MIC2007 datasheet(13/17 Pages) MICREL

13 / 17 page

Micrel

MIC2007/MIC2017

October 2005

M9999-102805

Application Information

Setting ILIMIT

The MIC2007/2017’s current limit is user programmable

and controlled by a resistor connected between the ILIMIT

pin and Ground. The value of this resistor is determined

by the following equation:

ILIMIT = Current Limit Factor (CLF)

RSET

RSET (Ω) = Current Limit Factor (V)

ILIMIT (A)

Example:

Set ILIMIT = 1.25A

Looking in the Electrical specifications we will find CLF

at ILIMIT = 1A. For the sake of this example, we will say

the typical value of CLF at an IOUT of 1A is 235V.

Applying the equation above:

RSET (Ω) = 235 V

1.25 A

RSET = 188 Ω

Designers should be aware that variations in the

measured ILIMIT for a given RSET resistor, will occur

because of small differences between individual ICs

(inherent in silicon processing) resulting in a spread of

ILIMIT values. In the example above we used the typical

value of CLF to calculate RSET. We can determine ILIMIT’s

spread by using the minimum and maximum values of

CLF and the calculated value of RSET.

RSET = 187 Ω

(the closest standard 1% value)

ILIMIT_MIN = 210V = 1.12A

187Ω

ILIMIT_MIN = 260V = 1.39A

187Ω

Giving us a maximum ILIMIT variation over temperature

of:

ILIMIT_MIN

ILIMIT_TYP

ILIMIT_MAX

1.12A

1.25A

1.39A

1.25A ±11%

ILIMIT vs. IOUT measured

The

MIC2007/2017’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 the

MIC2007/2017.

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, the MIC2007/2017’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.

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