Datasheet

LTC3813
16
3813fb
Operating Frequency
The choice of operating frequency is a tradeoff between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3813 applications is de-
termined implicitly by the one-shot timer that controls
the on-time t
OFF
of the synchronous MOSFET switch.
The on-time is set by the current into the I
OFF
pin and the
voltage at the V
OFF
pin according to:
t
OFF
=
V
VOFF
I
IOFF
76pF
()
Tying a resistor R
OFF
from V
OUT
to the I
OFF
pin yields a
synchronous MOSFET on-time inversely proportional to
V
OUT
. This results in the following operating frequency
and also keeps frequency constant as V
OUT
ramps up at
start-up:
f =
V
IN
V
VOFF
•R
OFF
(76pF)
(Hz)
The V
OFF
pin can be connected to INTV
CC
or ground or
can be connected to a resistive divider from V
IN
. The V
OFF
pin has internal clamps that limit its input to the one-shot
timer. If the pin is tied below 0.7V, the input to the one-
shot is clamped at 0.7V. Similarly, if the pin is tied above
2.4V, the input is clamped at 2.4V. Note, however, that
if the V
OFF
pin is connected to a constant voltage, the
operating frequency will be proportional to the input
voltage V
IN
. Figures 4a and 4b illustrate how R
OFF
relates
to switching frequency as a function of the input voltage
and V
OFF
voltage. To hold frequency constant for input
voltage changes, tie the V
OFF
pin to a resistive divider from
V
IN
, as shown in Figure 5. Choose the resistor values so
that the V
RNG
voltage equals about 1.55V at the mid-point
of V
IN
as follows:
V
IN,MID
=
V
IN(MAX)
+ V
IN(MIN)
2
= 1.55V • 1+
R1
R2






Figure 4a. Switching Frequency vs R
OFF
(V
OFF
= INTV
CC
) Figure 4b. Switching Frequency vs R
OFF
(V
OFF
Connected to a Resistor Divider from V
IN
)
APPLICATIONS INFORMATION
R
OFF
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
3813 F04a
V
IN
= 5V
V
IN
= 24V
V
IN
= 12V
R
OFF
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
3813 F04b
1+R1/R2 = 3.2
(V
IN
,
MID
= 5V)
1+R1/R2 = 7.7
(V
IN
,
MID
=12V)
1+R1/R2 = 15.5
(V
IN
,
MID
= 24V)