MIC20XX_1108 Datasheet, PDF (24/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
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 Ω’s law by the
load and the limiting current.
MIC20xx Family
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.
Normalized Output Current
vs. Output Voltage (5V)
1.2
1.0
0.8
0.6
0.4
0.2
0
0123456
OUTPUT VOLTAGE (V)
Figure 6. Normalized Output Current vs. Output Voltage
Figure 4. I_OUT in Current Limiting for V_IN - V_OUT < 1V
August 2011
24
M9999-080211-D