fore, for low reverse leakage and part availability, some
applications may prefer a silicon diode. If a silicon diode
is necessary
, be sure to select a diode with a specified low
reverse recovery time to maximize efficiency.
C
IN
and C
OUT
Selection
The input capacitor, C
IN
, is needed to filter the trapezoidal
current at the source of the high side MOSFET. C
IN
should
be sized to provide the energy required to magnetize the
inductor without causing a large decrease in input voltage
(∆V
IN
). The relationship between C
IN
and ∆V
IN
is given by:
C
IN
>
PEAK
2
2• V
• ∆V
It is recommended to use a larger value for C
IN
than
calculated by the previous equation since capacitance
decreases with applied voltage. In general, a 1µF X7R ce
-
ramic capacitor is a good choice for C
IN
in most LTC7138
applications.
To prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by:
I
RMS
=I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–1
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 because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based only on 2000 hours of life
which 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
size or height requirements in the design.
The output capacitor, C
OUT
, filters the inductor’s ripple
current and stores energy to satisfy the load current when
the LTC7138 is in sleep. The output ripple has a lower limit
of V
OUT
/160 due to the 5mV typical hysteresis of the feed-
back comparator. The time delay of the comparator adds
an additional ripple voltage that is a function of the load
current. During this delay time, the L
TC7138 continues to
switch and supply current to the output. The output ripple
