Datasheet

Micrel, Inc. MIC20xx Family
August 2011 24
M9999-080211-D
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 Figure 5 this test is repeated but with V
IN
- V
OUT
exceeding 1V.
When V
IN
- V
OUT
> 1V, switch’s current limiting circuitry
responds by decreasing I
OUT
, as can be seen in Figure
5. In this demonstration, V
OUT
is being controlled and
I
OUT
is the measured quantity. In real life applications
V
OUT
is determined in accordance with ’s law by the
load and the limiting current.
Figure 4. I
OUT
in Current Limiting for V
IN
- V
OUT
< 1V
Figure 5. I
OUT
in Current Limiting for V
IN
- V
OUT
> 1V
This folding back of I
LIMIT
can be generalized by plotting
I
LIMIT
as a function of V
OUT
, as shown below in Figures 6
and 7. The slope of V
OUT
between I
OUT
= 0V and I
OUT
=
I
LIMIT
(where I
LIMIT
= 1A) is determined by R
ON
of the
switch and I
LIMIT
.
0
0.2
0.4
0.6
0.8
1.0
1.2
0123456
NORMALIZED OUTPUT CURRENT (A)
OUTPUT VOLTAGE (V)
Normalized Output Current
vs. Output Voltage (5V)
Figure 6. Normalized Output Current vs. Output Voltage