MIC20XX Datasheet, PDF (23/29 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
MIC20xx Family
Application Information
Setting I_LIMIT
The MIC2009/2019’s current limit is user programmable
and controlled by a resistor connected between the I_LIMIT
pin and Ground. The value of this resistor is determined
by the following equation:
I_OUT
0.1A
0.2A
0.3A
0.4A
R_SET
1928Ω
993Ω
673Ω
511Ω
I_{LIMIT_MIN}
0.063A
0.137A
0.216A
0.296A
I_{LIMIT_MAX}
0.136A
0.265A
0.391A
0.515A
I_LIMIT
=
CurrentLimitFactor(CLF)
R _SET
or
R _SET
=
CurrentLimitFactor(CLF)
I_LIMIT (A)
For example: Set I_LIMIT = 1.25 A
Looking in the Electrical specifications we will find CLF
at I_LIMIT = 1 A.
0.5A
0.6A
0.7A
0.8A
0.9A
413Ω
346Ω
299Ω
263Ω
235Ω
0.379A
0.463A
0.548A
0.634A
0.722A
0.637A
0.759A
0.880A
1.001A
1.121A
Table 2. MIC20x9A R_SET Table
I_OUT
0.2A
R_SET
1125Ω
I_{LIMIT_MIN}
0.127A
I_{LIMIT_MAX}
0.267A
Min
Typ
Max
Units
190
243
293
V
0.3A
0.4A
0.5A
765Ω
582Ω
470Ω
0.202A
0.281A
0.361A
0.390A
0.510A
0.629A
Table 1. CLF at I_LIMIT = 1A
For the sake of this example, we will say the typical
value of CLF at an I_OUT of 1A is 243V. Applying the
equation above:
R_SET (Ω)
=
243V
1.25A
= 194.4Ω
R_SET = 196Ω
(the closest standard 1% value)
Designers should be aware that variations in the
measured I_LIMIT for a given R_SET resistor, will occur
because of small differences between individual ICs
(inherent in silicon processing) resulting in a spread of
I_LIMIT values. In the example above we used the typical
value of CLF to calculate R_SET. We can determine I_LIMIT’s
spread by using the minimum and maximum values of
CLF and the calculated value of R_SET.
I_{LIMIT_MIN}
=
190V
196Ω
=
0.97A
I_{LIMIT_MAX}
=
293V
196Ω
= 1.5A
Giving us a maximum I_LIMIT variation over temperature of:
I_{LIMIT_MIN}
I_{LIMIT_TYP}
I_{LIMIT_MAX}
0.97A (-22%)
1.25A
1.5A (+20%)
0.6A
0.7A
0.8A
0.9A
1A
1.1A
1.2A
1.3A
1.4A
1.5A
395Ω
341Ω
300Ω
268Ω
243Ω
222Ω
204Ω
189Ω
176Ω
165Ω
0.443A
0.526A
0.610A
0.695A
0.781A
0.868A
0.956A
1.044A
1.133A
1.222A
0.746A
0.861A
0.976A
1.089A
1.202A
1.314A
1.426A
1.537A
1.647A
1.757A
Table 3. MIC20x9 R_SET Table
I_LIMIT vs. I_OUT 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, V_OUT drops and the input to output
voltage differential increases. When V_IN - V_OUT exceeds
1V, I_OUT drops below I_LIMIT to reduce the drain of fault
current on the system’s power supply and to limit internal
heating of the switch.
When measuring I_OUT 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 V_OUT is pulled
below V_IN, with the test terminating when V_OUT is 1V
below V_IN. Observe that once I_LIMIT is reached I_OUT
remains constant throughout the remainder of the test. In
February 2011
23
M9999-020311-D