MIC2582_09 Datasheet, PDF (18/25 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
For additional information on MOSFET thermal
considerations, please see MOSFET Selection text and
subsequent sections.
Auto-Restart - 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:
MIC2582/MIC2583
( )( ) ^{RSENSE(MAX )}
=
1.03
42mV
I _{LOAD(CONT )}
= ^40.8mV
I _{LOAD(CONT )}
(11)
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 (12)
R_{SENSE(NOM )}
As an example, if an output must carry a continuous 2A
without nuisance trips occurring, Equation 11
yields:
R_{SENSE(MAX )}
=
40.8mV
2A
= 20.4mΩ.
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
20.0mΩ
=
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.
April 2009
Figure 8. 5V Switch/3.3V LDO Application
18
M9999-043009-C