Datasheet

LTC3609
12
3609fb
applicaTions inForMaTion
Figure 1a. Switching Frequency vs R
ON
(V
ON
= 0V)
Figure 1b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)
Because the voltage at the I
ON
pin is about 0.7V, the cur-
rent into this pin is not exactly inversely proportional to
V
IN
, especially in applications with lower input voltages.
To correct for this error, an additional resistor R
ON2
con-
nected from the I
ON
pin to the 5V INTV
CC
supply will further
stabilize the frequency.
R
V
V
R
ON ON2
5
0 7
=
.
Changes in the load current magnitude will also cause
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 lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
TH
pin to the V
ON
pin and V
OUT
. The values
required will depend on the parasitic resistances in the
specific 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 2a. Place capacitance on the V
ON
pin
to filter out the I
TH
variations at the switching frequency.
The resistor load on I
TH
reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 2b.
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3609 F01a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3609 F01b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V