Datasheet

LTC3813
18
3813fb
The required inductance can then be calculated to be:
L =
V
IN(MIN)
•D
MAX
f•I
L
The required saturation of the inductor should be chosen
to be greater than the peak inductor current:
I
L(SAT)

I
O(MAX)
1 D
MAX
+
I
L
2
Once the value for L is known, the type of inductor must be
selected. High effi ciency converters generally cannot afford
the core loss found in low cost powdered iron cores, forcing
the use of more expensive ferrite, molypermalloy or Kool Mμ
®
cores. A variety of inductors designed for high current, low
voltage applications are available from manufacturers such
as Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in the front page schematic
conducts during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diode of the synchronous MOSFET from turning on
and storing charge during the dead time, which can cause
a modest (about 1%) effi ciency loss. The diode can be
rated for about one half to one fi fth of the full load current
since it is on for only a fraction of the duty cycle. The peak
reverse voltage that the diode must withstand is equal to
the regulator output voltage. In order for the diode to be
effective, the inductance between it and the synchronous
MOSFET must be as small as possible, mandating that
these components be placed adjacently. The diode can
be omitted if the effi ciency loss is tolerable.
Output Capacitor Selection
In a boost converter, the output capacitor requirements
are demanding due to the fact that the current waveform
is pulsed. The choice of component(s) is driven by the
acceptable ripple voltage which is affected by the ESR,
ESL and bulk capacitance as shown in Figure 8e. The total
output ripple voltage is:
V
OUT
= I
O(MAX)
1
f•C
OUT
+
ESR
1–D
MAX






where the fi rst term is due to the bulk capacitance and
second term due to the ESR.
For many designs it is possible to choose a single capacitor
type that satisfi es both the ESR and bulk C requirements
for the design. In certain demanding applications, however,
the ripple voltage can be improved signifi cantly by con-
necting two or more types of capacitors in parallel. For
example, using a low ESR ceramic capacitor can minimize
the ESR step, while an electrolytic capacitor can be used
to supply the required bulk C.
Figure 8. Switching Waveforms for a Boost Converter
APPLICATIONS INFORMATION
V
IN
LD
SW
8a. Circuit Diagram
8b. Inductor and Input Currents
C
OUT
V
OUT
R
L
I
IN
I
L
8c. Switch Current
I
SW
t
ON
8d. Diode and Output Currents
8e. Output Voltage Ripple Waveform
I
O
I
D
V
OUT
(AC)
t
OFF
ΔV
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔV
COUT
3813 F08