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參數(shù)資料
| 型號(hào)： | LT1111MJ8-5 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Micropower DC/DC Converter Adjustable and Fixed 5V, 12V |
| 中文描述： | 1.5 A SWITCHING REGULATOR, 88 kHz SWITCHING FREQ-MAX, CDIP8 |
| 封裝： | 0.300 INCH, HERMETIC, CERDIP-8 |
| 文件頁(yè)數(shù)： | 9/16頁(yè) |
| 文件大?。?/td> | 337K |
| 代理商： | LT1111MJ8-5 |
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9
LT1111
capacitors provide still better performance at more ex-
pense. We recommend OS-CON capacitors from Sanyo
Corporation (San Diego, CA). These units are physically
quite small and have extremely low ESR. To illustrate,
Figures 1, 2, and 3 show the output voltage of an LT1111
based converter with three 100
μ
F capacitors. The peak
switch current is 500mA in all cases. Figure 1 shows a
Sprague 501D, 25V aluminum capacitor. V
OUT
jumps by
over 120mV when the switch turns off, followed by a drop
in voltage as the inductor dumps into the capacitor. This
works out to be an ESR of over 0.24
.
Figure 2 shows the
same circuit, but with a Sprague 150D, 20V tantalum
capacitor replacing the aluminum unit. Output jump is
now about 35mV, corresponding to an ESR of 0.07
.
Figure 3 shows the circuit with a 16V OS-CON unit. ESR
is now only 0.02
.
U
S
A
O
PPLICATI
IU
U
switch closes, current in the inductor builds according to
I
t
V
R
e
L
L
′
R t
L
( )
=
′
1
15
(
–
)
–
where R
′
= 0.65
+ DCR
L
V
L
= V
IN
– 0.75V
As an example, suppose –5V at 50mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
P
L
=
(
-5V
+0.5V
)(
50mA
)
= 275mW
Energy required from the inductor is:
(16)
P
f
OSC
mW
kHz
J
L
=
=
275
72
3 8
.
17
(
.
)
μ
Picking an inductor value of 56
μ
H with 0.2
DCR results
in a peak switch current of:
)
+
(
I
V
V
e
mA
PEAK
s
H
=
(
)
=
×
4 5
.
–0 75
0 65
.
0 2
.
1
445
18
(
0 85
– .
7
56
–
.
)
μ
μ
Substituting I
PEAK
into Equation (04) results in:
E
H
A
J
L
=
(
)(
)
=
1
2
56
0 445
.
5 54
.
19
(
2
μ
μ
.
)
Since 5.54
μ
J > 3.82
μ
J, the 56
μ
H inductor will work.
With this relatively small input range, R
LIM
is not usually
necessary and the I
LIM
pin can be tied directly to V
IN
. As in
the step-down case, peak switch current should be limited
to ~650mA.
Capacitor Selection
Selecting the right output capacitor is almost as important
as selecting the right inductor. A poor choice for a filter
capacitor can result in poor efficiency and/or high output
ripple. Ordinary aluminum electrolytics, while inexpensive
and readily available, may have unacceptably poor Equiva-
lent Series Resistance (ESR) and ESL (inductance). There
are low ESR aluminum capacitors on the market specifi-
cally designed for switch mode DC/DC converters which
work much better than general-purpose units. Tantalum
5
5
μ
s/DIV
LT1111 F01
Figure 1. Aluminum
5
5
μ
s/DIV
LT1111 F02
Figure 2. Tantalum
5
5
μ
s/DIV
LT1111 F01
Figure 3. OS-CON
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