FAC1534 datasheet(11/15 Pages) FS

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Function Description

Pin Functions

VIN

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 transients and to

supply the switching currents needed by the regulator

Gnd

Circuit ground.

Internal switch. The voltage at this pin switches between

(VIN – VSAT) and approximately – 0.5V, with a duty cycle

of approximately VOUT / VIN. To minimize coupling to

sensitive circuitry, the PC board copper area connected

to this pin should be kept a minimum.

Senses the regulated output voltage to complete the

feedback loop.

Allows the switching regulator circuit to be shutdown

using logic level signals thus dropping the total input

supply current to approximately 30uA. Pulling this pin

below a threshold voltage of approximately 0.7 V turns

the regulator down, and pulling this pin above 1.3V

(up

to a maximum of 12V) shuts the regulator on. For

automatic starup condition , can be implemented by the

addition of a resistive voltage divider from VIN to GND.

Thermal Considerations

The FAC1534 is available in SOP-8 package.

The SOP8 package needs a heat sink under most

conditions. The size of the heat sink depends on the

input voltage, the output voltage, the load current and the

ambient temperature.The FAC1534 junction temperature

rises above ambient temperature for a 2A load and

different input and output voltages. The data for these

curves was taken with the FAC1534(SOP8 package)

operating as a buck-switching regulator in an ambient

temperature of 25oC (still air). These temperature rise

numbers are all approximate and there are many factors

that can affect these temperatures. Higher ambient

temperatures require more heat sinking.

For the best thermal performance, wide copper traces

and generous amounts of printed circuit board copper

should be used in the board layout. (Once 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.

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 board 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, multi-layer board and

the amount of solder on the board.

The effectiveness of the PC board to dissipate heat also

depends on the size, quantity and spacing of other

components on the board, as well as whether the

surrounding air is still or moving. 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.

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to FB. The

voltage divider divides the

FAC1534

2015.06. 17

11/15

Revision No : 0

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