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參數(shù)資料
| 型號(hào)： | LT3681EDE |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 4.4 A SWITCHING REGULATOR, 3100 kHz SWITCHING FREQ-MAX, PDSO14 |
| 封裝： | 4 X 3 MM, PLASTIC, DFN-14 |
| 文件頁數(shù)： | 24/24頁 |
| 文件大?。?/td> | 367K |
| 代理商： | LT3681EDE |
LT3681
9
3681f
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
VOUT
12
1 265
1
=
.
–
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3681 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 RT value for a desired switching
frequency is in Figure 1.
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
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
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
efciency, 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 efciency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (fSW(MAX)) for a
given application can be calculated as follows:
f
VV
tV
V
SW MAX
D
OUT
ON MIN
DIN
SW
()
=
+
()
–
where VIN is the typical input voltage, VOUT is the out-
put voltage, VD is the power Schottky catch diode drop
(~0.55V), VSW is the internal switch drop (~0.5V at max
load). This equation shows that slower switching frequency
is necessary to safely accommodate high VIN/VOUT ratio.
Also, as shown in the next section, lower frequency al-
lows a lower dropout voltage. The reason input voltage
range depends on the switching frequency is because the
LT3681 switch has nite 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:
DC
f
t
DC
f
t
MIN
SW ON MIN
MAX
SW OFF MIN
=
()
1–
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on time (~150ns), and the tOFF(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 LT3681 applications de-
pends on switching frequency, the Absolute Maximum Rat-
ings on VIN and BOOST pins, and on operating mode.
If the output is in start-up or short-circuit operating modes,
then VIN must be below 34V and below the result of the
following equation:
V
VV
ft
VV
IN MAX
OUT
D
SW ON MIN
DSW
()
=
+
–
where VIN(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VD is the catch diode drop
(~0.55V), VSW is the internal switch drop (~0.5V at max
load), fSW is the switching frequency (set by RT), and
tON(MIN)istheminimumswitchontime(~150ns).Notethat
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
Figure 1. Switching Frequency vs. RT Value
APPLICATIONS INFORMATION
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