Comparator-Gated LTE-M Radio

• Comparator-gated LTE-M radio is a power management system that uses analog voltage detection to connect high-power LTE-M modems only when sufficient energy is stored.
• The design employs a TLV431-based comparator with hysteresis to precisely define turn-on and turn-off thresholds, preventing premature brown-out and maximizing harvested energy by 20–30%.
• Integrated with a boost converter, supercapacitor storage, and a Nordic Thingy:91 modem, the architecture supports autonomous, battery-less LTE-M communication as demonstrated in a water leak detector.

A comparator-gated LTE-M radio is a power management subsystem enabling energy-efficient, battery-less operation of long-range cellular IoT devices. It uses precise analog voltage detection to connect a high-power LTE-M modem only when sufficient harvested energy is stored in a supercapacitor, ensuring successful communication cycles and minimizing wasted energy. This architecture, demonstrated in a battery-less LTE-M water leak detector, integrates a TLV431-based comparator gating circuit, ME2108 boost conversion, supercapacitive storage, and a Nordic Thingy:91 LTE-M radio module to deliver fully autonomous, 3GPP-compliant communications powered exclusively by environmental electrochemical harvesting (Nepal et al., 25 Jan 2026).

1. Comparator Gating Circuit Architecture

The comparator gating circuit is based on a Diodes Inc. TLV431 programmable shunt regulator, configured as a voltage comparator with an internal 2.495 V reference. Its threshold is set by a resistor divider (R1 = 100 kΩ, R2 = 52 kΩ) between the supercapacitor (V_SC) and the TLV431 REF pin, with additional hysteresis provided by a 1 MΩ resistor (R3) from the TLV431 cathode to REF. A Diodes Inc. ZXM62P02E6 P-channel MOSFET (20 V, R_DS(on) ≈ 70 mΩ) acts as the load switch.

The following table summarizes the key circuit elements:

Component	Part Number	Function
Comparator	TLV431	Voltage threshold detection
Load Switch	ZXM62P02E6 (P-FET)	Power gating of LTE-M modem
R1 (divider top)	100 kΩ	Sets upper threshold
R2 (divider bot)	52 kΩ	Sets lower threshold w/ R1
R3 (hysteresis)	1 MΩ	Positive feedback, defines ΔV

The TLV431 output pulls the P-FET gate low and closes the switch when V_SC exceeds the upper threshold ($V_\text{on}\approx4.87$ V). As the radio draws current, V_SC drops; once it crosses the lower threshold ($V_\text{off}\approx3.67$ V), the comparator opens the switch, disconnecting the load and returning to a charging-only state (Nepal et al., 25 Jan 2026).

2. Threshold Calculation, Energy Dynamics, and Hysteresis

Threshold voltages derive from resistor divider ratios and comparator operation. With $V_\text{REF}=2.495$ V and resistor values $R_1=100$ kΩ, $R_2=52$ kΩ, the turn-on voltage is:

$$V_\text{on} = V_\text{REF} \left( 1 + \frac{R_1}{R_2} \right ) \approx 4.87\ \mathrm{V}$$

Positive feedback via R3 introduces $\approx1.2$ V hysteresis:

$$V_\text{off} = V_\text{on} - (I_\text{hys}\cdot R_2) \approx 3.67\ \mathrm{V}$$

The 1.5 F supercapacitor at 4.87 V stores up to approximately 17.8 J:

$$E_\text{SC} = \frac{1}{2} C V_\text{SC}^2 \approx 17.8\,\mathrm{J}$$

Each LTE-M beacon transmission, including network attach and payload, requires roughly 1.25 J at a peak power of 1.25 W (5 V × 0.25 A) with a duration of 1 s. Up to eight such transmissions are issued per charge (total ≈10 J), maintaining a ≈7.8 J energy margin. The enforced gating prevents premature brown-out and allows extraction of 20–30% more harvested energy by avoiding inefficient partial discharge cycles (Nepal et al., 25 Jan 2026).

3. Power System Integration and Operational Sequence

The power subsystem architecture integrates a dual-compartment electrochemical harvester (OCV up to 2.7 V, SCC ≈ 450 mA), ME2108 boost converter, the comparator gating switch, and a 1.5 F supercapacitor. The TLV431-based comparator continuously monitors V_SC:

• If $V_\text{SC} < 4.87$ V: Load switch open; Thingy:91 is electrically isolated.
• If $V_\text{SC} \geq 4.87$ V: Comparator closes the P-FET, applying 5 V to both the MCU and LTE-M radio.

The MCU-controlled operational sequence is:

1. On gate closure, MCU powers up and confirms V_SC > V_on.
2. Initiates LTE-M attach sequence (≈14 s on average).
3. Sends an MQTT payload (∼50 B JSON) to Azure IoT Hub.
4. Sleeps/idles for 2 min (fixed time gate) to allow supercap recharge.
5. If $V_\text{SC}\geq V_\text{off}$ at wake, repeat transmission.
6. Once $V_\text{SC}<V_\text{off}$, comparator opens; MCU and radio power down to deep sleep until next charge cycle.

An ASCII timing diagram in the source visually illustrates these cycling dynamics, showing characteristic charge and discharge waveforms (Nepal et al., 25 Jan 2026).

4. Laboratory Performance and Efficiency Metrics

Empirical evaluation demonstrates autonomous operation triggered by water-induced harvesting:

• Time to first activation (charge to 4.87 V): ~23 min in a 0.5 mm water layer.
• LTE-M network acquisition (attach time): 10–20 s, mean 14 s.
• Beacons per wetting event: 6–9 (mean 8), each separated by ∼2 min.
• Duty cycle: transmission (attach+TX) ≈0.83% (1 s every 2 min); dormant/charging ≈99.17%.
• Energy per charge-discharge cycle: 17.8 J stored; ∼10 J expended over 8 beacons; residual margin ∼7.8 J.
• Power savings by comparator gating: prevents immediate high-current draw below 3.67 V, avoiding brown-out and unnecessary recharge. Comparator gating recovers an estimated 2–3 J per cycle (approximately 20% of total harvested energy), compared to direct connection (Nepal et al., 25 Jan 2026).

5. LTE-M Radio Parameters, Protocol Compliance, and Forward Compatibility

The LTE-M subsystem operates in LTE Cat-M1 mode (3GPP Rel-13, half-duplex FDD) using band 66 (AWS, 1.4 MHz bandwidth), with QPSK modulation. Extended discontinuous reception (eDRX) and power saving mode (PSM) are both disabled to simplify wake-up procedures. Standard 3GPP-compliant attach (including NAS security, AS PDN activation) and MQTT over IPv4 are employed. Peak TX current is ∼250 mA at 5 V; idle RX is ∼10 mA.

The use of Nordic nRF9160 enables firmware upgrades for 5G NB-IoT/non-terrestrial network (NTN) support (3GPP Rel-17). Comparator gating and large energy storage provide headroom for satellite-based NB-IoT operation, which demands up to 1.5× terrestrial attach energy. This architecture is positioned to enable battery-less, gateway-less cloud beaconing from infrastructure-scarce regions, via both terrestrial and low-earth-orbit NTN links (Nepal et al., 25 Jan 2026).

6. Technical Significance and Applications

The comparator-gated LTE-M radio provides an effective solution to the challenge of high-peak, bursty power requirements in low-duty-cycle, battery-less IoT applications. By strictly enforcing high and low voltage thresholds for modem operation, it ensures reliable network transactions and maximized energy utilization from environmental harvesting. This approach enables truly autonomous, maintenance-free, wide-area sensor deployments—specifically evidenced in the battery-less LTE-M water leak detector, where water activation alone suffices to drive multiple cloud communications without batteries or local gateways.

A plausible implication is that comparator gating schemes with programmable hysteresis, when integrated with high-capacity supercapacitors and modern low-threshold modems, offer a generalizable template for energy-aware, event-driven cellular IoT devices—particularly those seeking compliance with both LTE-M and future 5G-NTN standards (Nepal et al., 25 Jan 2026).