Datasheet

Output voltage selection for adjustable version ST715XX, ST715XX25, ST715XX33
8/19 Doc ID 14414 Rev 7
5 Output voltage selection for adjustable version
The ST715xx features an adjustable output voltage using two external resistors connected
as a voltage divider to the FB pin as shown in the typical application circuit Figure 4. The
output voltage is set using the following equation:
Equation 1
V
OUT
= V
FB
(1 + R
1
/R
2
)
where typically V
FB
= 1.2 V. Choose R
2
≥ 5 kΩ in order to optimize quiescent current,
accuracy, and high-frequency power-supply rejection. To simplify resistor selection use the
following equation:
Equation 2
R
1
= R
2
x (V
OUT
/V
FB
- 1)
5.1 External capacitor requirements
A 0.1 µF or larger input bypass capacitor, connected between IN and GND and located
close to the device, is recommended to improve transient response and noise rejection of
the power supply as a whole. A higher-value input capacitor may be necessary if large, fast-
rise-time load transients will be present in the application and if the device is located several
inches away from the power source.
The ST715xx 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 will properly stabilize this loop.
5.2 Power dissipation and junction temperature
To ensure reliable operation, worst-case junction temperature should not exceed 125 °C.
This restriction limits the power dissipation the regulator can handle in any given application.
To ensure that the junction temperature is within acceptable limits, calculate the maximum
allowable dissipation, P
D(max)
, and the actual dissipation, P
D
, which must be less than or
equal to P
D(max)
.
The maximum power dissipation limit is determined using the following equation:
Equation 3
P
D(max)
= (T
JMAX
- T
A
) / R
thJA
where:
T
JMAX
is the maximum allowable junction temperature.
R
thJA
is the thermal resistance junction-to-ambient for the package.
T
A
is the ambient temperature.