LM2575D2T-005 Datasheet, PDF (21/28 Pages) ON Semiconductor – 1.0 A, Adjustable Output Voltage, Step-Down

LM2575, NCV2575
If the input voltage is greater than −12 V, the output will rise
above −12 V accordingly, but will not damage the regulator.
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.
C_in
100 mF
/50 V
4
+V_in
LM2575−12
Feedback
1
Output ^D1
C_out
1000 mF
/16 V
2
3 GND 5 ON/OFF
1N5817 ^Regulated
Output
V_out = -12 V
L1
Unregulated
DC Input
-V_in = -5.0 V to -12 V
150 mH
Load Current from
200 mA for V_in = -5.2 V
to 500 mA for V_in = -7.0 V
Figure 29. Negative Boost Regulator
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 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 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 the input voltage
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.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
+V_in
C_in
100 mF
+V_in
1 ^{LM2575−XX}
C1
0.1 mF 5 ON/OFF 3 GND
R1
47 k
R2
47 k
NOTE: This picture does not show the complete circuit.
Figure 30. Delayed Startup Circuitry
Undervoltage Lockout
Some applications require the regulator to remain off until
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:
ǒ Ǔ Vth [ VZ1 )
1
)
R2
R1
VBE (Q1)
+V_in
R2
R3
10 k 47 k
+V_in
LM2575−5.0
1
C_in
100 mF 5 ON/OFF 3 GND
Z1
1N5242B
R1
10 k
Q1
2N3904
V_th ≈ 13 V
NOTE: This picture does not show the complete circuit.
Figure 31. Undervoltage Lockout Circuit for
Buck Converter
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