MIC2003 datasheet(13/16 Pages) MICREL

13 / 16 page

Micrel

MIC2003/MIC20013

October 2005

M9999-102605

hbwhelp@micrel.com or (408) 955-1690

Application Information

ILIMIT vs. IOUT measured

MIC2003/2013’s’s current limiting circuitry 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 MIC2003/2013.

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 Figures

4 and 5.

In Figure 4 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 this test is repeated but with VIN - VOUT

exceeding 1V.

When VIN - VOUT > 1V, MIC2003/2013’s current limiting

circuitry responds by decreasing IOUT, as can be seen in

Figure 5. 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 4. IOUT in Current Limiting for VOUT ≤1V

Figure 5. IOUT in Current Limiting for VOUT >1V

This folding back of ILIMIT can be generalized by plotting

ILIMIT as a function of VOUT, as shown below. The slope

of VOUT between IOUT = 0 and IOUT = ILIMIT (where ILIMIT =

1) is determined by RON of MIC2003/2013 and ILIMIT.

0.2

0.4

0.6

0.8

1.0

1.2

OUTPUT VOLTAGE (V)

Normalized Output Current

vs. Output Voltage (5V)

Figure 6.

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