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參數(shù)資料
| 型號(hào)： | LT1111IS8-5#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, PDSO8 |
| 封裝： | LEAD FREE, SOIC-8 |
| 文件頁(yè)數(shù)： | 15/16頁(yè) |
| 文件大小： | 237K |
| 代理商： | LT1111IS8-5#PBF |
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8
LT1111
1111fd
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
VD = diode drop (0.5V for a 1N5818)
IOUT = output current
VOUT = output voltage
VIN = minimum input voltage
VSW is actually a function of switch current which is in turn
a function of VIN, L, time, and VOUT. To simplify, 1.5V can
be used for VSW as a very conservative value.
Once IPEAK is known, inductor value can be derived from:
L
VV
V
I
t
IN MIN
SW
OUT
PEAK
ON
=
( )
11
where tON = switch-on time (7s).
Next, the current limit resistor RLIM is selected to give
IPEAK from the RLIM Step-Down Mode curve. The addition
of this resistor keeps maximum switch current 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),
I
mA
PEAK =
()
+
=
2 300
050
50 5
12 15
05
600
12
.
–.
.
()
Next, inductor value is calculated using Equation (11):
L
mA
sH
==
12 1 5
5
600
764
13
–.–
.( )
Use the next lowest standard value (56
H).
Then pick RLIM from the curve. For IPEAK = 600mA, RLIM
= 56
.
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,
PL = (VOUT+ VD)(IOUT)
(14)
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
Picking an inductor value of 47
H with 0.2 DCR results
in a peak switch current of:
I
V
emA
PEAK
s
H
=
=
45
10
1
623
8
10
7
47
.
–.
( )
–.
Substituting IPEAK into Equation 4 results in:
EH
A
J
L =
()(
) =
1
2
47
0 623
9 1
9
2
..
( )
Since 9.1
J > 6.7J, the 47H 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,
consider using the LT1110. The 70% duty cycle of the
LT1110 allows more energy per cycle to be stored in the
inductor, resulting in more output power.
A resistor can be added in series with the ILIM pin to invoke
switch current limit. The resistor should be picked so the
calculated IPEAK at minimum VIN is equal to the Maximum
Switch Current (from Typical Performance Characteristic
curves). Then, as VIN 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 ~650mA
in this mode. Higher current can be obtained by using an
external switch (see Figure 6). The ILIM 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
DC
VV
V
PEAK
OUT
D
IN
SW
D
=
+
2
10
–
()
where DC = duty cycle (0.50)
VSW = switch drop in step-down mode
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