MIC2095_11 Datasheet, PDF (18/25 Pages) Micrel Semiconductor – Current-Limiting Power Distribution Switches

Micrel, Inc.
shuts down, and cools, it will re-start itself if the Enable
signal retains true (high on the ENABLE parts, low on
the ENABLE/ parts).
In Figure 2, die temperature is plotted against I_OUT
assuming a constant ambient temperature of 85°C. The
plot also assumes the maximum specified switch
resistance at high temperature.
Die Temperature vs Output Current
(Ambient Temperature = 85°C)
130
120
110
100
90
Tamb=85°C
80
70
0.0 0.2 0.4 0.6 0.8 1.0 1.2
IOUT (A)
Figure 2. Die Temperature vs. I_OUT
Setting I_LIMIT
The current limit of the MIC2097 and MIC2099 parts are
user programmable and controlled by a resistor
connected between the I_LIMIT pin and Ground. The value
of the current limit resistor is determined by the following
equations:
I_LIMIT
=
CurrentLimitFactor(CLF)
R_LIMIT
or
R_LIMIT
=
CurrentLimitFactor(CLF)
I_LIMIT
Eq. 4
Eq. 5
The Current-Limit Factor (CLF) is a number that is
characteristic to the MIC2097/9 switches. The CLF is a
product of the current-setting resistor value, and the
desired current-limit value. Please note that the CLF
varies with the current output current, so caution is
necessary to use the correct CLF value for the current
that you intend to use the part at. For example: If one
wishes to set a I_LIMIT = 1.1A, looking in the electrical
specifications we will find CLF at I_LIMIT = 1.1 A, as noted
in Table 1.
Min.
175
Typ.
215
Max.
263
Table 1. CLF at I_LIMIT = 1.1A
Units
V
MIC2095/97/98/99
For the sake of this example, the typical value of CLF at
an I_OUT of 1.1A is 215V. Applying Equation 5:
R_LIMIT (Ω) =
215V
1.1A
= 195Ω
Eq. 6
Choose R_LIMIT = 196Ω (the closest standard 1% value)
Designers should be aware that variations in the
measured I_LIMIT for a given R_LIMIT 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_LIMIT. We can determine
I_LIMIT’s spread by using the minimum and maximum
values of CLF and the calculated value of R_LIMIT:
I_{LIMIT_MIN}
=
175V
196Ω
=
0.89A
Eq. 7
I_{LIMIT _ MAX}
=
263V
196Ω
= 1.34A
Eq. 8
Giving us a maximum I_LIMIT variation of:
I_{LIMIT_MIN}
0.89A (-19%)
I_{LIMIT_TYP}
1.1A
I_{LIMIT_MAX}
1.34A (+22%)
For convenience, Table 2 lists the resistance values for
the R_SET pin, for various current limit values.
Nominal
I_LIMIT
0.1A
R_LIMIT
1920
I_{LIMIT_MIN}
0.063
I_{LIMIT_MAX}
0.137
0.2A
1000
0.138
0.263
0.3A
672
0.211
0.391
0.4A
508
0.288
0.517
0.5A
412
0.369
0.638
0.6A
344
0.448
0.764
0.7A
298
0.533
0.884
0.8A
263
0.620
1.002
0.9A
235
0.709
1.118
1.0A
213
0.801
1.233
1.1A
195
0.895
1.346
Table 2. MIC2097 and MIC2099 R_LIMIT Table
August 2011
18
M9999-080211-C