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參數(shù)資料
| 型號(hào)： | LT1111MJ8 |
| 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)： | 11/16頁(yè) |
| 文件大?。?/td> | 337K |
| 代理商： | LT1111MJ8 |
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11
LT1111
U
S
A
When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground. D1 MUST BE A SCHOTTKY
DIODE The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1111. Output voltage is deter-
mined by:
O
PPLICATI
IU
U
Figure 6. Q1 Permits Higher Current Switching.
LT1111 Functions as Controller.
V
R
R
V
OUT
=
+
(
)
1
2
1
1 25
.
23
(
)
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100
resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1111 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode con-
nected to SW2).
Higher Current Step-Down Operation
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
current limit sense. When the voltage drop across R1
equals a V
BE
, the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
the I
LIM
pin. This also lowers the maximum drop across R1
to V
BE
– V
D
, increasing efficiency. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the “Inductor Selection — Step-
Down Converter” section with the following conservative
expression for V
SW
:
V
V
V
V
SW
R
Q SAT
1
=
+
≈
1
1 0
.
24
(
)
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V
IN
is between 10V and 20V.
LT1111 TA08
D1
1N5821
+
+
V
OUT
V
IN
30V
MAX
L1
R1
0.3
R2
220
Q1
MJE210 OR
ZETEX ZTX749
R3
330
R4
R5
C1
V
OUT
= 1.25V
(
1 +
)
LT1111
GND
SW2
SW1
V
IN
I
L
FB
C2
In Figure 8, the input is negative while the output is
positive. In this configuration, the magnitude of the input
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
Inverting Configurations
The LT1111 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, the arrangement is very similar to
a step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and
V
OUT
should be less than 6.2V. More negative out-
put voltages can be accommodated as in the prior section.
LT1111 F07
–V
OUT
+
C2
+
C1
D1
1N5818
V
IN
R1
R2
L1
GND
SW2
SW1
LIM
I
IN
V
R3
FB
LT1111
Figure 7. Positive-to-Negative Converter
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