MIC2582 Datasheet, PDF (17/22 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

MIC2582/MIC2583
Micrel
V_IN
5V
GND
Q2
Si4876DY
(SO-8)
Backplane PCB Edge
Connector Connector
D1
(9V)
Long
Pin
1
2
C1 ³
0.47 F
R_SENSE 4
0.010Ω
2%
R1
47kΩ
8
VCC
2 _ON
7
SENSE
MIC2582
Open
Circuit
Short
Pin
3 _CPOR
GND
4
Q1
Si4876DY
(SO-8)
GATE ⁶
/POR ¹
5
FB
R2
R3
10Ω
10Ω
R4
1.2MΩ
C2
0.022 F
V_IN
R8
20kΩ
C3
4700pF
R9
750Ω
Long
Pin
Undervoltage (Output) = 3.3V
All resistors 5% unless specified otherwise
V_OUT
C6 ^3.3V@0.5A
100 F
V_OUT
5V@3.5A
C5
330 F
R5
510kΩ
R6
Q3
20kΩ
PN2222 1%
C4
0.1 F
R7
11.8kΩ
1%
Figure 8. 5V Switch/3.3V LDO Application
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 toler-
ances 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)
=
(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 cur-
rent is found using a 59mV trip voltage and a sense resistor
that is 3% low in value. The resulting equation is:
( )_{( )} ^{ILOAD(CONT,MAX) =}
0.97
59mV
R_SENSE(NOM)
= ^60.8mV
R_SENSE(NOM)
(12)
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 ex-
tremes for the output in question,
I_{LOAD(CONT,MAX)}
=
60.8mV
20.0mΩ
=
3.04A
,
approximately
3A.
Knowing this final datum, 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 ¹/₄W sense resistor is sufficient.
April 2003
17
MIC2582/MIC2583