LM2574, NCV2574
http://onsemi.com
20
The circuit in Figure 31 shows the negative boost
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.
1N5817
330 mH
Output
7
1
Feedback
V
out
= −12 V
Load Current
60 mA for V
in
= −5.2 V
120 mA for V
in
= −7.0 V
V
in
L1
D1
C
out
1000 mF
C
in
22 mF
LM2574−12
5
34
ON
/OFFPwr
Gnd
+V
in
2
Sig
Gnd
−5.0 to −12 V
Figure 31. Negative Boost Regulator
(5)
(12)
(3)
(14)
(4)(6)
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 what would be required for
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.
Another important point is that these negative boost
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
There are some applications, like the inverting regulator
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.
To provide a time delay between the time when the input
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.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
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.
R1
47 k
LM2574−XX
5
2
and
4
3 Gnds
Pins
ON/OFF
R2
47 k
+V
in
+V
in
C1
0.1 mF
C
in
22 mF
NOTE: This picture does not show the complete circuit.
Figure 32. Delayed Startup Circuitry
(5)
(12)
(4)
and
(6)
Undervoltage Lockout
Some applications require the regulator to remain off until
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:
V
th
[ V
Z1
)
ǒ
1.0 )
R2
R1
Ǔ
V
BE
(
Q1
)
R1
10 k
Z1
1N5242B
R2
10 k
Q1
2N3904
R3
47 k
C
in
22 mF
LM2574−XX
5
2
and
4
3 Gnds
Pins
ON/OFF
+V
in
+V
in
NOTE: This picture does not show the complete circuit.
Figure 33. Undervoltage Lockout Circuit for
Buck Converter
(5)
(12)
(4)
and
(6)
