Datasheet

LM3722, LM3723, LM3724
SNVS154E –MAY 2001–REVISED MARCH 2013 www.ti.com
MANUAL RESET INPUT (MR)
Many µP-based products require a manual reset capability, allowing the operator to initiate a reset. The MR input
is fully debounced and provides an internal 22 kΩ pull-up. When the MR input is pulled below V
IL
(0.25V
CC
) for
more than 100 ns, reset is asserted after a typical delay of 2 µs. Reset remains active as long as MR is held low,
and releases after MR rises above V
IH
and the reset timeout period expires. Use MR with digital logic to assert
reset or to daisy chain supervisory circuits.
APPLICATIONS INFORMATION
BENEFITS OF PRECISION RESET THRESHOLDS
A microprocessor supply supervisor must provide a reset output within a predictable range of the supply voltage.
A common threshold range is between 5% and 10% below the nominal supply voltage. The 4.63V, 3.08V and
2.32V options of the LM3722/LM3723/LM3724 use highly accurate circuitry to ensure that the reset threshold
occurs only within this range (for 5.0V, 3.3V and 2.5V supplies). Table 1 shows how the standard reset
thresholds apply to 5.0V, 3.3V, and 2.5V nominal supply voltages.
Table 1. Monitored Tolerance Table
Supply Voltage
Reset Threshold
2.5V 3.3V 5.0V
4.63 ± % 90.8-94.4%
3.08 ± % 91.8-95.2%
2.32 ± % 90.8-94.8%
ENSURING A VALID RESET OUTPUT DOWN TO V
CC
= 0V
When V
CC
falls below 1V, the LM3722 RESET output is unable to sink the rated current. A high-impedance
CMOS logic input connected to RESET can therefore drift to undetermined voltages. To prevent this situation, a
100kΩ resistor should be connected from the RESET output to ground, as shown in Figure 13.
A 100kΩ pull-up resistor to V
CC
is also recommended for the LM3723, if RESET is required to remain valid for
V
CC
< 1V.
Figure 13. Circuit for RESET Valid from V
CC
= 0V
OPEN DRAIN OUTPUT (LM3724)
An open drain output allows easy paralleling of multiple microprocessor reset circuits without requiring additional
logic gates. Open drain outputs also allow interfacing devices of differing logic levels or families, since the output
pull-up resistor may be connected to any supply voltage up to 5.5V, regardless of LM3724 V
CC
.
The pull up resistor is calculated so that maximum current flow into RESET is less than 10 mA when activated.
The resistor must be small enough so that the leakage current of all connected devices does not create an
excessive voltage drop when the output is not activated. A resistor value of 100 kΩ will generally suffice.
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