FAC2596 datasheet(11/16 Pages) FS

11 / 16 page

Function Description

Pin Functions

+VIN

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

Ground

Circuit ground.

Output

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 connect

ed to this pin should

be kept a minimum.

Feedback

Senses the regulated output voltage to complete

the feedback loop.

ON/OFF

Allows the switching regulator circuit to be

shutdown using logic level signals thus dropping

the total input supply current to approximately

80uA. 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

25V) shuts the regulatordown. If this shutdown

feature is not needed, the ON /OFF pin can be

wired to the ground pin or it can be left open, in

either case the regulator will be in the ON

condition.

Thermal Considerations

The FAC2596 is available in two packages, a 5-pin

TO-220B/TO-220 and a 5-pin surface mount

TO-263.

The TO-220B/TO-220 package needs a heat sink

under most conditions. The size of the heatsink

depends on the input voltage, the output voltage,

the load current and the ambient temperature.

The junction temperature rises above

ambient temperature for a 3A load and different

input and output voltages

The data for these

curves

was

taken

with

the

FAC2596

(TO-220B/TO-220 package) operating as a buck

switching regulator in an ambient temperature of

oC (still air). These temperature rise numbers

are all approximate and there are many factors

that can affect these temperatures. Higher

ambient temperatures requi re more heat sinking.

The TO-263 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 other

heat producing components,such as the catch

diode and inductor. The PC board copper area

that the package is soldered to should be at least

0.4 in

2, and ideally should have 2 or more square

inches of 2 oz. Additional copper area improves

the thermal characteristics, but with copper areas

greater than approximately 6 in

, only small

improvements in heat dissipation are realized. If

further thermal improvements are needed, double

sided, multilayer PC board with large copper

areas and/or airflow are recommended.

The

FAC2596

(TO-263

package)

junction

temperature rise 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 on a 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 be aware that there are many

factors that can affect the junction temperature.

When load currents higher than 3A are used,

double sided or multilayer PC boards with large

copper areas and/or airflow might be needed,

especially for high ambient temperatures and

high output voltages.

For the best thermal performance, wide copper

traces and generous amount s 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.

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to

FB(FAC2596-ADJ) The voltage divider divides the

FAC2596

2008. 02. 18

11/16

Revision No : 1

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