AN1441 datasheet(2/13 Pages) STMICROELECTRONICS

2 / 13 page

Functional description

AN1441

2/13

Doc ID 8422 Rev 2

Functional description

1.1

Switch resistance

The internal block diagram of the ST890 is shown in Figure 1. The high side power

MOSFET, used as a switch, has an RON lower than 90 mΩ at VCC = 3.0 V. In the case of a

load current of 1 A, the drop voltage between the IN and OUT pins is lower than 1 A x 0.1

= 0.1 V and, for a switch, it is very important to keep this drop voltage low.

Figure 2 displays the RON versus the VIN supply voltage. The RON depends on the VIN

because the P-channel MOSFET is driven by the VOUT of the internal error amplifier.

Figure 2.

ON resistance vs. VIN supply voltage

1.2

Short-circuit protection

The low resistance of the MOS channel is important. However, the control circuitry that must

quickly drive the MOS to provide enough VGS voltage to turn on the MOS channel, has

greater importance. If the control circuitry does not drive the MOS quickly enough, the

current of the card could be broken, as shown in Figure 3: ST890 in a short-circuit condition.

100

2.5

33.5

4.5

5.5

VIN (V)

TA = 25°C

AM00159v1

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