LM4871M/NOPB Design Guide for Portable Mono Audio Amplification

Bottom Line: When a portable consumer or industrial product needs a single-supply 3 W mono speaker driver under $0.30 per unit, the LM4871M (sold as the lead-free LM4871M/NOPB suffix) — TI's Boomer-series Class-AB BTL amplifier in SOIC-8 — delivers 3 W into 3 Ω at 5 V with 0.5% THD, click/pop suppression, and a hardware shutdown pin that drops to under 1 µA. Three rules carry the design: (1) keep the bridge-tied-load (BTL) speaker connection floating — never ground one terminal, (2) size CB and Ci capacitors against your low-frequency cutoff target, and (3) thermally couple the SOIC-8 ground pins to a copper pour of at least 50 mm² to reach the 3 W rating without thermal foldback.

What the LM4871M/NOPB Actually Does

The LM4871M/NOPB is a fixed-supply mono Class-AB audio power amplifier in TI's Boomer family, delivering 3 W into a 3 Ω BTL load at VDD = 5 V with 0.5% total harmonic distortion plus noise (THD+N). It uses a bridge-tied-load topology, meaning the two output pins drive the speaker differentially and the speaker has no ground reference — this doubles the voltage swing for the same supply and removes the need for a DC-blocking output capacitor. Internal click/pop suppression mutes the outputs during shutdown transitions, eliminating the audible thumps that plague unsuppressed audio chains.

Compared with the older LM386, the LM4871M/NOPB roughly doubles the output power at the same supply because the BTL output stage swings rail-to-rail differentially. Pin compatibility within the LM4871 family makes substitution straightforward: LM4871MM is the MSOP-8 variant for tighter footprints, while the LM4871MX is the tape-and-reel variant of the same SOIC-8 die.

Design Considerations

Supply Voltage, Output Power, and Speaker Impedance

Output power scales as V² / (2 × R_L) for a BTL amplifier, so the chosen supply rail and speaker impedance set the achievable acoustic output. The LM4871M/NOPB is rated for 2.0 V to 5.5 V VDD; at 5 V into 3 Ω BTL, it produces 3 W at 0.5% THD+N (and roughly 3.4 W at 10% THD). At 3.3 V into 8 Ω, expect about 600 mW — well-suited to compact handheld audio. Below 2 V the part stops meeting the click/pop suppression spec, so do not run the amplifier from a discharging single-cell lithium primary without a boost converter.

Sizing speaker impedance is a thermal balance. A 3 Ω load extracts maximum power but more than doubles the IDD compared with 8 Ω, dropping efficiency to about 60% near full output and forcing the SOIC-8 to dissipate over 1 W internally. For battery-powered portable products that rarely run at full output, 8 Ω trades peak loudness for runtime and cooler operation. Pick 3 Ω only when peak SPL drives the design and the power budget allows it.

Coupling Capacitors and Low-Frequency Cutoff

The input AC-couples through Ci into the inverting input — the capacitor and the input bias resistor (Ri) form a high-pass filter with cutoff f_C = 1 / (2 × π × Ri × Ci). For a target -3 dB at 100 Hz with Ri = 20 kΩ, choose Ci = 0.082 µF (use 0.1 µF X7R for cost and tolerance). Smaller Ci shifts the cutoff up and excludes bass; larger Ci adds turn-on click that the bypass network must absorb.

The bypass capacitor (CB) on the BYPASS pin sets the click/pop suppression time constant. Larger CB extends the soft-start ramp and drops audible turn-on noise; the datasheet recommends 1.0 µF as a starting value. Reduce only to 0.47 µF if board area is tight and turn-on click is verified inaudible at your final speaker. Pair CB with a 1.0 µF supply bypass (Cs) on VDD to keep the high-frequency PSRR specs.

Gain Setting and Stability

Closed-loop gain is set by the external Rf / Ri divider on the inverting input; a typical 20 dB gain uses Rf = 200 kΩ and Ri = 20 kΩ. Above 20 dB gain the amplifier remains stable but increases noise floor and reduces phase margin into highly capacitive loads. Below 6 dB gain the input must already be near full-scale to drive 3 W output, which usually means the upstream DAC output is the noise-limiting factor, not the amplifier.

For class-D conversion considerations, see the tps61291 wearable boost converter design guide; when stepping up from 1.5 V to 5 V, supply ripple at the boost switching frequency couples directly into the audio output unless filtered by Cs.

Thermal Design and Shutdown Sequencing

The SOIC-8 (D) package thermal resistance to ambient is approximately 110 °C/W in still air without copper pour. At 1.0 W internal dissipation that is a 110 °C rise — exceeding the 150 °C junction limit if ambient is over 40 °C. Adding a 50 mm² copper pour under the GND pins drops θ_JA to roughly 80 °C/W, which keeps junction temperature within spec at full output up to 50 °C ambient. For thermal foldback margin, layout the pour as a continuous, unbroken plane, and tie all four ground vias directly without thermal reliefs.

Shutdown current is < 1 µA when SHUTDOWN is held above 1.4 V (typically tied to VDD through a 100 kΩ pull-up). Drive SHUTDOWN low to enable the amplifier; the click/pop circuit ramps over CB × 4 ms typical. Sequence shutdown at least 50 ms before powering down VDD to avoid speaker pop, and never float the pin — a floating SHUTDOWN draws unpredictable current and may oscillate.

Recommended Solutions

Option A: Battery-Powered Speaker Toy or Toaster Notification

A 3 V to 4.2 V single-cell Li-ion product seeking compact mono audio gets the LM4871M/NOPB at VDD ≈ 4 V, 8 Ω speaker, 200 kΩ feedback. Quiescent current under 6.5 mA preserves runtime; shutdown drops below 1 µA. Pair with LM386N only if a single-supply ≥ 6 V is available — at lower rails, the LM4871 BTL topology wins decisively.

Option B: Industrial Voice Annunciator at 5 V Rail

For 5 V industrial systems (alarm panels, kiosks) the LM4871M/NOPB drives a 3 Ω full-range driver to 3 W with 0.5% THD. Tie SHUTDOWN through a logic-level signal from the host MCU so the amplifier idles at < 1 µA between announcements. Use 1 µF Cs and 1 µF CB; ensure the 50 mm² thermal pour under the SOIC-8 if peak duty cycle exceeds 30%.

Option C: USB-Powered Desktop Speaker Module

USB host-powered (5 V, 500 mA) desktop speakers with mono playback get full 3 W headroom from the LM4871M/NOPB. Add ferrite-bead filtering on the VDD rail to reject USB host noise above 100 kHz, since the amplifier's PSRR drops above audio band. The shutdown pin can be controlled by USB suspend signaling so the speaker self-mutes when the host sleeps.

Option	Supply	Speaker	Output	Notes
A — Li-ion handheld	3.0–4.2 V	8 Ω	600 mW at 4 V	Choose for runtime
B — 5 V industrial	5 V	3 Ω	3 W	Need thermal pour
C — USB desktop	5 V	4–8 Ω	1.0–1.5 W	Filter USB noise

Common Pitfalls and Troubleshooting

Pitfall 1: Grounding one speaker terminal. BTL outputs require the speaker float between OUTA and OUTB. Symptom: zero output volume and overheating. Fix: never connect either OUTA or OUTB to GND — the speaker must be a two-wire floating load.

Pitfall 2: Undersized bypass capacitor causing turn-on pop. CB too small lets the click/pop circuit miss its ramp window. Symptom: audible "thump" each time SHUTDOWN deasserts. Fix: use 1.0 µF X7R and verify pop level on the actual speaker.

Pitfall 3: Insufficient thermal copper. SOIC-8 without thermal pour cannot dissipate full-power load. Symptom: thermal foldback engages at < 1 W output and music distorts after a minute. Fix: 50 mm² copper pour under GND pins, no thermal reliefs.

Pitfall 4: Shutdown left floating. Unconnected SHUTDOWN pin draws unpredictable current and can latch the amplifier off. Symptom: device works on the bench but not in production after handling. Fix: tie SHUTDOWN to a 100 kΩ pull-up to VDD or actively drive it from logic.

Pitfall 5: Wrong supply for input range. Driving the amplifier input from a 3.3 V source when VDD = 5 V is fine; driving 5 V signal into VDD = 3 V causes input clipping. Symptom: audible distortion at low volume settings. Fix: keep input AC-coupled and DC-bias to mid-supply via the resistor network.

FAQ

What output power does the LM4871M/NOPB deliver at 5 V?

At VDD = 5 V driving a 3 Ω BTL speaker the LM4871M/NOPB delivers 3.0 W at 0.5% THD+N and roughly 3.4 W at 10% THD. Into 8 Ω at 5 V the output drops to about 1.1 W at 0.5% THD+N. These figures assume the BYPASS capacitor is 1.0 µF and the supply ripple is below 50 mV.

What input bias resistor and coupling capacitor should I use?

For a flat audio response down to 100 Hz with 20 dB gain, use Rf = 200 kΩ, Ri = 20 kΩ, Ci = 0.1 µF, CB = 1.0 µF, and Cs = 1.0 µF. Lower Ri tightens the input filter cutoff but increases load on the upstream source; values below 10 kΩ are usually unnecessary unless paired with a low-impedance line driver.

Is the LM4871M/NOPB pin-compatible with the older LM386?

No. The LM4871M/NOPB uses a BTL output (no output coupling capacitor required) while the LM386 is single-ended (needs a series 220 µF output cap). Pinouts and supply ranges also differ — LM386 needs 4 V minimum, LM4871 starts at 2 V. Substitution requires PCB rework. For applications already locked into LM386 footprints, LM386N remains the second-source path.

How do I prevent turn-on/turn-off pop noise?

The LM4871M/NOPB integrates click/pop suppression that ramps the BTL midpoint over a CB × 4 ms time constant during SHUTDOWN transitions. Use a 1.0 µF CB and ensure SHUTDOWN transitions through 0.8 V to 1.4 V monotonically — a clean digital edge from a microcontroller GPIO works. Avoid analog SHUTDOWN circuits that may dwell in the linear region.

What is the shutdown current and how is it activated?

Shutdown current is below 1 µA typical when SHUTDOWN is held above 1.4 V (active-high disable). Tie SHUTDOWN through a 100 kΩ pull-up to VDD with a digital pull-down for active control, or directly drive from a microcontroller. Shutdown disables both output drivers, mutes audibly clean, and disconnects the BTL from VDD bypass paths.

Bottom Line and Next Steps

The LM4871M/NOPB earns its place because it solves portable mono audio with one part: 3 W output, click/pop free transitions, and < 1 µA standby. Honor the BTL-floating-speaker rule, size CB / Ci against your audio band, and pour thermal copper under the SOIC-8 — the rest is layout details.

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