LT3684
9
3684fa
For more information www.linear.com/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:
R1=R2
OUT
1.265
−1
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
T
value for a desired switching
frequency is in Figure 1.
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.2
0.3
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
267
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
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
SW(MAX)
=
D
OUT
t
ON(MIN)
V
D
+ V
IN
− V
SW
( )
where V
IN
is the typical input voltage, 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). 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 LT3684 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:
MIN
SW
ON(MIN)
DC
MAX
= 1− f
SW
t
OFF(MIN)
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 LT3684 applications
depends on switching frequency, the Absolute Maximum
Ratings on V
IN
and BOOST pins, and on operating mode.
If the output is in start-up or short-circuit operating modes,
then V
IN
must be below 34V and below the result of the
following equation:
V
IN(MAX)
=
OUT
D
1− f
SW
t
ON(MIN)
− V
D
+ V
SW
where V
IN(MAX)
is the maximum operating input voltage,
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 (~150ns). Note that
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
Figure 1. Switching Frequency vs R
T
Value
APPLICATIONS INFORMATION
