LT3684
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the output
IN
OUT
SW
voltage,isthecatchdiodedrop(~0.5V),V istheinternal
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
switch drop (~0.5V at max load). This equation shows
that slower switching frequency is necessary to safely
accommodate high V /V
ratio. Also, as shown in
IN OUT
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
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:
OUT
R1=R2
–1
⎜
⎝
⎟
1.265
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3684 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom300kHzto2.8MHz
by using a resistor tied from the RT pin to ground. A table
DCMIN = fSWtON MIN
(
)
DCMAX =1– fSWtOFF MIN
showing the necessary R value for a desired switching
T
(
)
frequency is in Figure 1.
where f is the switching frequency, the t
is the
ON(MIN)
SW
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
minimum switch on time (~150ns), and the t
is
OFF(MIN)
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
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
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
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-
pendsonswitchingfrequency,theAbsoluteMaximumRat-
ings on V and BOOST pins, and on operating mode.
IN
Figure 1. Switching Frequency vs R Value
T
Iftheoutputisinstart-uporshort-circuitoperatingmodes,
Operating Frequency Tradeoffs
then V must be below 34V and below the result of the
IN
following equation:
Selection of the operating frequency is a tradeoff between
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
VOUT + VD
V
=
) – VD + VSW
IN MAX
(
)
fSWtON MIN
(
where V
OUT
is the maximum operating input voltage,
IN(MAX)
V
is the output voltage, V is the catch diode drop
D
highest acceptable switching frequency (f
given application can be calculated as follows:
) for a
SW(MAX)
(~0.5V), V is the internal switch drop (~0.5V at max
SW
load), f is the switching frequency (set by R ), and
SW
ON(MIN)
T
VD + VOUT
t
istheminimumswitchontime(~150ns).Notethat
fSW MAX
=
(
)
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
tON MIN V + V – V
(
)
D
IN
SW
(
)
3684f
10
Linear [ Linear ]
