LM2575 datasheet(21/28 Pages) ONSEMI

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LM2575, NCV2575

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If the input voltage is greater than −12 V, the output will rise

above −12 V accordingly, but will not damage the regulator.

Figure 29. Negative Boost Regulator

1N5817

150

Output

Feedback

Regulated

Output

Vout = -12 V

Load Current from

200 mA for Vin = -5.2 V

to 500 mA for Vin = -7.0 V

Unregulated

DC Input

-Vin = -5.0 V to -12 V

Cout

1000

/16 V

Cin

100

/50 V

LM2575−12

ON/OFF

GND

+Vin

Design Recommendations:

The same design rules as for the previous inverting

buck−boost converter can be applied. The output capacitor

Cout 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

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.

Figure 30. Delayed Startup Circuitry

47 k

LM2575−XX

GND

ON/OFF

47 k

+Vin

0.1

Cin

100

NOTE: This picture does not show the complete circuit.

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:

Vth [ VZ1 ) 1 )

VBE (Q1)

Figure 31. Undervoltage Lockout Circuit for

Buck Converter

10 k

1N5242B

10 k

2N3904

47 k

Vth ≈ 13 V

Cin

100

LM2575−5.0

GND

ON/OFF

+Vin

NOTE: This picture does not show the complete circuit.

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