AN557 datasheet(42/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

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Figure 56.

Figure 57.

Figure 58.

POWER SUPPLY WITH 0 TO 25V ADJUSTABLE OUTPUT VOLTAGE

a) It is a classical solution with high performance that make use of a negative reference voltage equal to

the value of the internal voltage of the device (5.1V).

To generate this negative reference voltage, it is useful to equip the mains trnsformer with another sec-

ondary winding at a low voltage of around 8Vac and capable of delivering a few dozen of mA.

During the phase of starting up and stopping of the mains, it is important to avoid generating oscillations

around the value of the output voltage , including the zero voltage.

For this reason a network consisting of two NPN signal transistors TR1 and TR2 and some resistors

has been introduced.

The transistor TR2 remains is saturation when TR1 is off, untill the output voltage of the negative reg-

ulator reaches 4.3V At this point TR1 goes in saturation, sending off TR2. In this way the soft start is

AN557 bảng dữ liệu(PDF) 42 Page - STMicroelectronics