參數(shù)資料
型號(hào)： LT1107IS8
英文描述： DC to DC Converter
中文描述： DC至DC轉(zhuǎn)換器
文件頁(yè)數(shù)： 8/16頁(yè)
文件大?。?/td> 206K
代理商： LT1107IS8
8
LT1107
1107fa
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
switch closes, current in the inductor builds according to:
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),
(
Energy required from the inductor is:
Picking an inductor value of 100
μ
H with 0.2
DCR results
in a peak switch current of:
(
+
(
=
325
Substituting I
PEAK
into Equation (04) results in:
L
=
(
2
Since 5.28
μ
J > 3.82
μ
J, the 100
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.
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
P
f
OSC
mW
kHz
J
L
=
=
275
63
4 4
.
17
( )
μ
I
t
V
R
e
L
L
′
L
( )
=
′
1
15
( )
where DC = duty cycle (0.50 in step-down mode)
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
IN MIN
SW
OUT
=
where t
ON
= switch ON time (7
μ
s).
Next, the current limit resistor R
LIM
is selected to give
I
PEAK
from the Maximum Switch Current vs R
LIM
curve.
The addition of this resistor keeps maximum switch cur-
rent constant as the input voltage is increased.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10):
PEAK
=
(
Next, inductor value is calculated using Equation (11):
=
600
L
mA
s
H
=
12 1 5
5
7
64
13
( )
.
μ
μ
Use the next lowest standard value (56
μ
H).
Then pick R
LIM
from the curve. For I
PEAK
= 600mA, R
LIM
= 56
.
Inductor Selection –– Positive-to-Negative Converter
Figure 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
(
P
(14)
L
=
+
)
(
)
V
V
I
OUT
D
OUT
L
I
t
PEAK
ON
×
)
( )
11
I
mA
mA
)
+
+
=
2 300
0 50
.
5
0 5
.
12 1 5
0 5
.
600
12
( )
.
P
=275mW
(16)
L
=
+
)
(
)
5
0 5
.
50
V
V
mA
I
V
V
e
mA
PEAK
s
H
=
)
)
4 5
.
0 75
.
0 65
.
0 2
.
1
18
( )
0 85
100
9
.
μ
μ
E
H
A
J
)(
)
=
1
100
0 325
.
5 28
.
19
( )
2
μ
μ
U
S
A
O
PPLICATI
U
U
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