LM2575EP datasheet(19/26 Pages) NSC

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Application Hints (Continued)

Since the lead frame is solid copper, heat from the die is

readily conducted through the leads to the printed circuit

board copper, which is acting as a heat sink.

For best thermal performance, the ground pins and all the

unconnected pins should be soldered to generous amounts

of printed circuit board copper, such as a ground plane.

Large areas of copper provide the best transfer of heat to the

surrounding air. Copper on both sides of the board is also

helpful in getting the heat away from the package, even if

there is no direct copper contact between the two sides.

Thermal resistance numbers as low as 40˚C/W for the SO

package, and 30˚C/W for the N package can be realized with

a carefully engineered pc board.

Included on the Switchers Made Simple design software is

a more precise (non-linear) thermal model that can be used

to determine junction temperature with different input-output

parameters or different component values. It can also calcu-

late the heat sink thermal resistance required to maintain the

regulators junction temperature below the maximum operat-

ing temperature.

Additional Applications

INVERTING REGULATOR

Figure 10 shows a LM2575-12EP in a buck-boost configu-

ration to generate a negative 12V output from a positive

input voltage. This circuit bootstraps the regulator’s ground

pin to the negative output voltage, then by grounding the

feedback pin, the regulator senses the inverted output volt-

age and regulates it to −12V.

For an input voltage of 12V or more, the maximum available

output current in this configuration is approximately 0.35A. At

lighter loads, the minimum input voltage required drops to

approximately 4.7V.

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 1.5A. 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 de-

sign 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 68 µH and 220

µH, 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 f

osc = 52 kHz. Under normal continuous inductor

current operating conditions, the minimum V

IN represents

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

current anticipated.

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 LM2575EP is

+28V, or +48V for the LM2575HVEP.

The Switchers Made Simple (version 3.3) 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.

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

switch current is relatively high, especially at low input volt-

ages. 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.

20113215

FIGURE 10. Inverting Buck-Boost Develops −12V

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