LM2575D2T-12 ON Semiconductor, LM2575D2T-12 Datasheet - Page 19

• Manufacturer Part Number: LM2575D2T-12
• Description: EASY SWITCHERE 1.0 A STEP-DOWN VOLTAGE REGULATOR
• Manufacturer: ON Semiconductor

Unregulated
DC Input
12 V to 25 V
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
applied to a buck–boost converter is shown in Figure 26.
Figure 32 in the “Undervoltage Lockout” section describes an
undervoltage lockout feature for the same converter
topology.
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 (1.4 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 27 and 28.
100 F
5.0 V
Figure 27. Inverting Buck–Boost Regulator Shut Down
MOTOROLA ANALOG IC DEVICE DATA
/50 V
It has been already mentioned above, that in some
With the inverting configuration, the use of the ON/OFF pin
C in
0
On
0.1 F
NOTE:
C1
Off
Figure 26. Inverting Buck–Boost
+V in
Regulator with Delayed Startup
Shutdown
Input
+V in
Circuit Using an Optocoupler
This picture does not show the complete circuit.
1
47 k
R1
470
R3
5 ON/OFF
LM2575–12
R2
47 k
C in
100 F
3
MOC8101
Gnd
47 k
R1
4
Output
2
Feedback
+V in
1
D1
1N5819
100 H
5
L1
LM2575–XX
ON/OFF
R2
47 k
–12 V @ 0.35 A
Regulated
C out
1800 F
/16 V
Output
3
Gnd
–V out
LM2575
Negative Boost Regulator
is called a 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.
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.
Unregulated
DC Input
–V in = –5.0 V to –12 V
100 F
Figure 28. Inverting Buck–Boost Regulator Shut Down
/50 V
This example is a variation of the buck–boost topology and
The circuit in Figure 29 shows the negative boost
C in
+V in
NOTE:
+V
+V in
Figure 29. Negative Boost Regulator
0
1
Circuit Using a PNP Transistor
On
This picture does not show the complete circuit.
C in
100 F
3
Off
LM2575–12
150 H
5.6 k
Gnd
R2
L1
Q1
2N3906
Shutdown
Input
5
+V in
1
ON/OFF
5
Feedback
Output
4
2
LM2575–XX
ON/OFF
R1
12 k
Load Current from
200 mA for V in = –5.2 V
to 500 mA for V in = –7.0 V
1N5817
D1
3
Gnd
–V out
V out = –12 V
C out
1000 F
/16 V
Regulated
Output
19