BM1507

150KHz, 3A PWM BUCK DC/DC CONVERTER

Application Information

Pin Functions

This is the positive input supply for the IC switching regulator.

A suitable input bypass capacitor must be present at this pin to

minimize voltage transie s and to supply the switching currents

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needed by the regulator.

than approximately 6in , only small improvements in heat

dissipation are realized. If further thermal improvements are

needed, double sided, multi-layer PC board with large copper

areas and/or airflow will be recommended.

2

Ground

Circuit ground.

The BM1507 (TO-252 package) junction temperature rises

above ambient temperature with a 3A load for various input and

output voltages. This data was taken with the circuit operating

as a buck-switching regulator with all components mounted n a

o

PC board to simulate the junction temperature under actual

operating conditions. This curve can be used for a quick check

for the approximate junction temperature for various conditions,

but there are many factors that can affect the junction

temperature. When load currents higher than 3A are used,

double sided or multi-layer PC boards with large copper are s

a

and/or airflow might be needed, especi ly for high ambient

al

temperatures and high output voltages.

Output

Internal switch. The voltage at this pin switches between

circuitry, the PC board copper area connected to this pin should

be kept at a minimum.

Feedback (FB)

For the best thermal performance, wide copper traces and

generous amounts of printed circuit board copper should be

used in the board layout. (One exception to this is the output

(switch) pin, which should not have large areas of copper.)

Large areas of copper provide the best transfer of heat (lower

thermal resistance) to the surrounding air, and moving air lowers

the thermal resistance even further.

Sens s the regulated output voltage to complete the feedback

e

loop.

ON/OFF (SD)

Allows the switching regulator circuit to be shutdown using logic

level signals thus dropping the total input supply current to

approximately 150uA. Pulling this pin below a threshold voltage

of approximately 1.3V turns the regulator on, and pulling this pin

above 1.3V (up to a maximum of 18V) shuts the regulator dow .

n

If this shutdown feature

not needed, the ON/OFF pin can be

is

wired to the ground pin.

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors that

will affect these numbers. Some of these factors include b ard

o

size, shape, thickness, position, location, and even board

temperature. Other factors are trace width, total printed circuit

copper area, copper thickness, single or double-sided, ulti-

m

layer board and the amount of solder on the board. The

effectiveness of the PC board to dissipate heat also depen s on

d

the size, quantity and spacing of other components on the

board, as well as whether the surrounding air is still or mov g.

in

Furthermore, some of these components such as the catch

diode will add heat to the PC board and the heat can vary as the

input voltage changes. For the inductor, depending on the

physical size, type of core material and the DC resistance, it

could either act as a heat sink taking heat away from the board,

or it could add heat to the board.

Thermal Considerations

The TO-252 surface mount package tab is designed to be

soldered to the copper on a printed circuit board. The copper

and the board are the heat sink for this package and the o her

t

heat producing components, such as the catch diode and

inductor. The PC board copper area that the package is

2

soldered to should be at least 0.8 in ,

and ideally should have

2 or more square inches of 2 oz. additional copper area which

improves the thermal characteristics. With copper areas greater

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