lm2574dw ON Semiconductor, lm2574dw Datasheet - Page 20

• Manufacturer Part Number: lm2574dw
• Description: 0.5 A, Adjustable Output Voltage, Stepdown Switching Regulator
• Manufacturer: ON Semiconductor

configuration. The input voltage in this application ranges
from −5.0 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.
V
Design Recommendations:
buck−boost converter can be applied. The output capacitor
C
a 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 any 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.
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 32 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.
−5.0 to −12 V
in
22 mF
out
The circuit in Figure 31 shows the negative boost
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 when the input
C
in
must be chosen larger than what would be required for
+V
Figure 31. Negative Boost Regulator
in
5
4
(12)
Pwr
Gnd
(6)
LM2574−12
330 mH
2
L1
Sig
Gnd
(4)
3
(14)
(3)
ON/OFF
(5)
1
Feedback
Output
7
Load Current
60 mA for V
120 mA for V
1N5817
D1
in
in
V
LM2574, NCV2574
= −5.2 V
C
1000 mF
out
= −7.0 V
out
http://onsemi.com
= −12 V
20
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 33
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck−boost converter
is shown in Figure 34. 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:
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
Some applications require the regulator to remain off until
1N5242B
Figure 33. Undervoltage Lockout Circuit for
+V
NOTE: This picture does not show the complete circuit.
NOTE: This picture does not show the complete circuit.
+V
22 mF
10 k
10 k
R1
Z1
R2
Figure 32. Delayed Startup Circuitry
in
in
C
in
V
th
47 k
R3
[ V
Buck Converter
Q1
2N3904
Z1
C1
0.1 mF
47 k
R1
) 1.0 ) R2
+V
in
5
C
22 mF
+V
in
(12)
3
in
5
LM2574−XX
ON/OFF
R2
47 k
(12)
R1
3
(5)
LM2574−XX
ON/OFF
(5)
V
and
BE
2
4
( Q1 )
and
Gnds
Pins
2
4
Gnds
Pins
and
(4)
(6)
and
(4)
(6)