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LM2594
+V in
+V in
7
LM2594
+V
0
On
Off
R2
Shutdown
Input
5.0 V
0
On
Off
Shutdown
Input
R3
470
C in R1
100 m F 47 k
5
ON/OFF 6
R2
GN
D
+V in
5.6 k
C in
100 m F
Q1
+V in
7
LM2594
47 k
2N3906
5
ON/OFF 6
GN
-V out
D
MOC8101
R1
12 k
-V out
NOTE: This picture does not show the complete circuit.
Figure 25. Inverting Buck ? Boost Regulator Shutdown
Circuit Using an Optocoupler
With the inverting configuration, the use of the ON/OFF
pin requires some level shifting techniques. This is caused
by the fact, that the ground pin of the converter IC is no
longer at ground. Now, the ON/OFF pin threshold voltage
(1.3 V approximately) has to be related to the negative
output voltage level. There are many different possible shut
down methods, two of them are shown in Figures 25 and 26.
NOTE: This picture does not show the complete circuit.
Figure 26. Inverting Buck ? Boost Regulator Shutdown
Circuit Using a PNP Transistor
Negative Boost Regulator
This example is a variation of the buck ? boost topology
and it is called negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results
in lower output load current capability.
The circuit in Figure 27 shows the negative boost
configuration. The input voltage in this application ranges
from ? 5.0 V to ? 12 V and provides a regulated ? 12 V output.
If the input voltage is greater than ? 12 V, the output will rise
above ? 12 V accordingly, but will not damage the regulator.
R4
Feedback
C out
470 m F
+V in
LM2594
C in
100 m F/
50 V
ON/OFF
GND
D1
1N5822
R3
? 12 V @ 0.25 A
Regulated
Output
? 12 V
Unregulated
DC Input
L1
100 m H
Figure 27. Negative Boost Regulator
Design Recommendations:
The same design rules as for the previous inverting
buck ? boost converter can be applied. The output capacitor
C out must be chosen larger than would be required for a what
standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of m F). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.
Another important point is that these negative boost
converters cannot provide current limiting load protection in
the event of a short in the output so some other means, such
as a fuse, may be necessary to provide the load protection.
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