Application information STLQ50
10/18 DocID13205 Rev 6
7 Application information
The STLQ50 is a BiCMOS linear regulator specifically designed for operating in
environments with very low power consumption requirements. The very low quiescent
current of 3 µA is obtained through the use of CMOS technology which makes the device
suitable for application that have long standby time. Its very low power consumption allows
extended battery life and the tiny packages (SOT323-5L) satisfy the space-saving
requirements of battery-powered equipment. Moreover, the STLQ50 provides wide input
voltage operation from 2.5 V up to 12 V.
The PMOS pass element also permits a very good dropout values of 0.7 V at full load and at
125 °C without affecting consumption characteristics.
7.1 External components
The typical application schematic of the STLQ50 is shown in Figure 4 - Figure 5, 1 µF input
and output capacitors placed close to the device are required for proper operation. The
device is stable with electrolytic and ceramic output capacitors having values higher than 1
µF (see Figure 11 for stability details).
In the adjustable version the output voltage is programmed using an external resistor
divider, as shown in Figure 5. The output voltage can be adjusted from 1.22 to 11 V and it
can be calculated using the following equation:
Equation 1
V
O
= V
FB
x (1+R
1
/R
2
)
where V
FB
= 1.222 V is the internal reference voltage.
The sum of the R
1
and R
2
resistors should be chosen in order to guarantee at least 1 µA of
divider current. Lower value resistors improve the noise performance but the quiescent
current will increase. Higher value resistors should be avoided because the ADJ leakage
current will influence the voltage set by the resistor divider, rendering the formula above no
longer valid.
The suggested design procedure is to choose R
2
= 1 MΩ and then calculate R
1
using the
following equation:
Equation 2
R
1
= (V
O
/V
FB
-1) x R
2
7.2 Power dissipation
In order to ensure proper operation, the STLQ50 junction temperature should never exceed
125 °C; this limits the maximum power dissipation the regulator can sustain in any
application. The maximum power dissipation can be calculated as:
Equation 3
P
DMAX
= (T
JMAX
- T
A
)/R
thJA
where T
JMAX
= 125 °C;
