LM2575 datasheet(20/28 Pages) MOTOROLA

20 / 28 page

LM2575

MOTOROLA ANALOG IC DEVICE DATA

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

µF). 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

µF

Cin

100

µF

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:

th [

Z1 )

) R2

(Q1)

Figure 31. Undervoltage Lockout Circuit for

Buck Converter

10 k

1N5242B

10 k

2N3904

47 k

Vth ≈ 13 V

Cin

100

µF

LM2575–5.0

Gnd

ON/OFF

+Vin

NOTE:

This picture does not show the complete circuit.

Figure 32. Undervoltage Lockout Circuit for

Buck–Boost Converter

15 k

1N5242B

15 k

2N3904

68 k

Vth ≈ 13 V

Cin

100

µF

LM2575–5.0

Gnd

ON/OFF

+Vin

Vout = –5.0 V

NOTE:

This picture does not show the complete circuit.

Adjustable Output, Low–Ripple Power Supply

A 1.0 A output current capability power supply that

features an adjustable output voltage is shown in Figure 33.

This regulator delivers 1.0 A into 1.2 V to 35 V output. The

input voltage ranges from roughly 8.0 V to 40 V. In order to

achieve a 10 or more times reduction of output ripple, an

additional L–C filter is included in this circuit.

LM2575 Datasheet(PDF) 20 Page - Motorola, Inc

LM2575 Datasheet(HTML) 20 Page - Motorola, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL