Seeking Schematic Review & Power Optimization Insights! - Page 1
Forum section: electronics / projects

author: kalpacha420
author_rank: Newbie
timestamp: September 05, 2025, 05:52:30 pm
content: Deep Dive into Ultra-Low-Power Design: STM32U0 + E-Paper + SHT45 (CR2450)I am engaged in a project focused on extreme power efficiency, and I am currently at the schematic review stage prior to PCB layout. The objective is to build an ultra-low-power, battery-powered thermometer/hygrometer utilizing an e-paper display. This design necessitates minimizing current consumption to the lowest possible microampere level. I would appreciate a review and expert feedback on the power management design, with a particular focus on sleep mode optimization.### Project Overview:The device is designed to periodically acquire data from an SHT45 (U3) sensor and update a 1.54" e-paper display (GDEM0154D67WT). The microcontroller unit is an STM32U031x8 (U2), chosen for its ultra-low-power capabilities.#### Power Architecture:1. Primary Power: A single CR2450 coin cell (BT1). This 3V nominal, ~600mAh cell necessitates maximum battery life optimization.2. Main Regulator: The TPS62842DGRR (U1) was selected as the high-efficiency, low-quiescent-current buck converter, configured to output 2.5V. R2 (4.3kΩ) is used for its VSET pin. The EN pin of U1 is connected directly to the +3V0 battery rail. This 2.5V rail supplies power to the MCU, the sensor, and the e-paper display's logic.3. Display High-Voltage Generation: The e-paper requires specific positive (VGH, VPP) and negative (VGL) high voltages. These are generated via a discrete charge pump circuit. Q1 (Si1308EDL) is involved in the voltage generation, while Q2 (Si2301CDS) is used for power gating, and MBR0530 diodes (D1, D2, D3) complete the boosting from the 2.5V rail.### Design Philosophy: Sleep Mode OptimizationThe system is designed to operate primarily in deep sleep mode, with brief awakenings (e.g., once per minute) for sensor measurements and display updates. This low-duty-cycle operation renders quiescent current (Iq) during sleep mode as the primary optimization concern. Efficiency during active operation is secondary to minimizing quiescent current.### Technical Questions and Areas for Review:This power design has been developed with careful consideration; however, I am seeking critical analysis and insights into potential issues or methods for further microampere reduction.1. Buck Converter (U1 - TPS62842) vs. LDO: Optimal Choice for Ultra-Low Power?The TPS62842 was selected for its high efficiency (~90%+ at expected loads) and low quiescent current (typical 350 nA). With its EN pin tied high and MODE pin connected to GND, the device operates in automatic Power-Save Mode (PFM/PWM auto-transition). This configuration allows for extremely low quiescent current (typically 60 nA) during light or no-load conditions, enabling it to remain 'always on' while maintaining high power efficiency for sleep-dominant applications. The primary question is: Given this very low IQ in Power-Save Mode, is a high-efficiency buck converter, constantly enabled, truly the optimal choice for an ultra-low-power, sleep-dominant application, or would a very low-Iq LDO (e.g., the TPS7A02) provide superior overall battery life? Consideration is given to the CR2450's (BT1) discharge curve, where an LDO's lower dropout voltage might offer an advantage as the battery depletes. Comments on this trade-off are welcome.2. Quiescent Current (Iq) & Sleep Mode Optimization: Identification of Current Drains.I aim to identify any subtle current drains.a. TPS62842 EN Pin (U1, Pin 4): This pin is tied directly to the +3V0 battery rail, meaning the buck converter is always enabled. The datasheet indicates a very low quiescent current (typical 60 nA) in Power-Save Mode at light or no load. Given this, are there any further implications or potential disadvantages of having the regulator *always enabled* in a sleep-dominant, ultra-low-power design.b. STM32 Crystal Loading Caps (C22, C23): The STM32U0 (U2) utilizes an X1 (32.768 kHz crystal) for its LSE/RTC. C22 (18pF) and C23 (18pF) are used as loading capacitors. Are these values appropriately sized for optimal low-power operation and reliable oscillation stability, particularly given the focus on extended sleep?3. Discrete Display Driver Power Efficiency: Assessment of Charge Pump Performance.A discrete charge pump was selected for generating the e-paper's high voltages. Q1 (Si1308EDL) is directly involved in voltage generation, while Q2 (Si2301CDS) is used for power gating (to enable/disable the charge pump circuit), and MBR0530 diodes (D1, D2, D3) complete the boosting from the 2.5V rail. This design was chosen based on its theoretical zero sleep current for the charge pump components when Q2 is off.a. The question arises: Is this discrete charge pump sufficiently efficient during the active display update phase? Are there significant switching losses or shoot-through currents within the Q1 (Si1308EDL) and MBR0530 (D1, D2, D3) diode network that require mitigation?b. Conversely, would a dedicated, integrated e-paper power IC (e.g., from Texas Instruments' TPS6518x family) provide a superior overall power profile, despite its inherent quiescent current? Or is the theoretical zero sleep current of the discrete approach, contingent on Q2's effective power gating, still justifiable despite potential complexity during active phases?c. Diode Selection (MBR0530): These are 0.5A Schottky diodes. Their leakage current performance during deep sleep, particularly for high-voltage generation, is a concern. Are more efficient alternatives available, or should a 1A diode be considered if current spikes during display updates exceed expectations?4. Transient Current Handling: Risk of Brown-Out During Display Updates.The CR2450 (BT1) exhibits a relatively high internal impedance. During a display update, the combined current draw from the MCU, the charge pump, and the e-paper can be significant (potentially tens of mA, albeit briefly).Are the bulk capacitors (C1=10µF, C13=10µF, C1=10µF) sufficient to manage these current transients without significant voltage droop on the 2.5V rail? Concerns exist regarding potential MCU brown-out or system reset. Guidance on optimizing their placement or sizing is requested.5. General Power Integrity & Noise: Decoupling and Filtering.a. Decoupling: Is the 100nF decoupling strategy (C8, C9, C10, C11, C12, C14, C21) for the STM32U0 (U2) and other ICs adequate for low-noise operation and minimizing radiated emissions? Specific consideration is given to the rapid switching associated with e-paper drive voltages.### Requested Feedback:1. Schematic review (PDF attached): Identification of any critical issues or subtle optimization opportunities for ultra-low power.2. Practical recommendations for quiescent current reduction: Ideas for the regulator (considering its always-enabled state and low IQ), and general leakage from diodes/MOSFETs.3. Insights into the efficiency of the discrete charge pump, and whether integrated solutions are advisable.4. Guidance on transient response management: Methods to ensure stability under peak current loads from the CR2450 (BT1).5. General best practices: Including but not limited to decoupling layout, test points, and ESD protection for the display connector.I extend my gratitude for your expertise and time in reviewing this. Your insights will be invaluable prior to PCB layout finalization.P.S.: Sorry for my bad English, it is not my main language.

author: mariush
author_rank: Super Contributor
timestamp: September 05, 2025, 08:35:42 pm
content: A CR2450 battery is a bad choice. It's not rechargeable, and it has very low recommended output current, at around 4-5mA (acceptable bursts up to 15mA).If possible, you should aim for rechargeable flat lithium cells, you can get 4-5mm thick cells for a few dollars, like for example :https://www.digikey.com/en/products/detail/adafruit-industries-llc/1578/5054539orhttps://www.digikey.com/en/products/detail/sparkfun-electronics/13851/6605199At 29mm by 37mm , they don't use that much space compared to a CR2450 cell in a holder (at least a 24mm diameter circle)You have a project that will use at most maybe 50-100mA? Do you think you need a regulator "optimized" for up to 750mA output current and which runs at 1.8 Mhz ?Your microcontroller SMT32U0 can run down to 1.7v , your SHT45 advertises 1.08 3.3 3.6v ... so in theory it could function with 1.8vI'd suggest looking at a regulator like Renesas ISL9122 for example :https://www.renesas.com/en/document/dst/isl9122a-datasheet8WLCSPhttps://www.digikey.com/en/products/detail/renesas-electronics-corporation/ISL9122AIINZ-T/124296358DFNhttps://www.digikey.com/en/products/detail/renesas-electronics-corporation/ISL9122AIRNZ-T/12429632It's buck-boost, it has a force bypass mode to just pass battery voltage through (120nA quiescent current in that mode), and it has configurable output voltage through i2c (starts initially at 3.3v, then adjustable in 25mV steps from 1.8v to 5v+). So for example, you could measure the battery voltage and if it's below 3.6v (maximum supported by your STM32 and sensor) you could pass through voltage instead of using the regulator in step-down mode.When you want to actually update the ePaper you could configure the output voltage to 3.3v or higher to make it easier for that charge pump, and outside that period, maybe step down to 1.8v.Such regulator would also allow you to use batteries that output lower voltage, like for example let's say 2 AAA batteries (1.2v per cell, 2.4v in total) - it would be thicker at around 11mm (due to the AAA diameter of 10mm) or easily replaceable alkaline / heavy duty cells.If you want buck only, there's regulators that will you close to 95% efficiency and still have quite low quiescent currentFor example TPS62743https://www.digikey.com/en/products/detail/texas-instruments/TPS62743YFPR/5323984orhttps://www.digikey.com/en/products/detail/texas-instruments/TPS62746YFPT/5344304will do up to 300mA output current with 360nA quiescent current and the output voltage is selectable with 3 pins, you have these options : 1.2 V, 1.5 V, 1.8 V, 2.1 V, 2.5 V, 2.8 V, 3.0 V, 3.3 VTPS627431 will do up to 400mA and has slightly different output voltage presets : 1.3 V, 1.4 V, 1.6 V, 1.7 V, 1.9 V, 2.0 V, 2.9 V, 3.1 Vhttps://www.digikey.com/en/products/detail/texas-instruments/TPS627431YFPR/6098071orhttps://www.digikey.com/en/products/detail/texas-instruments/TPS627431YFPT/6110564Richtek RT5707 / RT5707A is super similar TPS627431 down to the preset voltages, but has higher output currents :RT5707 (up to 600mA, up to 3.3v)https://www.digikey.com/en/products/detail/richtek-usa-inc/RT5707WSC/9178344RT5707A (up to 400mA, up to 3.1v)https://www.digikey.com/en/products/detail/richtek-usa-inc/RT5707AWSC/9178343TPS628438 is another option , 275nA quiescent current, up to 600mA, voltage configurable between 0.8V to 3.6V using a single resistor : (sot563)https://www.digikey.com/en/products/detail/texas-instruments/TPS628438DRLR/24708443or (balls)https://www.digikey.com/en/products/detail/texas-instruments/TPS628438YKAR/18179071but the efficiency will be lower at 90% peak.

author: kalpacha420
author_rank: Newbie
timestamp: September 06, 2025, 12:36:25 am
content: Thank you very much for taking the time to provide such comprehensive feedback on my schematic and power design. I truly appreciate the suggestions. I've gone through each point, done some further research, and here are my thoughts:CR2450 Battery Choice: You raise a very valid point regarding the CR2450's current limitations and its suitability for this application, especially with the e-paper display's transient current requirements.I agree that the typical pulse current ratings (e.g., 15mA) of a CR2450 are indeed a significant concern, as our e-paper display updates can momentarily exceed this.The suggestion of rechargeable flat lithium cells (like the Adafruit/SparkFun examples) is excellent.My primary design constraint for this project is extreme compactness, often dictating the use of coin cell form factors. However, your points make it clear that if any flexibility in physical dimensions can be achieved, moving to a higher-capacity, lower-impedance rechargeable cell would be a superior solution. This is a critical trade-off that I will re-evaluate based on the mechanical design constraints.TPS62842 Regulator Suitability (750mA rating, 1.8MHz): I agree that the 750mA current rating is indeed over-specified for the average or continuous current draw of this device. My primary rationale for selecting the TPS62842 was its ultra-low quiescent current (typically 60nA) in Power-Save Mode (PFM), which is crucial for maximizing battery life in a sleep-dominant application. The 1.8MHz switching frequency was also a plus for enabling smaller inductor/capacitor sizes, contributing to overall compactness. While the current capability is higher than strictly necessary, its excellent low-IQ performance for the sleep state was the driving factor.Alternative Regulator Suggestions: Thank you for the list of alternative regulators. I've analyzed each of them with respect to my project's primary goal of ultra-low quiescent current.Renesas ISL9122 (Buck-Boost with I2C control and Bypass Mode): This is an extremely promising suggestion.Buck-Boost Capability: This is a major advantage, allowing full energy extraction from the battery as its voltage drops, and opening possibilities for other low-voltage battery chemistries (e.g., AAAs).I2C Configurable Output Voltage: This feature is particularly compelling. It would enable dynamic voltage scaling: running the MCU/sensor at the lowest stable voltage (e.g., 1.8V) during deep sleep to further reduce static and dynamic power consumption, and then boosting it to a higher voltage (e.g., 3.3V) only when actively updating the e-paper display. This could lead to significant overall system power savings.Bypass Mode (120nA Iq): While its 120nA Iq in bypass mode is higher than the TPS62842's 60nA Iq in PFM, the system-level optimization offered by the dynamic voltage adjustment and buck-boost capability could potentially yield a better overall power profile. I will be doing a much deeper dive into this part.Other Buck-Only Regulators (TI TPS6274x series, Richtek RT5707/A, TI TPS628438): I appreciate these suggestions as well, as they are more appropriately sized for current delivery. However, upon reviewing their datasheets:TI TPS6274x series (e.g., TPS62743 at 360nA Iq): These have a quiescent current significantly higher than the TPS62842's 60nA.Richtek RT5707/A: Generally similar Iq performance to the TPS6274x series.TI TPS628438 (275nA Iq): Also features a quiescent current substantially higher than the TPS62842. For a sleep-dominant, ultra-low-power application, the absolute lowest possible quiescent current from the main regulator is paramount.Therefore, while these are capable regulators, their higher Iq values make them less suitable than the TPS62842 for my core objective of minimizing sleep current.Thank you once again for your input.

author: mariush
author_rank: Super Contributor
timestamp: September 06, 2025, 10:38:15 am
content: Excellent, glad I could be of help.You may want to give it some more thought to ePaper vs raw lcd (lcd made with raw segments, not controller). If you update the ePaper once a minute, the energy spent to make the update (and what you lose in the charge pumps / boosters) may be more than what a raw lcd display would use to work for half an hour or more.It will use uA worth of energy to keep alive. If the microcontroller doesn't have built in lcd controller, you can get controllers for very cheap (10-20 cents) which use very little power (uA worth of power)For example, see a 4 digit Lumex LCD-S401M16KR :https://www.digikey.com/en/products/detail/lumex-opto-components-inc/LCD-S401M16KR/7364561orhttps://eu.mouser.com/ProductDetail/Lumex/LCD-S401M16KR?qs=0JU57JYmZjvIRhzkrYuX%252BQ%3D%3DNote the datasheet on mouser is for another model and Digikey just goes to website, the correct one is here :https://www.rxelectronics.cz/datasheet/ea/lcd-s401m16kr.pdfThe LCD has 4 digits + extra ":" for scenario where you'd want to make a clock. So you could display temperatures and use last digit as "c" or "F" or you could alternative between displaying time and temperature.Or you could go overkill with a 8 digit 14 segment display likehttps://www.digikey.com/en/products/detail/varitronix/VIM-878-DP-RC-S-LV/1118605-it's around 52mm wide, 22mm tall ... about the size of a AAA/AA battery in width.Both of these should run fine with 3v DC, possibly even lessPCF8553 is 40 x 4 driver that can go down to 1.8v and consumes less than 6uA :https://www.lcsc.com/product-detail/C3194504.htmlOther examples ... Siproin SSP9750 or SSP55080 work down to 2.5v and do 35 segments x 8 commons and go lower than 10uA.https://www.lcsc.com/product-detail/C5444289.htmlhttps://www.lcsc.com/product-detail/C5444288.htmlhttps://www.lcsc.com/product-detail/C5444287.htmlhttps://www.lcsc.com/product-detail/C5444286.htmlTM1729 / GVN1729 / *1729 with 36 segments x 4 com will go down to 2.5v and consume less than 10uA...Lower pin count ICs ... lCD Segment drivers like GN1621DThttps://www.lcsc.com/product-detail/C3294396.htmlor HTM1621-TDhttps://www.lcsc.com/product-detail/C47637389.htmlcan drive up to 32 segments (depends on chip pinout) and 4 commons and claim to consume as little as 60uA and run with as little as 2.4vSpeaking of AAA / AA batteries and you also mentioned compactness...Didn't cover the option of running your product on a single AAA/AA cell. You have 1v to 1.25v for rechargeable, 1v to 1.65v for alkaline ... with the right boost regulator you may be able to boost to 1.8v or higher with close to 95-97% efficiency.Don't get hanged up on quiescent current that much, think also about easy of battery replacement, how easy to get a new battery to swap old one with new, and how much original energy you have in the cell.AAA (10.5mm x 44.5mm ) go up to 1.4Ah but commonly available ones go to 1.2 Ah :https://www.digikey.com/short/j72nvrnvAA (14.5mm x 50.5mm) can go up to 3.1Ah possibly more if you go for the Lithium based ones :https://www.digikey.com/short/nd33zmt9Also worth noting that it's also possible to source LiFePO4 cells in AA footprint (14mm x 50mm), it's the 14500 standard.. LiFePO4 has 3.2v nominal voltage, 3.6v charge voltage... example :https://www.digikey.com/en/products/detail/dantona-industries/LIFEO4-14505/13692638If you use a charge pump or boost regulator to boost 1.5v to 2.5v or 3v , you'll get 500-600mAh equivalent out of a 1200mAh AAA cell, about as much as a CR2450 claims to give you. If you manage to get the raw lcd working down to 1.8v or 2.5v even better.Note there's also AAAA (8.3mm x 42mm ) can go up to 700mAh but they're harder to buy in stores :https://www.digikey.com/en/products/detail/energizer-battery-company/E96VP/1955840Some examples of boosters (because they'll pretty much run 24/7, stuff like the rds(on) of internal mosfets will probably matter more than quiescent current)fixed 3.3v outTPS613221 :https://www.digikey.com/en/products/detail/texas-instruments/TPS613221ADBVT/9371269TPS61221 :https://www.digikey.com/en/products/detail/texas-instruments/TPS61221DCKR/2232832Torex XC9142 :https://www.digikey.com/en/products/detail/torex-semiconductor-ltd/XC9142B33CMR-G/6148747TLV61225https://www.digikey.com/en/products/detail/texas-instruments/TLV61225DCKR/2696400fixed 2.5v outTPS610992Yhttps://www.digikey.com/en/products/detail/texas-instruments/TPS610992YFFR/7776779

author: kalpacha420
author_rank: Newbie
timestamp: September 06, 2025, 06:27:59 pm
content: Thank you for your latest detailed reply.Here are my thoughts on the points you've raised:ePaper vs. Raw LCD (efficiency, energy consumption): You make a very astute observation regarding the energy spent on e-paper updates versus the constant, but very low, consumption of a raw LCD. I fully agree that for a simple numerical display, a raw LCD combined with a low-power segment driver (like the PCF8553 or GN1621DT you mentioned) offers an extremely compelling power profile for continuous operation.However, my decision to go with e-paper was influenced by practical, real-world constraints: sourcing suitable raw LCDs in my region. Even for inexpensive modules, the total cost after factoring in import duties and shipping became disproportionately high, making them less viable than the e-paper module I ultimately chose. In fact, my initial MCU choice was an STM32U073 specifically for its integrated LCD driver and this custom LCD display (https://pt.aliexpress.com/item/1005008929216562.html?spm=a2g0o.cart.0.0.45867f061OMoe7&mp=1&pdp_npi=5%40dis%21BRL%21BRL%2014.89%21BRL%2014.89%21%21BRL%2014.89%21%21%21%402103244417571754537653102ed9f4%2112000047242171251%21ct%21BR%21868463811%21%211%210&gatewayAdapt=glo2bra), but I switched to the STM32U031 due to the difficulty of economically sourcing a compatible raw LCD. This situation forced a trade-off where the inherent advantages of e-paper (zero power retention, excellent readability) became the more accessible solution, despite its update power consumption.AAA/AA/AAAA Batteries and LiFePO4 Cells: Thank you for expanding on the options for higher-capacity batteries. Your point about not solely focusing on quiescent current and also considering total energy, ease of replacement, and the usable capacity is very well taken. The detailed information on AAA, AA, AAAA, and especially the LiFePO4 14500 cells is extremely valuable.My project's core requirement for extreme compactness heavily dictates the use of the coin cell form factor (or something very similar). While the cylindrical cells you mentioned offer significantly higher energy density, their larger dimensions (e.g., 10.5mm diameter for AAA) present a considerable challenge for my current mechanical design. Nevertheless, your comprehensive breakdown offers excellent insights for future designs where compactness might be a slightly less critical constraint.Boost Regulator Examples: I appreciate you providing specific examples of boost regulators (TPS613221, TPS61221, Torex XC9142, TLV61225, TPS610992Y). This information is certainly noted for potential future iterations or related projects.Thank you once again for your consistent and helpful input.