FS1084 datasheet(8/9 Pages) FORTUNE

8 / 9 page

FS1084

Rev. 1.0

8/9

14. Application Description

The FS1117 keeps a constant 1.25V between the

output pin and the adjust pin. By placing a resister

R1 across these two pins a constant current flows

through R1,assign to the ladj current and into the R2

resister producing a voltage equal to the

(1.25/R1)*R2+ladj*R2 which will be added to the

1.25V to set the output voltage. This is summarized

in the above equation. Since the minimum load

current requirement of be FS1117 is 10mA, R1 is

typically selected to be 121Ω resistor so that it

automatically satisfies the minimum current

requirement . Notice that since ladj is typically in the

range and should only be considered when a very

precise output voltage setting is required. For

example, in a typical 3.3V application where

R1=121Ω and R220Ω the error due to ladj is only

0.3% of the nominal set point.

Stability

The FS1117 requires the use of an output

capacitor as part of the frequency compensation

in order to make the regulator stable. For most

applications a minimum of 100μ F aluminum

electrolytic capacitor insures both stability and

good transient response.

Thermal Design

The FS1117 incorporates an internal thermal

shutdown that protects the device when the junction

temperature exceeds the maximum allowable

junction temperature. Although this device can

operate with junction temperatures in the range of

150℃, it is recommended that the selected heat sink

be chosen such that during maximum continuous

load operation the junction temperature is kept

below the temperature.

Pd=VOUT × IOUT

TJ=TA+Pd × θ JA < 150℃

Layout Consideration

The output capacitors must be located as close to

the VOUT terminal of the device as possible. It is

recommended to use a section of a layer of the PC

board as a plane to connect the VOUT pin to the

output capacitors that may result due to excessive

trace inductance.

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