lm2575-5 ON Semiconductor, lm2575-5 Datasheet - Page 21

• Manufacturer Part Number: lm2575-5
• Description: 1.0 A, Adjustable Output Voltage, Step-down Switching Regulator
• Manufacturer: ON Semiconductor

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
standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of mF). 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 Startup
already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
limited, this delayed startup feature becomes very useful.
is applied and the time when the output voltage comes up,
the circuit in Figure 30 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 1.4 V, the regulator starts
up. Resistor R1 is included to limit the maximum voltage
applied to the ON/OFF pin, 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
100 mF
Unregulated
DC Input
-V
out
/50 V
The same design rules as for the previous inverting
Another important point is that these negative boost
There are some applications, like the inverting regulator
To provide a time delay between the time the input voltage
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
in
C
= -5.0 V to -12 V
in
must be chosen larger than would be required for a
+V
Figure 29. Negative Boost Regulator
in
1
3
LM2575−12
150 mH
GND
L1
5
ON/OFF
4
Feedback
Output
2
Load Current from
200 mA for V
to 500 mA for V
1N5817
D1
V
in
Regulated
out
C
1000 mF
/16 V
= -5.2 V
in
Output
out
http://onsemi.com
= -7.0 V
= -12 V
21
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 31
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck−boost converter
is shown in Figure 32. Resistor R3 pulls the ON/OFF pin
high and keeps the regulator off until the input voltage
reaches a predetermined threshold level, which is
determined by the following expression:
Some applications require the regulator to remain off until
NOTE: This picture does not show the complete circuit.
NOTE: This picture does not show the complete circuit.
1N5242B
Figure 31. Undervoltage Lockout Circuit for
100 mF
+V
+V
in
10 k
10 k
C
Figure 30. Delayed Startup Circuitry
R2
Z1
R1
in
in
V
th
[ V
47 k
R3
Z1
Buck Converter
Q1
2N3904
C1
0.1 mF
) 1 ) R2
47 k
R1
+V
in
1
C
100 mF
+V
in
5
R1
in
LM2575−XX
1
ON/OFF
R2
47 k
V
5
BE
LM2575−5.0
V
ON/OFF
th
( Q1 )
≈ 13 V
3
GND
3
GND