ST715 datasheet(8/24 Pages) STMICROELECTRONICS

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Output voltage selection for adjustable version

ST715

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DocID14414 Rev 9

Output voltage selection for adjustable version

The ST715 features an adjustable output voltage due to two external resistors connected as

a voltage divider to the FB pin as shown in Figure 4. The output voltage is set by the

following equation:

Equation 1

VOUT = VFB (1 + R1/R2)

where typically VFB = 1.2 V. Choose R2 ≥ 5 kΩ to optimize the quiescent current, accuracy,

and high-frequency power supply rejection. The resistor selection is given by the following

equation:

Equation 2

R1 = R2 x (VOUT/VFB - 1)

5.1

External capacitor requirements

A 0.1 µF or a larger input bypass capacitor, connected between IN and GND and located

close to the device, is recommended. In this manner, the transient response and noise

rejection of the power supply, as a whole, improve. A higher value of the input capacitor may

be necessary if large, fast-rise-time load transients are present in the application and if the

device is several inches far from the power source.

The ST715 requires an output capacitor connected between OUT and GND to stabilize the

internal control loop. Any capacitor, (including ceramic and tantalum) with a value higher

than 0.47 µF, stabilizes this loop.

5.2

Power dissipation and junction temperature

For a reliable operation, junction temperature should not exceed 125 °C. This limits the

power dissipation the regulator can handle in any application. To guarantee that the junction

temperature is within acceptable limits, calculate the maximum allowable dissipation,

PD(max), and the dissipation, PD, which must be less than or equal to PD(max).

The maximum power dissipation limit is given by the following equation:

Equation 3

PD(max) = (TJMAX - TA) / RthJA

where:

TJMAX is the maximum allowable junction temperature

RthJA is the thermal resistance junction-to-ambient for the package

TA is the ambient temperature

The regulator dissipation is calculated by the following equation:

ST715 Fiches technique(PDF) 8 Page - STMicroelectronics