Datasheet

LTC3814-5
14
38145fc
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






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.
APPLICATIONS INFORMATION
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
)
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
R
OFF
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
38145 F04a
V
IN
= 5V
V
IN
= 24V
V
IN
= 12V
R
OFF
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
38145 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)
R2
R1
V
IN
V
OFF
LTC3814-5
38145 F05
1000pF
R
OFF
R
ITH
V
OUT
I
OFF
I
TH
LTC3814-5
38145 F06
10R
OFF
V
OUT
R
ITH
=