Datasheet

LTC3813
17
3813fb
With these resistor values, the frequency will remain
relatively constant at:
f =
1+ R1/ R2
R
OFF
(76pF)
(Hz)
for the range of 0.45V
IN
to 1.55 • V
IN
, and will be propor-
tional to V
IN
outside of this range.
Changes in the load current magnitude will also cause
a frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By shortening the off-time slightly
as current increases, constant-frequency operation can be
maintained. This is accomplished with a resistor connected
from the I
TH
pin to the I
OFF
pin to increase the I
OFF
current
slightly as V
ITH
increases. The values required will depend
on the parasitic resistances in the specifi c application. A
good starting point is to feed about 10% of the R
OFF
cur-
rent with R
ITH
as shown in Figure 6.
Minimum On-Time and Dropout Operation
The minimum on-time t
ON(MIN)
is the smallest amount of
time that the LTC3813 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 350ns. The
minimum on-time limit imposes a minimum duty cycle
of t
ON(MIN)
/(t
ON(MIN)
+ t
OFF
). If the minimum duty cycle is
reached, due to a rising input voltage, for example, then
the output will rise out of regulation. The maximum input
voltage to avoid dropout is:
V
IN(MAX)
= V
OUT
t
OFF
t
ON(MIN)
+ t
OFF
A plot of maximum duty cycle vs switching frequency is
shown in Figure 7.
Figure 7. Maximum Duty Cycle vs Switching Frequency
Figure 6. Correcting Frequency Shift with Load Current Changes
Figure 5. V
OFF
Connection to Keep the Operating
Frequency Constant as the Input Supply Varies
Inductor Selection
An inductor should be chosen that can carry the maximum
input DC current which occurs at the minimum input volt-
age. The peak-to-peak ripple current is set by the inductance
and a good starting point is to choose a ripple current of
at least 40% of its maximum value:
I
L
= 40% •
I
O(MAX)
1 D
MAX
APPLICATIONS INFORMATION
R2
R1
V
IN
V
OFF
LTC3813
3813 F05
1000pF
R
OFF
R
ITH
V
OUT
I
OFF
I
TH
LTC3813
3813 F06
10R
OFF
V
OUT
R
ITH
=
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3813 F07
1.0
DROPOUT
REGION
V
IN
/V
OUT
SWITCHING FREQUENCY (MHz)