MIC2753/2754

Micrel

MIC2753/2754

10

June 1999

Microcontroller

Manual

Reset

MIC2754

VCC

HV

MR

RST

IN

RST IN

GND

Fuse

100k

MCR708A

SCR

2200pF

10k

MIC94030

1k

22

Ω

GND

Figure 2a. MIC2754 Valid Reset Below 1.5V

Microcontroller

Manual

Reset

MIC2753

VCC

HV

MR

RST

IN

RST IN

GND

Fuse

100k

MCR708A

SCR

2200pF

10k

MIC94030

1k

22

Ω

GND

Figure 2b. MIC2753 Valid Reset Below 1.5V

Microcontroller

Manual

Reset

MIC2754

VCC

HV

MR

RST

IN

RST IN

GND

GND

Fuse

MCR708A

SCR

4.7k

BUFFER

/RST signal to

other circuits

2200pF

10k

MIC94030

1k

22

Ω

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

V

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 10k

Ω 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 1k

Ω 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.