LM2574 datasheet(19/24 Pages) NSC

19 / 24 page

Application Hints (Continued)

The data sheet thermal resistance curves and the thermal

model in

Switchers Made Simple software (version 3.3)

can estimate the maximum junction temperature based on

operating conditions. ln addition, the junction temperature

can be estimated in actual circuit operation by using the fol-

lowing equation.

j = Tcu +(θj-cu xPD)

With the switcher operating under worst case conditions and

all other components on the board in the intended enclosure,

measure the copper temperature (T

cu ) near the IC. This can

be done by temporarily soldering a small thermocouple to

the pc board copper near the IC, or by holding a small ther-

mocouple on the pc board copper using thermal grease for

good thermal conduction.

The thermal resistance (

j-cu) for the two packages is:

j-cu = 42˚C/W for the N-8 package

j-cu = 52˚C/W for the M-14 package

The power dissipation (P

D) for the IC could be measured, or

it can be estimated by using the formula:

Where I

S is obtained from the typical supply current curve

(adjustable version use the supply current vs. duty cycle

curve).

Additional Applications

INVERTING REGULATOR

Figure 11 shows a LM2574-12 in a buck-boost configuration

to generate a negative 12V output from a positive input volt-

age. 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 voltage and regu-

lates it to −12V.

For an input voltage of 8V or more, the maximum available

output current in this configuration is approximately 100 mA.

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 0.6A. 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 to select the in-

ductor 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 cur-

rent operating conditions, the minimum V

IN represents the

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

rent 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 LM2574 is +28V,

or +48V for the LM2574HV.

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 12 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.

DS011394-19

Note: Pin numbers are for the 8-pin DIP package.

FIGURE 11. Inverting Buck-Boost Develops −12V

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