Datasheet

LT3980
9
3980fa
For more information www.linear.com/LT3980
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 LT3980 uses a constant frequency PWM architecture
that can be programmed to switch from 100kHz to 2.4MHz
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.
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.1
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
432
215
137
97.6
76.8
60.4
51.1
43.2
35.7
32.4
24.9
20
16.2
14
11
Figure 1. Switching Frequency vs R
T
Value
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off 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 dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3980 switch has
finite minimum on and off times. The switch can turn on
for a minimum of ~200ns and turn off for a minimum of
~200ns. 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 (~200ns), and the t
OFF(MIN)
is
the minimum switch off time (~200ns). These equations
show that duty cycle range increases when switching
frequency 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 V
oltage Range
The maximum input voltage for LT3980 applications de-
pends on switching frequency, Absolute Maximum Ratings
of the V
IN
and BOOST pins, and the operating mode.
The LT3980 can operate from input voltages of up to 58V,
and withstand voltages up to 80V. Note that while V
IN
is
above 61V typical (58V minimum and 64V maximum)
the part will keep the switch off and the output will not
be in regulation.
T
he 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 typical operating input
voltage, V
OUT
is the output voltage, V
D
is the catch diode
applicaTions inFormaTion