MIC20XX datasheet(21/38 Pages) MICROCHIP

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DS20006486C-page 21

MIC20XX

5.2

ILIMIT vs. IOUT Measured

The MIC20XX’s current-limiting circuitry, during current

limiting, is designed to act as a constant current source

to the load. As the load tries to pull more than the

allotted current, VOUT drops and the input to output

voltage differential increases. When VIN – VOUT

exceeds 1V, IOUT drops below ILIMIT to reduce the drain

of fault current on the system’s power supply and to

limit internal heating of the switch.

When measuring IOUT it is important to bear this

voltage dependence in mind, otherwise the

measurement data may appear to indicate a problem

when none really exists. This voltage dependence is

illustrated in Figure 5-1 and Figure 5-2.

In Figure 5-1, output current is measured as VOUT is

pulled below VIN, with the test terminating when VOUT

is 1V below VIN. Observe that once ILIMIT is reached

IOUT remains constant throughout the remainder of the

test. In Figure 5-2 this test is repeated but with

VIN – VOUT exceeding 1V.

When VIN – VOUT > 1V, the switch’s current limiting

circuitry responds by decreasing IOUT, as can be seen

in Figure 5-2. In this demonstration, VOUT is being

controlled and IOUT is the measured quantity. In real life

applications VOUT is determined in accordance with

ohm’s law by the load and the limiting current.

FIGURE 5-1:

IOUT in Current Limiting for

VIN - VOUT < 1V.

FIGURE 5-2:

IOUT in Current Limiting for

VIN - VOUT > 1V.

This folding back of ILIMIT can be generalized by

plotting ILIMIT as a function of VOUT, as shown below in

Figure 5-3 and Figure 5-4. The slope of VOUT between

IOUT = 0V and IOUT = ILIMIT (where ILIMIT = 1A) is

determined by RON of the switch and ILIMIT.

FIGURE 5-3:

Normalized Output Current

vs. Output Voltage (5V).

FIGURE 5-4:

Normalized Output Current

vs. Output Voltage (2.5V).

0.2

0.4

0.6

0.8

1.0

1.2

2345

OUTPUT VOLTAGE (V)

0.2

0.4

0.6

0.8

1.0

1.2

0.5

1.0

1.5

2.0

2.5

3.0

OUTPUT VOLTAGE (V)

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