LM2576-12 MOTOROLA [Motorola, Inc], LM2576-12 Datasheet - Page 20

• Manufacturer Part Number: LM2576-12
• Description: Easy Switcher 3.0A Step-Down Voltage Regulator
• Manufacturer: MOTOROLA [Motorola, Inc]

Negative Boost Regulator
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.
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.
Design Recommendations:
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
V in
–5.0 V to –12 V
Figure 29. Inverting Buck–Boost Regulator Shutdown
100 F
20
This example is a variation of the buck–boost topology and
The circuit in Figure 30 shows the negative boost
The same design rules as for the previous inverting
C in
+V in
NOTE: This picture does not show the complete circuit.
+V
0
V in
Figure 30. Negative Boost Regulator
1
Circuit Using a PNP Transistor
On
C in
100 F
3
Off
LM2576–12
5.6 k
100 H
Gnd
R2
Q1
2N3906
Shutdown
Input
5
+V in
1
ON/OFF
5
4
Feedback
Output
2
LM2576–XX
ON/OFF
R1
12 k
Typical Load Current
400 mA for V in = –5.2 V
750 mA for V in = –7.0 V
1N5820
3
Gnd
–V out
V out = –12 V
C out
2200 F
Low Esr
LM2576
currents require a large value output capacitor (in the range
of thousands of F). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.
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.
Delayed Start–up
already mentioned above, which require a higher amount of
start–up current. In such cases, if the input power source is
limited, this delayed start–up feature becomes very useful.
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 31 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory.
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the ON/OFF
pin, the regulator could be switched periodically on and off
with the line (or double) frequency.
Undervoltage Lockout
the input voltage reaches a certain threshold level. Figure 32
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck–boost converter is
shown in Figure 33. Resistor R3 pulls the ON/OFF pin high
and keeps the regulator off until the input voltage reaches a
Another important point is that these negative boost
There are some applications, like the inverting regulator
To provide a time delay between the time when the input
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
Some applications require the regulator to remain off until
100 F
+V in
NOTE: This picture does not show the complete circuit.
C in
Figure 31. Delayed start–up Circuitry
MOTOROLA ANALOG IC DEVICE DATA
C1
0.1 F
47 k
R1
+V in
1
5
LM2576–XX
ON/OFF
R2
47 k
3
Gnd