L7800 SERIES
Figure 35: Adjustable Output Voltage with Temperature Compensation
R
R
2
V
= V (1+
) + V
BE
O
XX
1
NOTE: Q is connected as a diode in order to compensate the variation of the Q V with the temperature. C allows a slow rise time of the V .
2
1
BE
O
Figure 36: Light Controllers (V
= V + V
)
BE
Omin
XX
V
rises when the light goes up
O
V
falls when the light goes up
O
Figure 37: Protection against Input Short-Circuit with High Capacitance Loads
Application with high capacitance loads and an output voltage greater than 6 volts need an external diode (see fig. 33) to protect the device
against input short circuit. In this case the input voltage falls rapidly while the output voltage decrease slowly. The capacitance discharges by
means of the Base-Emitter junction of the series pass transistor in the regulator. If the energy is sufficiently high, the transistor may be de-
stroyed. The external diode by-passes the current from the IC to ground.
23/34
STMICROELECTRONICS [ ST ]
