Function Description

Pin Functions

This is the positive i

nput 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.

SW

Internal switch.The voltage at this pin switches

minimize coupling to sensitive circuitry, the PC

board copper area connected to this pin should

be kept a minimum.

FB

Senses the regulated output voltage to complete

the feedback loop.

EN

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 0.7V 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

Thermal Considerations

The FAC1591 is available in SOP8 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

FAC1591

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 FAC1591 (SOP8

package) operating as a buck-switching regulator

in an ambient temperature of 25

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

output voltage down by the ratio:

VFB = VOUT * R2 / (R1 + R2)

Thus the output voltage is:

VOUT = 1.222 * (R1 + R2) / R2

R2 can be as high as 100KΩ , but a typical

value is 10KΩ. Using that value, R1 is

determined by:

R1 ~= 8.18 * (VOUT – 1.222) (KΩ)

For example, for a 3.3V output voltage, R2 is

10KΩ, and R1 is 17KΩ.

2008.01. 28

9/12

Revision No : 0

FAC1591