PT7M6832 Datasheet, PDF (7/12 Pages) Pericom Semiconductor Corporation – Ultra Low Voltage Detectors

PT7M6832-6840
Ultra Low Voltage Detectors
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Application Information
Negative-Going VCC Transients
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, the PT7M6832–
PT7M6840 are relatively immune to short-duration negative-
going Vcc transients. A 0.1µF bypass capacitor mounted as
close as possible to the Vcc pin provides additional transient
immunity.
Valid Reset Output Down to Vcc = 0
When Vcc falls below 0.9V, the PT7M6832/6835/6838 push-
pull RESET output no longer sinks current for it becomes an
open circuit. Therefore, high-impedance CMOS logic inputs
connected to RESET can drift to undetermined voltages. This
presents no problem in most applications since most µP and
other circuitry are inoperative with Vcc lower than 0.9V.
However, in applications where RESET must be valid down to
0, adding a pull-down resistor to RESET causes any stray
leakage currents to flow to ground, holding RESET low(Fig.4).
R3’s value is not critical; 100kΩ is large enough not to load
RESET and small enough to pull RESET to ground.
A 100kΩ pull-up resistor to Vcc is also recommended for the
PT7M6833/PT7M6836/PT7M6839 if RESET is required to
remain valid for Vcc < 0.85V.
Interfacing to µPs with Bidirectional Reset Pins
Since the RESET output on the PT7M6834/6837/6840 is
open-drain, these devices interface easily with µPs that have
bidirectional reset pins. Connecting the µP supervisor’s
RESET output directly to the µP’s RESET pin with a single
pull-up resistor allows either device to assert a reset (Fig.5).
Open-Drain RESET Output with Multiple Supplies
In some systems the open-drain output is used to level-shift
from the monitored supply to reset circuitry powered by some
other supply (Fig.6).
Note: As the PT7M6834/6837/6840’s Vcc decreases, so does
the IC’s ability to sink current at RESET. Also, with any pull-
up, RESET will be pulled high as Vcc declines toward 0. The
voltage where this occurs depends on the pull-up resistor
value and the voltage to which it is connected.
PT7M6832
/6835
/6838
RESET
VCC
R3
100kΩ
VCC
VCC
PT7M6834
/6837
/6840
RESET
VCC
Bidirectional
µP
RESET
INPUT
Fig.4 Reset output valid to VCC=0 circuit
+1.2V
+5.0V
VCC
PT7M6834
/6837
/6840
RESET
VCC
5V System
RESET
INPUT
Fig.5 Interfacing to µPs with Bidirectional Reset I/O
Fig.6 Open-drain Reset output with multiple supplies
2013-11-0003
7
PT0236-6
11/18/13