MIC2753_02 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – Power Supply Supervisors

MIC2753/2754
Micrel
V_CC
Fuse
Manual
Reset
MIC2754
IN
RST
MR
HV
GND
Microcontroller
VCC
10k
GND
RST IN
2200pF
R_PULL-DOWN
100k
MIC94030
22Ω
1k
MCR708A
SCR
Figure 2a. MIC2754 Valid Reset Below 1.5V
V_CC
Fuse
Manual
Reset
R_PULL-UP
100k
MIC2753
IN
RST
MR
HV
GND
Microcontroller
VCC
10k
GND
RST IN
2200pF
MIC94030
22Ω
1k
MCR708A
SCR
Figure 2b. MIC2753 Valid Reset Below 1.5V
V_CC
Fuse
Manual
Reset
BUFFER
/RST signal to
other circuits
MIC2754 ^RSERIES
4.7k
IN
RST
MR
HV
GND
Microcontroller
VCC
10k
GND
RST IN
2200pF
MIC94030
22Ω
1k
MCR708A
SCR
Figure 3. MIC2754 With Processors Using Bidirectional Reset Inputs.
Crowbar Applications
The following MIC275x applications diagrams show an SCR
crowbar circuit. The function of this circuit is to protect system
components from damage caused by excessive supply volt-
age, as might be generated by a runaway power supply. The
low R_DS(on) of the MIC94030 P-channel MOSFET (<1Ω at
V_GS = 4.5V) and the 22Ω resistor assure adequate gate
current to trigger the SCR.
The crowbar circuit is designed to be functional over all
operating ranges of the MIC275x. Since the lowest upper-
threshold voltage of the MIC275x family is only 3.08V, and
since the SCR-gate forward voltage is about 1V, there is
about 90mA of gate-drive current available: 2.08V ÷ 23Ω =
0.09A. The highest upper-threshold voltage is 5.38V, so
available gate-drive current is about 190mA: 4.38V ÷ 23Ω =
0.190A. The maximum forward gate current of the SCR is
200mA. The SCR trigger characteristics and the gate-drive
capability provide a workable solution for the power-supply
voltage range in question.
The 10kΩ resistor is the pull-up for the open-drain /HV output
of the MIC275x, and the 2200pF capacitor avoids inadvertent
premature triggering of the SCR, which might be caused by
a very narrow spike on the /HV line. Similarly, the 1kΩ resistor
from the SCR-gate to ground keeps the SCR-gate biased off.
The fuse shown in the diagrams, which ultimately protects
both the system components and the crowbar SCR, should
be a fast-blow type. The SCR is rated at 4A(rms), but it can
withstand a brief 25A surge. The crowbar circuit shown
should, conservatively, be capable of blowing a 5A or 6A
fuse.
MIC2753/2754
10
June 1999