Datasheet

LT3682
11
3682f
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
RR
V
V
OUT
12
0 8
1=−
⎛
⎝
⎜
⎞
⎠
⎟
.
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
Setting the Switching Frequency
The LT3682 uses a constant frequency PWM architecture
that can be programmed to switch from 250kHz to 2.2MHz
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)
0.25 158
0.3 127
0.4 90.9
0.5 71.5
0.6 57.6
0.7 47.5
0.8 40.2
0.9 34
1.0 29.4
1.2 22.6
1.4 18.2
1.6 14.7
1.8 12.1
2.0 9.76
2.2 8.06
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
OUT D
ON MIN IN SW D
()
()
()
=
+
−+
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 Input Voltage Range section, lower frequency allows
a lower dropout voltage. Input voltage range depends on
the switching frequency because the LT3682 switch has
fi nite minimum on and off times. An internal timer forces
the switch to be off for at least t
OFF(MIN)
per cycle; This
timer has a maximum value of 210ns over temp. On the
other hand, delays associated with turning off the power
switch dictate the minimum on time t
ON(MIN)
before the
switch can be turned off; t
ON(MIN)
has a maximum value
of 150ns over temp. The minimum and maximum duty
cycles that can be achieved taking minimum on and off
times into account 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 (210ns). These equations show
that duty cycle range increases when switching frequency
is decreased.
A good choice of switching frequency should allow adequate
input voltage range (see Input Voltage Range section) and
keep the inductor and capacitor values small.
Input Voltage Range
The minimum input voltage is determined by either the
LT3682’s minimum operating voltage of ~3.6V (V
BD
> 3V)
or by its maximum duty cycle (see equation in Operating
APPLICATIONS INFORMATION