參數(shù)資料
型號(hào)： LTC1938IDD#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 4 A SWITCHING REGULATOR, 3300 kHz SWITCHING FREQ-MAX, PDSO10
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10
文件頁(yè)數(shù)： 2/24頁(yè)
文件大?。?/td> 272K
代理商： LTC1938IDD#TRPBF
LT1938
10
1938f
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 LT1938 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)
0.2
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
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 output
voltage, is the catch diode drop (~0.5V), 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 allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT1938 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 LT1938 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 25V 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
(~0.5V), 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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