Datasheet

LTC3810
18
3810fc
application. A good starting point is to feed about 25%
of the voltage change at the I
TH
pin to the V
ON
pin as
shown in Figure 8. Place capacitance on the V
ON
pin to
fi lter out the I
TH
variations at the switching frequency.
Figure 8. Correcting Frequency Shift with Load Current Changes
Minimum Off-Time and Dropout Operation
The minimum off-time, t
OFF(MIN)
, is the smallest amount
of time that the LTC3810 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of t
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
V
IN(MIN)
= V
OUT
t
ON
+ t
OFF(MIN)
t
ON
A plot of maximum duty cycle vs frequency is shown in
Figure 9.
Figure 9. Maximum Switching Frequency vs Duty Cycle
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
I
L
=
V
OUT
fL






1
V
OUT
V
IN






Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest effi ciency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, effi ciency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specifi ed maximum, the inductance
should be chosen according to:
L =
V
OUT
fI
L(MAX)






1
V
OUT
V
IN(MAX)






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 bottom 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. In order for the
APPLICATIONS INFORMATION
C
VON
0.01μF
R
VON2
30k
100k
R
VON1
200k
INTV
CC
10V
V
ON
I
TH
LTC3810
3810 F08
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3810 F09
1.0
DROPOUT
REGION
DUTY CYCLE (V
OUT
/V
IN
)
SWITCHING FREQUENCY (MHz)
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