LT3685
9
3685fb
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 LT3685 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the R
T
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
187
121
88.7
68.1
56.2
46.4
40.2
34
29.4
23.7
19.1
16.2
13.3
11.5
9.76
8.66
Figure 1. Switching Frequency vs. R
T
Value
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
SW(MAX)
) for a
given application can be calculated as follows:
f
VV
tVVV
SW MAX
D OUT
ON MIN D IN SW
()
()
–
=
+
+
()
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 LT3685 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 LT3685 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V
IN
and BOOST pins, and the operating
mode.
The LT3685 can operate from input voltages up to 38V,
and safely withstand input voltages up 60V. Note that
while V
IN
> 38V (typical), the LT3685 will stop switching,
allowing the output to fall out of regulation.
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following discussion.
For safe operation at inputs up to 60V the switching fre-
quency must be set low enough to satisfy V
IN(MAX)
≥ 40V
according to the following equation. If lower V
IN(MAX)
is
desired, this equation can be used directly.
APPLICATIONS INFORMATION
