MIC2582_14 Datasheet, PDF (19/27 Pages) Micrel Semiconductor – Single-Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC5283
For additional information on MOSFET thermal
considerations, please see MOSFET Selection text and
subsequent sections.
Auto-Restart for MIC2583R
The MIC2583R provides an auto-restart function. Upon an
overcurrent fault condition such as a short circuit, the
MIC2583R initially shuts off the GATE output. The
MIC2583R attempts to restart with a 12µA charge current
at a preset 10% duty cycle until the fault condition is
removed. The interval between auto-retry attempts is set
by capacitor C_FILTER.
Sense Resistor Selection
The MIC2582 and MIC2583 use a low-value sense resistor
to measure the current flowing through the MOSFET
switch (and therefore the load). This sense resistor is
nominally set at 50mV/I_LOAD(CONT). To accommodate worst-
case tolerances for both the sense resistor (allow ±3%
over time and temperature for a resistor with ±1% initial
tolerance) and still supply the maximum required steady-
state load current, a slightly more detailed calculation must
be used.
The current limit threshold voltage (i.e., the “trip point”) for
the MIC2582/83 may be as low as 42mV, which would
equate to a sense resistor value of 42mV/I_LOAD(CONT).
Carrying the numbers through for the case where the
value of the sense resistor is 3% high yields:
   ^RSENSE(MAX)

42mV
1.03 I_{LOAD(CONT )}
40.8mV

I_{LOAD(CONT )}
Eq. 12
Once the value of R_SENSE has been chosen in this manner,
it is good practice to check the maximum I_LOAD(CONT) which
the circuit may let through in the case of tolerance buildup
in the opposite direction. Here, the worst-case maximum
current is found using a 59mV trip voltage and a sense
resistor that is 3% low in value. The resulting equation is:
  ^{ILOAD(CONT,MAX )}

59mV
(0.97) R_SENSE(NOM)
60.8mV

R_{SENSE (NOM)}
Eq. 13
As an example, if an output must carry a continuous 2A
without nuisance trips occurring, Equation 12 yields:
R_{SENSE (MAX )}

40.8mV
2A
 20.4m.
The next lowest standard value is 20mΩ. At the other set
of tolerance extremes for the output in question,
I_{LOAD (CONT,MAX )}

60.8mV
20.0m

3.04A
approximately 3A. Knowing this final data, we can
determine the necessary wattage of the sense resistor
using P = I²R, where I will be I_{LOAD(CONT, MAX)}, and R will be
(0.97)(R_SENSE(NOM)). These numbers yield the following:
P_MAX = (3A)² (19.4mΩ) = 0.175W.
In this example, a ¼W sense resistor is sufficient.
May 23, 2014
Figure 7. 5V Switch/3.3V LDO Application
19
Revision 5.0