Always-On Wearables on a Single Cell: Architecting Battery Life with the TPS61291 15nA-Bypass Boost Converter
Design guide for always-on wearables on single AA or coin cells: how the TPS61291 15nA bypass mode unlocks multi-year battery life on a 3.3V rail.
Last updated: May 2026
Bottom Line: Architecting multi-year battery life for an always-on wearable on a single AA, AAA, or coin cell comes down to three decisions: (1) choose a boost converter whose bypass-mode quiescent current — not its active-mode IQ — sets the system floor; (2) operate the converter in bypass for >99% of the duty cycle, only switching when VIN drops below the regulated 3.3 V rail; and (3) budget burst loads (radio, OLED, sensor wake-ups) on a separate, higher-current rail. The TPS61291 with 15nA bypass mode makes (1) and (2) trivial: at 15 nA typical, the IC contributes effectively zero to the average drain, leaving battery life governed entirely by MCU sleep current.
Why 15 nA Changes the Wearable BOM Calculus
The defining constraint of an always-on wearable — a continuously running heart-rate monitor, a smart ring, an asset tag, a glucose patch — is that the system never fully shuts down. Power-management ICs that report 1–5 µA shutdown currents are useless here, because shutdown is exactly the state the MCU is not in. What matters is the converter's idle current while the load (an MCU in deep sleep, plus its leakage) is drawing tens to hundreds of nanoamps.
In that operating point, the converter's quiescent current dominates the system floor. A 5 µA-class converter pinned to a 50 nA MCU sleep current sets a 5.05 µA floor; the MCU's careful sub-µA budget has been thrown away by the PMIC. A 15 nA-class converter sets a 65 nA floor — virtually unchanged from the MCU alone. Across a multi-year deployment on a 200 mAh coin cell, that single design choice is the difference between an 18-month and a 6-year battery life.
This is why wearable device BOM optimization under aggressive battery targets always starts at the boost converter: it is the single component whose datasheet quiescent current directly multiplies into operational lifetime.
TPS61291 Bypass Mode vs Shutdown Mode: A Current Consumption Comparison
The TPS61291 is unusual among boost converters because it explicitly distinguishes bypass mode from shutdown:
- Bypass mode (EN = high, MODE = bypass): 15 nA typical IQ. The internal boost switch is disabled, the inductor is bypassed via an internal pass FET, and VOUT tracks VIN minus a small drop. The IC is alive — it can be re-enabled into boost mode within microseconds — but draws no switching current.
- Shutdown mode (EN = low): 5.7 µA typical, governed by leakage paths in the disabled FET, level shifters, and reference. The IC is fully off; coming out requires a startup sequence, soft-start, and re-establishing VOUT.
For an always-on wearable, bypass is dramatically lower than shutdown — about 380× lower — and recoverable in microseconds rather than milliseconds. This inverts the conventional wisdom that "off is the lowest-power state." With the TPS61291, the lowest-power state is bypass, which only works because the system tolerates VOUT tracking VIN during low-current windows.
The practical pattern: when VIN (a fresh 1.5 V alkaline) is above the 1.2–1.4 V minimum operating voltage of your MCU sleep state, run bypass. Boost only when (a) VIN sags below the MCU's brownout threshold, or (b) a peripheral demands the regulated 3.3 V rail. This pattern is what lets a single AA carry an asset tag for 5+ years.
How to Calculate Battery Life for Wearables With Ultra-Low IQ Boost Converters
The calculation is straightforward once you separate four current contributors:
- Converter quiescent (bypass) —
IQ_BYP. For the TPS61291 with 15nA bypass mode, 15 nA. Negligible. - MCU sleep current — typically 100 nA to 2 µA on modern Arm Cortex-M0+ MCUs (e.g. STM32L0, nRF52, EFR32).
- Active duty cycle — wake-up energy × wake-ups per hour, averaged over the hour.
- Boost mode duty cycle — fraction of the hour the converter is actively switching at ~22 µA active IQ + load current.
For an asset tag with a 1% wake duty cycle, an 800 µA active load, and a 200 nA sleep current:
I_avg = 0.01 × 800 µA + 0.99 × (0.2 µA + 0.015 µA) ≈ 8.21 µA
Battery life on 250 mAh CR2032: 250 / 0.00821 ≈ 30,500 h ≈ 3.5 years
If you replace the converter with a 5.7 µA-class part:
I_avg = 0.01 × 800 µA + 0.99 × (0.2 µA + 5.7 µA) ≈ 13.84 µA
Battery life on 250 mAh CR2032: 250 / 0.01384 ≈ 18,100 h ≈ 2.1 years
A single component swap delivers 40% more battery life with no architectural change. Multiplied across an installed fleet — service calls avoided, replacement cells saved — this is the single highest-leverage decision in wearable power design.
Designing a Single-Cell Alkaline-to-3.3 V Rail for Always-On IoT Nodes
The reference topology for a single-AA-powered always-on node is simple: one inductor, one input cap, one output cap, MODE driven by an MCU GPIO. Design choices that matter:
- Inductor: 4.7 µH, low-DCR shielded (e.g. Coilcraft LPS4018 or TDK VLF series). DCR directly multiplies into boost-mode efficiency at high VIN/VOUT ratios.
- Output capacitor: 10 µF X5R 0603/0805. Larger COUT reduces VOUT ripple in bypass mode when load steps occur.
- MODE pin control: drive from the MCU. Boost mode for radio/sensor active windows, bypass mode the rest of the time. The transition is gateable from any GPIO with no glue logic.
- Brownout monitor: set the MCU's BOR or PVD threshold above the IC's bypass-mode dropout. The IC will stop tracking VIN cleanly somewhere around 1.0 V on a depleted alkaline; the MCU should reset gracefully before the rail collapses.
For systems where the load occasionally needs more than the TPS61291's 200 mA peak — e.g. a BLE radio TX burst hitting 15 mA, or an OLED panel refreshing — the architectural answer is not to oversize the always-on converter, but to add a second rail. Pair the TPS61291 with the higher-current TPS61086 (2A) for short bursts or, for OLED and RF PA loads at the upper end, the 3.5A TPS61236 for OLED/RF burst loads. The burst converter sits in shutdown until the MCU enables it for the active window, then disables it again. The always-on rail keeps its 15 nA floor untouched.
When the Topology Breaks: Battery Voltage Crossing the Output
A single AA delivers 1.5 V → ~0.9 V across discharge. A single CR2032 delivers 3.0 V → ~2.2 V. A single Li-Po delivers 4.2 V → ~3.0 V. The first two are pure boost cases — VIN < VOUT throughout life — and the TPS61291 is the textbook fit.
Li-Po is different. A single Li-Po sourcing a 3.3 V rail crosses the regulation threshold during discharge: the cell starts above 3.3 V, drifts through 3.3 V, then drops below. A pure boost cannot regulate when VIN > VOUT, and a pure buck cannot regulate when VIN < VOUT. The correct topology is buck-boost. Use the TPS63020 synchronous buck-boost when input crosses output — its ~25 µA active IQ is higher than the TPS61291's bypass floor, so this is a different design point with a different battery-life ceiling, but it is the only correct answer when input voltage crosses the output rail.
The Multi-Rail Wearable: Combining Boost and Step-Down
Modern wearable SoCs increasingly demand sub-1.8 V core rails (1.0 V, 1.2 V) alongside the standard 3.3 V system rail. Generating 1.0 V directly from a 1.5 V AA is impossible with a boost; the architecture is two-stage: AA → 3.3 V (boost) → 1.0 V (step-down).
For the post-boost step-down stage, the TPS62203 step-down companion for the post-boost rail is the canonical choice — 300 mA, sub-µA shutdown, ~15 µA quiescent in active mode, fixed 1.8 V or adjustable variants for arbitrary core rails. Like the burst converter, it spends most of its time in shutdown; the MCU enables it only when the SoC core needs to wake.
The full chain for a 1-cell-AA, BLE-class wearable:
| Stage | Component | Role | Typical IQ |
|---|---|---|---|
| Always-on 3.3 V | TPS61291 | Sleep rail, sensor power | 15 nA bypass |
| Burst 3.3 V | TPS61086 or TPS61236 | Radio / OLED active window | 0 (shutdown) |
| Core 1.0–1.8 V | TPS62203 | SoC core, on-demand | 0 (shutdown) |
System floor: ~15 nA + MCU sleep. Burst converters are billed only against active-window energy, not the idle floor.
Recommended Solutions: Picking the Right Boost for Your Wearable
| Scenario | Recommended converter | Why |
|---|---|---|
| Always-on rail, single AA / coin cell, ≤200 mA peak | TPS61291 (boost, 15 nA bypass) | Lowest possible idle floor; bypass mode preserves MCU sleep budget |
| Burst rail, BLE radio TX or sensor wake-up at 1–2 A | TPS61086 (2 A boost) | High peak current, gateable shutdown for active-window-only operation |
| Burst rail, OLED panel or 2.4 GHz PA at 3+ A | TPS61236 (3.5 A boost) | Highest peak current in this lineup; sized for OLED refresh and RF PA bursts |
| Single-cell Li-Po sourcing a 3.3 V rail | TPS63020 (synchronous buck-boost) | Required topology when VIN crosses VOUT during discharge |
| SoC core 1.0–1.8 V, on-demand | TPS62203 (step-down) | Pairs cleanly with a 3.3 V boost rail to deliver sub-1.8 V core voltages |
Common Pitfalls in Always-On Wearable Boost Design
Pitfall 1 — Sizing the always-on converter for peak load. A designer sees that the BLE radio peaks at 15 mA and picks a 2 A boost converter for the always-on rail. The 2 A part has 50× higher IQ. The wearable now lives 12 months instead of 5 years. Fix: separate the always-on rail from the burst rail.
Pitfall 2 — Leaving the converter in active boost mode 24/7. Even with a 22 µA active IQ part, running it continuously costs 22 µA × 8760 h = 193 mAh per year, which is most of a CR2032. Fix: drive MODE to bypass via MCU GPIO during sleep windows.
Pitfall 3 — Ignoring inductor DCR at the cell-end-of-life boundary. A 200 mΩ DCR inductor on a 0.9 V cell at 15 mA load drops 3 mV; on a 0.6 V boost ratio, that's a measurable efficiency hit at exactly the moment the battery is most stressed. Fix: pick a shielded inductor with <100 mΩ DCR.
Pitfall 4 — Overspecifying COUT for "ripple." Some designs add 22 µF or 47 µF on VOUT to fight perceived ripple. The added bulk capacitor costs board area and adds leakage current. Fix: 10 µF X5R is sufficient for any sub-200 mA wearable load; add bulk only on the burst rail.
Pitfall 5 — Not gating the boost converter from the MCU. If MODE is tied to a fixed level, you forfeit the ability to opportunistically bypass. Fix: route MODE to an MCU GPIO with a default-bypass pull-up.
FAQ
Q: Can the TPS61291 actually start up from a depleted single AA at 0.9 V?
A: Yes — the TPS61291 has a startup voltage of about 0.7 V. Once running, it sustains operation down to roughly 0.5 V, well below any practical end-of-life cell voltage. The minimum input for starting the IC matters most when the wearable is first powered on with a partially discharged cell.
Q: Why does the spec sheet show 22 µA active IQ if you're claiming 15 nA?
A: 22 µA is the IQ when the converter is actively switching in boost mode. 15 nA is the IQ when MODE is set to bypass. The design assumption of this article is that the system spends >99% of its life in bypass; the 22 µA only accrues during burst windows, which are billed against the burst rail's energy budget, not the always-on floor.
Q: How does this compare to a low-IQ LDO on a 3 V coin cell?
A: An LDO on a 3 V coin cell wastes about 9% of stored energy because it cannot pull the cell below 3.3 V + dropout. A boost configured for bypass-by-default extracts useful energy down to 0.7 V, harvesting nearly the full coin-cell capacity. For a CR2032, this is the difference between roughly 220 mAh-equivalent and the full 250 mAh datasheet rating.
Q: What inductor saturation current do I need?
A: For a 200 mA peak boost output at 3.3 V from a 1.0 V cell, the inductor peak current is about 660 mA. Pick an inductor rated for ≥1 A saturation to leave 50% margin under cold-cell conditions. Coilcraft LPS4018-472MR (4.7 µH, 1.1 A Isat) is a typical fit.
Q: Is the MCU's GPIO drive strength enough to control MODE?
A: Yes — the MODE pin is a CMOS input drawing single-digit nA. Any MCU GPIO can drive it directly with a 1 MΩ default-state pull-up.
Conclusion
Always-on wearables live or die on their PMIC's quiescent current. A converter that delivers 15 nA in bypass mode — a regime where the IC remains alive and re-engageable in microseconds — fundamentally changes the BOM calculus: the always-on rail's IQ effectively disappears from the energy budget, and the MCU's careful sleep design is preserved end-to-end. Pair this always-on rail with task-specific burst converters and a step-down for SoC core rails, and the architecture hits 3–7 year battery life on coin cells and single AAs without exotic components.
If you're sourcing for an always-on wearable BOM, request a quote on the TPS61291DRVR plus the burst and core-rail companions discussed above — verified inventory across 200+ FindMyChip distributors with 24-hour pricing turnaround. For broader power-IC selection across boost, buck-boost, and LDO topologies, search the FindMyChip catalog by IQ, package, and current rating.