74ACT240 Datasheet - Specs, Timing & Design Tips

74ACT240 Datasheet: Complete Specs & Electrical Summary

5 February 2026 0

The first step in assessing any octal buffer for bus or memory applications is to extract the critical operating points and timing constraints from the manufacturer data. Typical datasheets show a V_CC centered near 5V and propagation delays in the low single-digit nanoseconds; these electrical specs determine whether the device can meet your bus timing and drive requirements. This introduction frames the actionable summary and design checklist that follows.

Design Logic

Point: Engineers need a concise map from datasheet tables to board-level decisions.

Evidence: Key tables list V_CC range, input thresholds, I/O drive, and timing under specified C_L and R_L.

Explanation: By focusing on those fields, you can size pull resistors, estimate worst-case path delays, and predict thermal load before committing to prototypes.

Background: What the 74ACT240 Is and Where It’s Used

74ACT240 Datasheet: Complete Specs & Electrical Summary

Function Summary — Octal Buffer with 3-State Outputs

Point: This device class is an inverting octal buffer/line driver with tri-state enables used to isolate or drive an 8-bit bus.

Evidence: Typical roles include memory/address drivers, bus transceivers, and clock buffering where inverted signals or controlled bus release are required.

Explanation: Treat it as a parallel driver with two enable halves; unused inputs should be tied to defined logic levels per the datasheet to avoid floating states and excess I_CC.

Choosing the 74ACT240 vs. Alternatives

Point: Selection hinges on V_CC compatibility, speed, output drive, and power.

Evidence: ACT variants offer TTL-compatible thresholds and low single-digit ns t_PD, while AC or HC families trade thresholds and drive for other properties.

Explanation: Choose ACT when TTL thresholds and fast switching are necessary; consider output I_OH/I_OL and I_CC impact for dense, multi-drop bus topologies.

74ACT240 Datasheet: Pinout & Functional Overview

Pin-by-Pin Summary

Point: A concise pin mapping reduces miswiring and speeds PCB layout.

Evidence: Typical pinout maps inputs, outputs, two enable pins, V_CC and GND with active-low enables noted.

Explanation: Note required states for unused inputs (tie high/low per datasheet), and document which pins control each 4-bit half when implementing enable sequencing.

Pinout Summary
Pin	Name	Function
1	A1	Input 1
2	A2	Input 2
3	A3	Input 3
4	A4	Input 4
5	G1	Enable (active-low) — half 1
7	Y1	Output 1 (inverting)
10	G2	Enable (active-low) — half 2
20	VCC	Supply

Package & Footprint Notes

Point: Package choice affects parasitics and thermal behavior.

Evidence: Common packages include DIP for bench prototyping and SOIC/TSSOP for production with smaller parasitic capacitance in leadless packages.

Explanation: Use DIP or SOIC when prototyping; for high-speed or dense boards prefer small outline packages with careful pad design to limit trace inductance and allow close decoupling.

74ACT240 Datasheet: Electrical Specs & Timing Summary

DC Electrical Analysis

Extract V_CC operating range, V_IH/V_IL, I_OH/I_OL, I_CC and input leakage to size your interface. Use typical figures for behavior and maximums for safety margins.

AC Timing Analysis

Timing depends on specified V_CC, C_L and R_L. Add worst-case t_PD for each driver in the path and budget margin for enable/disable to avoid bus contention.

Visualization: Propagation Delay (t_PD) Comparison

74ACT240 (Typical)3.5 ns

74ACT240 (Max @ 50pF)8.5 ns

Older HC Series (Reference)15.0 ns

Electrical & Timing Summary (Example Fields)
Parameter	Typical	Max / Test Condition
V_CC	~5 V nominal	4.5V to 5.5V Supply range
V_IH / V_IL	TTL-compatible	2.0V (High) / 0.8V (Low)
I_OH / I_OL	±24 mA class	Varies by Temperature
t_PD	Low single-digit ns	Depends on C_L (Typical 50pF)

Thermal, Absolute Maximum Ratings & Power Considerations

Absolute Maximums

Designing below absolute maximums preserves reliability. Operate with safety margins (e.g., several percent below max V_CC), protect against transients, and calculate derating for elevated ambient temperatures to prevent premature failure.

Power & Decoupling

Estimate power from quiescent I_CC and switching losses. Power ≈ V_CC × I_CC + Σ(C_load × V_CC² × f). Place a 0.1 μF decoupling capacitor adjacent to V_CC pin and add a 1 μF bulk cap nearby.

Design & Application Guide: Practical Implementation Tips

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Driving Multi-Drop Buses & Managing 3-State Enables

Proper wiring and enable sequencing prevent contention. Use dedicated enable lines per half, avoid simultaneous enables on multiple drivers, and sequence enables during power-up.
✔
Signal Integrity, Level Interfacing & Protection

Fast edges require damping. Add small series resistors (10–33 Ω) at outputs to reduce overshoot. Evaluate level translation if interfacing to non-TTL domains to avoid false switching.

PCB Layout Case Study & Troubleshooting Checklist

Mini Case Study: Layout for an 8-bit Bus Driver

Placement and decoupling determine performance. Place the device adjacent to its primary load, route 8 data traces with matched lengths, and place a 0.1 μF ceramic cap within 1–2 mm of the V_CC pin. Keep ground return short and wide.

Troubleshooting Quick Checklist (Common Failure Modes) +

Supply Check: Verify supply voltage and polarity at the chip pins.
Logic States: Confirm enable pin (G1, G2) states are pulled low for active mode.
Physical Inspection: Inspect for solder bridges between adjacent high-speed traces.
Loading: Measure outputs unloaded vs. loaded to check for excessive capacitive loading.
Thermal: Check for thermal anomalies with a contact thermometer or IR camera indicating internal shorts.

The S-8261AAG is presented here with the most relevant datasheet numbers up front: the device supports overcharge detection in the ~3.9–4.5 V range with 5 mV step granularity and voltage-detection accuracy around ±25 mV, combined with integrated delay logic and overcurrent/overdischarge protection. This short report decodes the S-8261AAG datasheet and turns key specs into design and troubleshooting guidance for engineers working on single-cell packs and portable systems. Target Audience Hardware engineers, battery-system designers, and technical writers who need a concise, data-first reference to convert the datasheet into a checklist and test plan. The report emphasizes measurable thresholds, likely battery-life impact, PCB and component choices, and pragmatic verification steps engineers can apply during prototype bring-up. S-8261AAG: Design Overview & Core Functions Pinout, package & basic block diagram Point: The IC typically exposes BAT, V-, VOUT and gate-drive pins plus an exposed thermal pad. Evidence: The package routes cell sense and MOSFET-drive signals through dedicated pins. Explanation: Designers should map BAT to the cell positive, V- to pack negative, and locate the thermal/exposed pad under the part for heat dissipation and reliable soldering. BOM notes: prioritize a footprint with a solid exposed pad and plated-through thermal vias. Protection features at a glance Point: The device integrates overcharge, overdischarge, overcurrent/short protection and auto-recovery behavior. Evidence: Internal comparators, delay circuits and MOSFET-drive logic implement these functions. Explanation: Overcharge/overdischarge thresholds are selectable in defined steps while some trip limits and internal timers are fixed; designers must pick setpoints with margin for cell tolerance and charger behavior to avoid nuisance trips while maintaining safety. S-8261AAG Electrical Specifications & Datasheet Breakdown Voltage detection details: thresholds, accuracy, and step resolution Detection Range (V) Accuracy: ±25 mV 0.0 V 3.9 V 4.5 V Point: Voltage detection supports roughly 3.9–4.5 V with 5 mV step granularity and ≈±25 mV accuracy. Evidence: The datasheet lists detection range, step size and typical accuracy under nominal temperature conditions. Explanation: This fine resolution enables precise charger termination and tight overcharge margins, but engineers must account for ADC tolerances, cell imbalance and measurement offset when selecting setpoints to avoid repeated toggling near thresholds. Current & timing specs: overcurrent thresholds, delay times, and timing diagrams Parameter Typical Performance Design Impact Overcurrent Detection Sense Resistor Triggered Prevents thermal runaway during shorts Delay Logic Integrated Timers Filters transient noise/current spikes Point: Overcurrent/short detection relies on sense resistor measurement and configured delay logic to distinguish pulses from sustained faults. Explanation: Choose sense resistor and delay settings to suppress harmless current spikes while ensuring fast enough response for thermal protection; verification should include both pulse and continuous current tests to validate trip behavior and energy dissipation. Performance Metrics, Thermal & Reliability Considerations Quiescent current, leakage, and battery life impact Point: Quiescent and leakage currents are key for shelf life and wearable applications. Evidence: The datasheet specifies standby drain in the low‑microampere range and leakage paths under protection conditions. Explanation: Estimate pack shelf drain by multiplying quiescent current by time and include inverter losses; recommended verification: long-duration DMM logging and periodic coulomb-count checks to confirm expected µA-range impact on months-long standby life. Thermal behavior & robustness in real use Point: Power dissipation during fault clearing concentrates in MOSFETs and the PCB thermal path. Evidence: The datasheet and block diagram identify FET-drive nodes and the thermal pad as the main heat path. Explanation: Design wide copper pours, multiple thermal vias, and derate FET Rds(on) at elevated temperature; include temperature cycling and sustained overcurrent soak tests in reliability checks to reveal marginal soldering or thermal runaway risk. Implementation Guide: PCB Integration & Application Circuit Tips Typical application circuit and layout best practices Point: The canonical single-cell application uses a sense resistor, back-to-back MOSFETs and bypass paths for charge/discharge. Evidence: Application diagrams place the sense resistor between V- and pack negative with MOSFETs controlled by gate-driver pins. Explanation: Layout priorities are short sense traces, ground-return segregation, and placing decoupling close to BAT; ensure wide copper for current paths and thermal vias under the exposed pad for heat spreading. Component selection: MOSFETs, sense resistors, and external filtering Point: Choose MOSFETs and sense resistors to balance conduction loss and detection resolution. Evidence: Datasheet timing and threshold behavior depend on the voltage drop across the sense resistor and FET Rds(on). Explanation: Use low Rds(on) FETs with adequate Vds margin and a sense resistor tolerance that preserves detection accuracy; add RC filtering only when needed to avoid slowing legitimate fault detection—verify with pulse tests. Use Cases, Troubleshooting & Quick Design Checklist Typical application scenarios & quick comparisons Point: Common targets include consumer handhelds, wearables and portable instruments where low part count and accurate thresholds matter. Explanation: For wearables prioritize lowest quiescent current and smallest footprint, while instruments may prioritize higher sustained discharge capability and thermal margin—select components accordingly. Troubleshooting flow & quick verification checklist ✔ confirm BAT/V- reference voltages ✔ log quiescent current ✔ apply calibrated pulses to validate timing ✔ inspect solder joints and thermal vias Summary 1 The S-8261AAG provides fine-grain voltage detection (3.9–4.5 V, 5 mV steps) and ≈±25 mV accuracy, enabling precise charger termination and tight protection margins for single-cell designs using this battery IC. 2 Integrated delay logic and MOSFET drive simplify BOM and reduce firmware complexity, but designers must validate trip timing with both pulse and continuous loads to avoid false trips. 3 Quiescent and leakage currents are low (µA-range typical), so long-term shelf drain is small if verified with extended logging; thermal vias and copper pour are critical for robustness. 4 Recommended next step: consult the official datasheet for exact tables and timing diagrams labeled “datasheet” and execute a short prototype test plan covering threshold validation and thermal soak. FAQ How does S-8261AAG handle charger termination and what datasheet items to verify? + The S-8261AAG uses fine voltage steps and accurate thresholds to detect full-charge; verify the threshold table and voltage accuracy entries in the datasheet, then test with a regulated charger and a precision meter. Confirm hysteresis and delay behavior to ensure stable termination without oscillation. What battery IC measurements should be captured during bring-up for S-8261AAG? + Key measurements include quiescent current over 24–72 hours, sense-resistor voltage drop under defined loads, trip timing for overcurrent events, and thermal rise during sustained discharge. Record each measurement with calibrated instruments and compare against datasheet typical/maximum values. How to debug frequent false trips with S-8261AAG? + Start by measuring the sense-node waveform with an oscilloscope during the event to distinguish pulses from sustained faults. Check layout for long sense traces, verify sense-resistor tolerance, and confirm delay settings. If needed, increase hysteresis or adjust the sense resistor and re-run pulse-vs-continuous tests to tune reliability.