Datasheet

LT3680
9
Rev C
For more information www.analog.com
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis
-
tors according to:
RR
V
V
OUT
12
079
1=
.
–
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3680 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
T
value for a desired switching
frequency is in Figure 1.
SWITCHING FREQUENCY (MHz)
R
T
VALUE (kΩ)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
215
140
100
78.7
63.4
53.6
45.3
39.2
34
26.7
22.1
18.2
15
12.7
10.7
9.09
Figure 1. Switching Frequency vs. R
T
Value
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
VV
tVVV
SW MAX
D OUT
ON MIN
DINSW
()
()
=
+
+
()
–
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V) and V
SW
is the
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
IN
/V
OUT
ratio. Also, as shown
in the next section, lower frequency allows a lower drop
-
out voltage. The reason input voltage range depends on
the switching frequency is because the LT3680 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. Typical minimum on time at 25°C is 80ns. This
means that the minimum and maximum duty cycles are:
DC ft
DC ft
MINSW
ON MIN
MAXSW
OFFMIN
=
=
()
()
1–
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on time (~150ns), and the t
OFF(MIN)
is
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching fre-
quency is decreased.
A good choice of switching frequency should allow ade
-
quate input voltage range (see next section) and keep the
inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3680 applications
depends on switching frequency and Absolute Maximum
Ratings of the V
IN
and BOOST pins (36V and 56V
respectively).
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following equation:
V
VV
ft
VV
IN MAX
OUT D
SW
ON MIN
DS
W
()
()
=
+
+–
where V
IN(MAX)
is the maximum operating input volt-
age, V
OUT
is the output voltage, V
D
is the catch diode
drop (~0.5V), V
SW
is the internal switch drop (~0.5V at
max load), f
SW
is the switching frequency (set by R
T
),
and t
ON(MIN)
is the minimum switch on time (~100ns).
Note that a higher switching frequency will depress the
APPLICATIONS INFORMATION
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