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 10μ 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

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.