Datasheet LM3722, LM3723, LM3724 (Texas Instruments) - 8

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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 VIL (0.25VCC) 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 VIH 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 VCC = 0V
When VCC 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 VCC is also recommended for the LM3723, if RESET is required to remain valid for VCC < 1V.
Figure 13. Circuit for RESET Valid from VCC = 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 VCC. 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. 8 Submit Documentation Feedback Copyright © 2001–2013, Texas Instruments Incorporated Product Folder Links: LM3722 LM3723 LM3724 Document Outline FEATURES APPLICATIONS DESCRIPTION Typical Application Circuits Connection Diagram Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Timing Diagram Circuit Information RESET OUTPUT RESET THRESHOLD MANUAL RESET INPUT (MR) Applications Information BENEFITS OF PRECISION RESET THRESHOLDS ENSURING A VALID RESET OUTPUT DOWN TO VCC = 0V OPEN DRAIN OUTPUT (LM3724) NEGATIVE-GOING VCC TRANSIENTS INTERFACING TO µPS WITH BIDIRECTIONAL RESET PINS Typical Application Circuits Revision History