AN1441 datasheet(3/13 Pages) STMICROELECTRONICS

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AN1441

Functional description

Doc ID 8422 Rev 2

3/13

Figure 3.

ST890 in a short-circuit condition

When in normal function, the OUTPUT current, divided by 1110 by the current mirror

circuitry (see Figure 1), flows in the external RSET resistor creating a VSET voltage. VSET is

compared to the internal reference voltage (1.24 V), and the error amplifier provides the VGS

voltage to drive the high side P-channel MOSFET. Due to this feedback, it is possible to limit

the output current at ILIMIT. When a short-circuit occurs, the VSET drops because of the

internal current mirror and the V+ input of the error amplifier becomes lower than 1.24 V

(internal reference voltage). In this case the error amplifier cannot work in the linear area

and current control is not possible. To ensure current limitation, even in short-circuit

conditions, the error amplifier is supported by a correction circuitry and the result is shown in

Figure 3. After the rising of the output current, the feedback circuitry begins to run and the

output current is then equal to or less than 1.2 times the ILIMIT value. This measurement was

taken with the test circuit seen in Figure 4.

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