LTC3608 LINER [Linear Technology], LTC3608 Datasheet - Page 12

• Manufacturer Part Number: LTC3608
• Description: 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3608
Because the voltage at the I
rent into this pin is not exactly inversely proportional to
V
To correct for this error, an additional resistor R
nected from the I
stabilize the frequency.
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
12
IN
R
, especially in applications with lower input voltages.
ON2
Figure 1b. Switching Frequency vs R
Figure 1a. Switching Frequency vs R
=
1000
1000
0.7V
100
100
5V
ON
R
V
100
100
OUT
ON
V
OUT
pin to the 5V INTV
= 3.3V
= 1.5V
R
R
ON
ON
1000
1000
ON
(kΩ)
(kΩ)
V
V
pin is about 0.7V, the cur-
OUT
OUT
V
V
OUT
= 3.3V
OUT
= 12V
= 5V
CC
= 2.5V
ON
ON
supply will further
(V
3608 F01a
3608 F01b
ON
(V
10000
10000
ON
= INTV
= 0V)
ON2
CC
)
con-
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
required will depend on the parasitic resistances in the
specifi c application. A good starting point is to feed about
25% of the voltage change at the I
as shown in Figure 2a. Place capacitance on the V
to fi lter out the I
The resistor load on I
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
time that the LTC3608 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 320ns.
The minimum off-time limit imposes a maximum duty
cycle of t
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:
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3608 develops a 0.6V reference voltage between
the feedback pin, V
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
To improve the frequency response, a feed forward capaci-
tor C1 may also be used. Great care should be taken to
route the V
inductor or the SW line.
V
V
IN(MIN)
OUT
= 0.6V 1+
ON
TH
FB
= V
/(t
ON
line away from noise sources, such as the
pin to the V
OUT
TH
+ t
FB
variations at the switching frequency.
t
OFF(MIN)
ON
R2
R1
, and the signal ground as shown in
TH
+ t
OFF(MIN)
reduces the DC gain of the error
t
ON
OFF(MIN)
ON
). If the maximum duty cycle
pin and V
is the smallest amount of
TH
pin to the V
OUT
. The values
ON
ON
3608fa
pin
pin