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参数资料
| 型号： | LIT1108CS8-12#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, SOIC-8 |
| 文件页数： | 10/12页 |
| 文件大小： | 266K |
| 代理商： | LIT1108CS8-12#PBF |
7
LT1108
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
315
19
16 6
07
.(
)
Picking an inductor value of 100
H with 0.2 DCR results
in a peak switch current of
I
V
emA
PEAK
s
H
=
=
×
2
10
1
605
08
10
36
100
.
–(
)
–.
Substituting IPEAK into Equation 04 results in
EH
A
J
L =
()(
) =
1
2
100
6 605
18 3
09
2
..
(
)
Since 18.3
J > 16.6J, the 100H inductor will work. This
trial-and-error approach can be used to select the optimum
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.
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.
Step-Down Converter
The step-down case (Figure 2) 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 3). 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 (0.60)
VSW = switch drop in step-down mode
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 (36s).
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
060
50 5
12
15
05
500
12
.
–.
.
()
Next, inductor value is calculated using Equation (11)
L
mA
sH
==
12
1 5
5
500
36
396
13
–. –
()
Use the next lowest standard value (330
H).
Then pick RLIM from the curve. For IPEAK = 500mA,
RLIM = 220.
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,
PL = (VOUT+ VD)(IOUT)
(14)
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