參數(shù)資料
型號(hào)： LT1683
廠商： Linear Technology Corporation
英文描述： Ultralow Noise Push-Pull DC/DC Controller
中文描述： 超低噪聲推挽式DC / DC控制器
文件頁(yè)數(shù)： 19/24頁(yè)
文件大小： 237K
代理商： LT1683
19
LT1683
1683f
For continuous operation at I
OUT(MIN)
= I
OUT(MAX)
/4,
inductor ripple (the same as output ripple):
=
=
I
A
4
A
L
2
2
1
The duty cycle for nominal input is:
DC
V
V
I
N V
(
5 5
.
R
NOM
OUT
F
IN NOM
(
SW
ON
=
+
)
=
=
2
2
.
6 1
47 5
35. %
.
)
Then
L
A
kHz
H
=
+
(
)
(
100
)
=
μ
5
0 5
.
1 2 35. %
1
16
Off-the-shelf components can be used for this inductor.
Say we choose a 22
μ
H inductor then ripple current at
maximum input (DC = 29.1%) is:
(
μ
F
kHz
22
100
=
+
)
(
)
=
I
A
L
5
0 5
.
1 2 29. %
1 03
.
The maximum inductor current is:
I
A
A
A
L MAX
(
)
.
.
=
+
=
2
1 03
2
2 52
Primary inductance should be greater than:
L
PRI
= 5 22
μ
H 6.1
2
= 4.1mH
The secondary inductance would then be:
4.1mH/6.1
2
= 110
μ
H
The magnetizing ripple current is approximately:
=
=
I
mH
kHz
mA
MAG
5 5
.
1
.
6 1
4 1
100
81
Peak switch current is:
I
A
mA
mA
SW PEAK
(
)
.
2 51
=
+
=
1
6 1
81
494
Note that you can discern your magnetizing ripple by
looking at the reflected inductance ripple and subtracting
it from the switch current ripple.
I
MAG
= N I
L
–
I
SW
The max ripple current on the switch is:
=
+
=
I
A
mA
A
SW MAX
(
)
.
.
.
1 03
6 1
81
0 25
Knowing the peak switch current we can go back and
iterate with a more accurate switch on voltage. We would
have to know the R
ON
of the FET. In our case our assump-
tions of a 0.5V switch on voltage is valid for
R
ON
+ R
SENSE
< 1
.
Capacitors
Correct choice of input and output capacitors is very
important to low noise switcher performance. Push-pull
topologies and other low noise topologies will in general
have continuous currents, which reduce the requirements
for capacitance. However, noise depends more on the ESR
of the capacitors. In addition lower ESR can also improve
efficiency.
Input capacitors must also withstand surges that occur
during the switching of some types of loads. Some solid
tantalum capacitors can fail under these surge conditions.
Design Note 95 offers more information but the following
is a brief summary of capacitor types and attributes.
Aluminum Electrolytic:Low cost and higher voltage. They
can be used in this application but in general you will need
higher capacitance to achieve low ESR. Additional
nonelectrolytic capacitors may be required to achieve
better performance.
Specialty Polymer Aluminum:Panasonic has come out
with their series CD capacitors. While they are only avail-
able for voltages below 16V, they have very low ESR and
good surge capability.
APPLICATIU
W
U
U
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