Datasheet

LTC3610
11
3610ff
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.
Minimum
Off-Time and Dropout Operation
The minimum off-time, t
OFF(MIN),
is the smallest amount
of time that the LTC3610 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
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle
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:
V V
t t
t
IN MIN OUT
ON OFF MIN
ON
( )
( )
=
+
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
Setting
the Output Voltage
The LTC3611 develops a 0.6V reference voltage between
the feedback pin, V
FB
, and the signal ground as shown in
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
V
OUT
= 0.6V 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
To improve the frequency response, a feedforward capaci-
tor C1 may also be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW trace.
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3610 F01a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3610 F01b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V