LT3972
9
3972fa
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 LT3972 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 Trade-Offs
Selection of the operating frequency is a trade-off between
effi ciency, 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 effi ciency, 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 LT3972 switch has fi nite
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 f t
DC f t
MIN SW
ON MIN
MAX SW
OFF MIN
=
=
()
()
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
frequency is decreased.
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 LT3972 applications de-
pends on switching frequency, Absolute Maximum Ratings
of the V
IN
and BOOST pins, and the operating mode.
The LT3972 can operate from input voltages of up to 33V,
and withstand voltages up to 62V. Note that while V
IN
is
above 35V typical (33V minimum and 37V maximum)
the part will keep the switch off and the output will not
be in regulation.
To safely allow inputs of up to 62V, be sure to choose a
Schottky diode, inductor size, and switching frequency
to allow safe operation at 37V according to the following
discussions.
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following equation:
APPLICATIONS INFORMATION
