LT3682 LINER [Linear Technology], LT3682 Datasheet - Page 11

• Manufacturer Part Number: LT3682
• Description: 1A Micropower Step-Down
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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:
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
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
R
SWITCHING FREQUENCY (MHz)
1
=
R
2
Figure 1. Switching Frequency vs. R
⎛
⎜
⎝
V
0 8
OUT
.
0.25
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
V
−
1
⎞
⎟
⎠
T
value for a desired switching
T
pin to ground. A table
R
T
T
VALUE (kΩ)
Value
90.9
71.5
57.6
47.5
40.2
29.4
22.6
18.2
14.7
12.1
9.76
8.06
158
127
34
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:
where V
voltage, V
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
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
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
switch can be turned off; t
of 150ns over temp. The minimum and maximum duty
cycles that can be achieved taking minimum on and off
times into account are:
where f
minimum switch on time (150ns), and the t
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
or by its maximum duty cycle (see equation in Operating
DC
DC
f
SW MAX
MIN
MAX
(
SW
IN
D
=
is the typical input voltage, V
= −
is the switching frequency, the t
)
is the catch diode drop (~0.5V) and V
f
1
=
SW ON MIN
t
ON MIN
f
t
SW OFF MIN
(
(
t
)
V
(
)
(
OUT
V
IN
ON(MIN)
−
IN
)
+
V
/V
V
D
SW
OUT
OFF(MIN)
+
has a maximum value
ratio. Also, as shown
V
D
ON(MIN)
)
OUT
per cycle; This
SW(MAX)
OFF(MIN)
LT3682
ON(MIN)
is the output
before the
SW
BD
11
) for a
is the
is the
> 3V)
is the
3682f