LM2576 datasheet(18/23 Pages) ARTSCHIP

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LM2576/LM2576HV

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Additional Applications (Continued)

The switch currents in this buck-boost configuration are higher

than in the standard buck-mode design, thus lowering the

available output current. Also, the start-up input current of the

buck-boost converter is higher than the standard buck-mode

regulator, and this may overload an input power source with a

current limit less than 5A. Using a delayed turn-on or an

undervoltage lockout circuit (described in the next section)

would allow the input voltage to rise to a high enough level

before the switcher would be allowed to turn on.

Because of the structural differences between the buck and the

buck-boost regulator topologies, the buck regulator design

procedure section can not be used to select the inductor or the

output capacitor. The recommended range of inductor values

for the buck-boost design is between 68μH and 220μH, and

the output capacitor values must be larger than what is normally

required for buck designs. Low input voltages or high output

currents require a large value output capacitor (in the thousands

of micro Farads).

The peak inductor current, which is the same as the peak

switch current, can be calculated from the following formula:

Where fOSC=52 kHz. Under normal continuous inductor current

operating conditions, the minimum VIN represents the worst

case. Select an inductor that is rated for the peak current

anticipated.

FIGURE 10. Inverting Buck-Boost Develops-12V

Also, the maximum voltage appearing across the regulator is

the absolute sum of the input and output voltage. For a -12V

output, the maximum input voltage for the LM2576 is +28V, or

+48V for the LM2576HV.

The Switchers Made Simple (version 3.0) design software can

be used to determine the feasibility of regulator designs using

different topologies, different input-output parameters, different

components, etc.

NEGATIVE BOOST REGULATOR

Another variation on the buck-boost topology is the negative

boost configuration. The circuit in Figure 11 accepts an input

voltage ranging from -5V to -12V and provides a regulated -12V

output. Input voltages greater than -12V will cause the output to

rise above -12V, but will not damage the regulator.

Typical Load Current

400mA for VIN = -5.2V

750mA for VIN = -7V

Note: Heat sink may be required.

FIGURE 11. Negative Boost

Because of the boosting function of this type of regulator, the

switch current is relatively high, especially at low input voltages.

Output load current limitations are a result of the maximum

current rating of the switch. Also, boost regulators can not

provide current limiting load protection in the event of a shorted

load, so some other means (such as a fuse) may be necessary.

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off until

the input voltage reaches a certain threshold. An under voltage

lockout circuit which accomplishes this task is shown in Figure

, while Figure 13 shows the same circuit applied to a

buck-boost configuration. These circuits keep the regulator off

until the input voltage reaches a predetermined level.

01147616

Note: Complete circuit not shown.

FIGURE 12. Undervoltage Lockout for Buck Circuit.

LM2576 Datasheet(PDF) 18 Page - ARTSCHIP ELECTRONICS CO.,LMITED.

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