TLE 4279

Semiconductor Group

5

1998-11-01

Circuit Description

The control amplifier compares a reference voltage, made highly accurate by resistance

balancing, with a voltage proportional to the output voltage and drives the base of the

series PNP transistor via a buffer. Saturation control as a function of the load current

prevents any over-saturation of the power element.

In the reset generator block a comparator compares a reference voltage independent of

the input voltage with the scaled-down output voltage. If the output voltage reaches 4.6 V

the reset delay capacitor is discharged and the reset output is set to low. This low is

guaranteed down to an output voltage of 1 V. As the output voltage increases again,

from 4.6 V onward the reset delay capacitor is charged with constant current. When the

capacitor voltage reaches the upper switching threshold

V

choosing the value of this capacitor, the reset delay time can be selected over a wide

range. With the reset threshold input RE it is possible to lower the reset threshold

V

pin RE is connected to pin Q via a voltage divider, for example, the reset condition is

reached when this voltage is decreased below the switching threshold

V

Another comparator compares the signal of the pin SI, normally fed by a voltage divider

from the input voltage, with the reference and gives an early warning on the pin SO. It is

also possible to superwise an other voltage e.g. of a second regulator, or to build a

watchdog circuit with few external components.

Application Description

The input capacitor

C

approx. 1

capacitance can be damped. The output capacitor C

regulating circuit. Stability is guaranteed at values

≥ 10 µF and an ESR ≤ 10 Ω within the

operating temperature range. Both reset output and sense output are open collector

outputs and have to be connected to 5 V output via external pull-up resistors

≥ 10 kΩ.

For small tolerances of the reset delay the spread of the capacitance of the delay

capacitor and its temperature coefficient should be noted.