LT3681
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the out-
OUT
IN
put voltage, V is the power Schottky catch diode drop
D
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
(~0.55V), V is the internal switch drop (~0.5V at max
SW
load).Thisequationshowsthatslowerswitchingfrequency
is necessary to safely accommodate high V /V
ratio.
IN OUT
Also, as shown in the next section, lower frequency al-
lows a lower dropout voltage. The reason input voltage
range depends on the switching frequency is because the
LT3681 switch has finite minimum on and off times. The
switch can turn on for a minimum of ~150ns and turn off
for a minimum of ~150ns. This means that the minimum
and maximum duty cycles are:
V
⎛
⎞
⎠
OUT
R1=R2
–1
⎜
⎝
⎟
1.265
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3681 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom300kHzto2.8MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Figure 1.
DCMIN = fSWtON MIN
(
)
DCMAX =1– fSWtOFF MIN
(
)
where f is the switching frequency, the t
is the
ON(MIN)
SW
minimum switch on time (~150ns), and the t
is
OFF(MIN)
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
0.3
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
187
133
84.5
60.4
45.3
36.5
29.4
23.7
20.5
16.9
14.3
12.1
10.2
8.66
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3681 applications de-
pendsonswitchingfrequency,theAbsoluteMaximumRat-
ings on V and BOOST pins, and on operating mode.
Figure 1. Switching Frequency vs. RT Value
IN
Iftheoutputisinstart-uporshort-circuitoperatingmodes,
Operating Frequency Tradeoffs
then V must be below 34V and below the result of the
IN
Selection of the operating frequency is a tradeoff between
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
following equation:
VOUT + VD
V
=
– VD + VSW
IN MAX
(
)
fSWtON MIN
(
)
where V
OUT
is the maximum operating input voltage,
IN(MAX)
V
is the output voltage, V is the catch diode drop
D
highest acceptable switching frequency (f
given application can be calculated as follows:
) for a
SW(MAX)
(~0.55V), V is the internal switch drop (~0.5V at max
SW
load), f is the switching frequency (set by R ), and
SW
ON(MIN)
T
t
istheminimumswitchontime(~150ns).Notethat
VD + VOUT
fSW MAX
=
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
(
)
tON MIN V + V – V
(
)
D
IN
SW
(
)
3681f
9
Linear [ Linear ]
