Micrel MIC2772
Functional Block Diagram
+
Reset
Threshold (V)
V
CC
1 (8)
RESET1 (7)
RESET
GENERATOR
MR1 (1)
GND1 (2)
GND2 (4)
V
CC
+
Reset
Threshold (V)
V
CC
2 (6)
RESET2 (5)
RESET
GENERATOR
MR2 (3)
V
CC
MIC2772 Block Diagram
Application Information
Reset Output, /RESET
The /RESET pin is active low (open drain) pin. The
/RESET pin is asserted whenever VCC falls below
the reset threshold voltage or if /MR (manual reset)
is forced low. The /RESET pin remains asserted for
the duration of the threshold timeout period after
VCC has risen above the reset threshold and/or /MR
has returned high. The reset function ensures the
microprocessor is properly reset and power up in a
known condition after a power failure. /RESET will
remain valid with VCC as low as 1.0V.
The /RESET output is a simple open-drain N-
channel MOSFET structure. A pull-up resistor must
be used to pull this output to some voltage. For
most applications, this voltage will be the same
power supply that supplies VCC to MIC2772. It is
possible however to tie this resistor to some other
voltage. This will allow the MIC2772 to monitor one
voltage while level-shifting the /RESET output to
some other voltage. The pull-up voltage must be
limited to 6.0V or less (absolute maximum) to avoid
damage to the MIC2772. The resistor must be small
enough to supply current to the inputs and leakage
paths that are driven by the /RESET output.
Manual Reset Input, /MR
The ability to initiate a reset via external logic or a
manual switch is provided in addition to the
MIC2772’s automatic supervisory functions. Driving
the /MR input to a logic low causes an immediate
and unconditional reset to occur. Assuming VCC is
within tolerance when /MR is released (returns high),
the /RESET output will be de-asserted no less than
t
RESET
later. /MR may be driven by a CMOS or TTL
logic signal, or a mechanical switch. Typically, a
momentary push-button switch is connected such
that /MR is shorted to ground when the switch
contacts close. Switch de-bouncing is performed
internally; the switch may be connected directly
between /MR and GND. /MR has an internal 20kΩ
pull-up resistor (typical) to VCC and may be left
open if unused.
April 2005 6
M9999-042605
(408) 955-1690
