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參數(shù)資料
| 型號： | LT1110CN8-5 |
| 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, 70 kHz SWITCHING FREQ-MAX, PDIP8 |
| 封裝： | MINI, PLASTIC, DIP-8 |
| 文件頁數(shù)： | 8/16頁 |
| 文件大?。?/td> | 310K |
| 代理商： | LT1110CN8-5 |
LT1110
8
U
S
A
O
PPLICATI
IU
U
Picking an inductor value of 47
μ
H with 0.2
DCR results
in a peak switch current of
I
V
W
e
mA
PEAK
s
H
=
=
4 5
1 0
.
1
862
08
(
1 0
.
10
47
.
.
)
W
m
m
Substituting I
PEAK
into Equation 05 results in
)(
2
Since 17.5
μ
J > 13.7
μ
J, the 47
μ
H inductor will work. This
trial-and-error approach can be used to select the opti-
mum inductor. Keep in mind the switch current maximum
rating of 1.5A. If the calculated peak current exceeds this,
an external power transistor can be used.
E
H
A
J
L
=
(
)
=
1
47
0 862
.
17 5
09
(
2
μ
μ
.
.
)
A resistor can be added in series with the I
LIM
pin to invoke
switch current limit. The resistor should be picked such
that the calculated I
PEAK
at minimum V
IN
is equal to the
Maximum Switch Current (from Typical Performance
Characteristic curves). Then, as V
IN
increases, switch
current is held constant, resulting in increasing efficiency.
Inductor Selection — Step-Down Converter
The step-down case (Figure 5) differs from the step-up in
that the inductor current flows through the load during
both the charge and discharge periods of the inductor.
Current through the switch should be limited to ~800mA
in this mode. Higher current can be obtained by using an
external switch (see Figure 6). The I
LIM
pin is the key to
successful operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula
I
I
DC
V
V
+
V
V
V
PEAK
OUT
OUT
–
D
IN
SW
D
=
+
2
10
(
)
where DC = duty cycle (0.69)
V
SW
= switch drop in step-down mode
V
D
= diode drop (0.5V for a 1N5818)
I
OUT
= output current
V
OUT
= output voltage
V
IN
= minimum input voltage
V
SW
is actually a function of switch current which is in turn
a function of V
IN
, L, time and V
OUT
. To simplify, 1.5V can
be used for V
SW
as a very conservative value.
Once I
PEAK
is known, inductor value can be derived from
V
V
V
I
PEAK
where t
ON
= switch ON time (10
μ
s).
Next, the current limit resistor R
LIM
is selected to give
I
PEAK
from the R
LIM
Step-Down Mode curve. The addition
of this resistor keeps maximum switch current constant as
the input voltage is increased.
L
t
INMIN
SW
OUT
ON
=
–
–
( )
11
As an example, suppose 5V at 250mA is to be generated
from a 9V to 18V input. Recalling Equation (10),
PEAK
=
Next, inductor value is calculated using Equation (11)
I
mA
mA
)
+
+
=
2 250
0 69
5
0 5
.
9
0 5
.
498
12
.
–1 5
. (
)
L
mA
s
H
=
=
9
1 5
5
498
10
50
13
(
– . –
.
)
μ
μ
Use the next lowest standard value (47
μ
H).
Then pick R
LIM
from the curve. For I
PEAK
= 500mA,
R
LIM
= 82
.
Inductor Selection — Positive-to-Negative Converter
Figure 7 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
(
In this mode the switch is arranged in common collector
or step-down mode. The switch drop can be modeled as
a 0.75V source in series with a 0.65
resistor. When the
P
V
V
I
L
OUT
D
OUT
=
+
)(
)
|
|
.
(
)
14
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