lt3684emse-trpbf Linear Technology Corporation, lt3684emse-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt3684emse-trpbf
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3684
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:
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3684 uses a constant frequency PWM architecture
that can be programmed to switch from 300kHz to 2.8MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
frequency is in Figure 1.
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff 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
given application can be calculated as follows:
10
R
f
SW MAX
SWITCHING FREQUENCY (MHz)
1
=
(
R
2
Figure 1. Switching Frequency vs R
⎛
⎝ ⎜
)
1 265
=
V
.
OUT
t
0.2
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
ON MIN
(
–
1
⎞
⎠ ⎟
)
V
(
D
V
+
T
D
value for a desired switching
+
V
OUT
V
IN
–
V
SW
R
)
T
T
VALUE (kΩ)
Value
SW(MAX)
84.5
60.4
45.3
36.5
29.4
23.7
20.5
16.9
14.3
12.1
10.2
8.66
267
187
133
) for a
where V
voltage, is the catch diode drop (~0.5V), V
switch drop (~0.5V at max load). This equation shows
that slower switching frequency is necessary to safely
accommodate high V
the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3684 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. This means that the minimum and maximum
duty cycles are:
where f
minimum switch on time (~150ns), and the t
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 LT3684 applications de-
pends on switching frequency, the Absolute Maximum Rat-
ings on V
If the output is in start-up or short-circuit operating modes,
then V
following equation:
where V
V
(~0.5V), V
load), f
t
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
ON(MIN)
OUT
DC
DC
V
IN MAX
is the output voltage, V
MIN
MAX
(
IN
SW
SW
IN
IN(MAX)
is the minimum switch on time (~150ns). Note that
must be below 34V and below the result of the
IN
=
SW
is the typical input voltage, V
=
is the switching frequency, the t
)
is the switching frequency (set by R
and BOOST pins, and on operating mode.
f
=
1–
SW ON MIN
is the internal switch drop (~0.5V at max
f
SW ON MIN
V
f
t
is the maximum operating input voltage,
SW OFF MIN
OUT
t
(
t
+
(
IN
V
)
(
D
/V
)
OUT
–
)
V
D
D
ratio. Also, as shown in
+
is the catch diode drop
V
SW
OUT
SW
ON(MIN)
is the internal
is the output
OFF(MIN)
T
), and
is the
3684f
is