LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 18

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3810
application. A good starting point is to feed about 25%
of the voltage change at the I
shown in Figure 8. Place capacitance on the V
fi lter out the I
Minimum Off-Time and Dropout Operation
The minimum off-time, t
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
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:
A plot of maximum duty cycle vs frequency is shown in
Figure 9.
18
Figure 8. Correcting Frequency Shift with Load Current Changes
V
IN(MIN)
Figure 9. Maximum Switching Frequency vs Duty Cycle
INTV
10V
ON
CC
2.0
1.5
1.0
0.5
= V
/(t
0
TH
0
R
ON
200k
VON1
OUT
variations at the switching frequency.
+ t
t
OFF(MIN)
R
30k
ON
0.25
VON2
DUTY CYCLE (V
+ t
OFF(MIN)
t
ON
OFF(MIN)
). If the maximum duty cycle
0.50
C
0.01μF
VON
TH
OUT
, is the smallest amount
DROPOUT
REGION
/V
pin to the V
IN
0.75
)
100k
V
I
3810 F09
TH
LTC3810
ON
1.0
3810 F08
ON
ON
pin as
pin to
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
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
occurs at the highest V
does not exceed a specifi ed maximum, the inductance
should be chosen according to:
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
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
L =
I
L
=
f I
V
V
®
L(MAX)
OUT
f L
OUT
cores. A variety of inductors designed for
1
OUT(MAX)
1
IN
V
V
OUT
. To guarantee that ripple current
IN
V
IN(MAX)
V
OUT
. The largest ripple current
3810fb