AN1441 datasheet(5/13 Pages) STMICROELECTRONICS

5 / 13 page

AN1441

Functional description

Doc ID 8422 Rev 2

5/13

The formula to calculate RSET is:

RSET = VSET / ISET

ISET = ILIMIT / 1110

then:

RSET = 1.24 x 1110 / ILIMIT

where VSET is the pin 5 voltage; in the case of current limitation, this voltage corresponds to

the internal VREF (see Figure 1). ISET is the current flowing into the RSET resistor.

Figure 6 shows the programmable current range joined with the RSET. The minimum ISC

value is up to 200 mA.

Figure 6.

RSET vs. ISC characteristics

1.4

Fault

The FAULT pin (pin 8) is an open drain output useful to warn the microprocessor that a fault

condition has occurred. The fault condition starts in the following conditions:

–

if the IOUT current exceeds the ILIMIT value set

–

if a short-circuit occurs

–

if the devices goes in thermal protection

The FAULT pin should be connected to the IN pin (VCC) by a 100 k pull-up resistor. This N-

channel MOSFET can drive a LED in pull-up configuration as shown in Figure 4: Test circuit.

In a typical USB application the ST890 typically supplies a load up to 44

Ω in parallel with 10

µF that represents any bypass capacitor directly connected across the power USB line.

When the power switch is turned ON, an inrush current flows through the capacitor and

causes an unwanted FAULT warning signal, as seen in Figure 7. To avoid the controller

going into an alarm state, an RC filter can be placed on the FAULT pin, as seen in Figure 8.

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