Switching Power Supplies¶
Purpose¶
This dual switching power supplies convert wide-range input voltage to regulated 5V and 3.3V rails. The system achieves extreme efficiency for what it does, which is important for this device to be used as a battery monitor.
Input Voltage Range¶
- VIN_2-60: 4.2V to 65V (recommended operating range)
- Absolute maximum: Up to 70V (stress rating)
- Typical operating voltage: 12V (automotive/industrial applications)
Output Rails¶
5V Rail Specifications¶
| Parameter | Value |
|---|---|
| Output Voltage | 5.0V ±2% |
| Maximum Current | 1A |
| Input Range | 4.2V - 65V |
| Peak Efficiency | 94% @ 300-500mA |
| Full Load Efficiency | 92% @ 1A |
| Power Dissipation @ 1A | 0.43W |
3.3V Rail Specifications¶
| Parameter | Value |
|---|---|
| Output Voltage | 3.3V ±2% |
| Maximum Current | 2A |
| Input Source | 5V rail |
| Peak Efficiency | 95.5% @ 500mA |
| Full Load Efficiency | 92% @ 2A |
| Power Dissipation @ 2A | 0.57W |
Main Components¶
5V Stage (LMR36510ADDAR)¶
The 5 V system rail is generated by U8, an LMR36510ADDAR — TI's 4.2–65 V, 1 A synchronous buck in HSOIC-8 with exposed pad. Selected for:
- 65 V max VIN (70 V transient). Battery rail can spike well above 48 V nominal during alternator load dump; the regulator must survive transients on VIN_2-60.
- 400 kHz switching. Low enough that inductor copper losses and EMI are easy to manage on a 4-layer board, high enough to keep output ripple and inductor size reasonable.
- 26 µA non-switching quiescent. A battery monitor that idles at the regulator must not bleed the bank.
| Component | Function |
|---|---|
| U8 — LMR36510ADDAR | 4.2–65 V input, 1 A synchronous buck. fSW = 400 kHz nominal. VFB = 1.0 V typical (0.985–1.015 V). IQ = 26 µA. |
| L3 — SMNR8040-150MT | 15 µH switching inductor. Changed from the 22 µH TI recommendation for better saturation current, lower cost, and smaller package (8 × 8 mm). Negligible efficiency penalty at our load. Typical measured ≈ 17–18 µH (within the ±20 % part tolerance). |
| R42 / R48 | Feedback divider — R42 = 100 kΩ (top, VOUT → FB), R48 = 24.9 kΩ (bottom, FB → GND). VOUT = VFB × (1 + R42/R48) = 1.0 × (1 + 100 / 24.9) = 5.02 V. Updated 2026-05-23 (previously documented as 10 kΩ / 24.9 kΩ — the top resistor is 100 kΩ, not 10 kΩ). |
| C55 — 100 nF, 100 V | Bootstrap capacitor between BOOT (pin 7) and SW (pin 8). Required for high-side MOSFET gate drive. |
| C54 — 1 µF | Internal LDO bypass on VCC (pin 6). 1 µF is the datasheet-recommended value. |
| C56, C57, C58 — 22 µF each | Output bulk filtering on the 5V_LMR rail. Three 22 µF X5R 0603 caps give ~50 µF total effective at 5 V (DC bias derated). |
| C47 (2.2 µF) + C50 (220 nF) | Input decoupling located directly at the LMR36510 VIN pin. Part of the downstream energy buffer — sized for fast transient demand, not for bulk. |
Why feedback resistors are in the 100 kΩ range, not lower: The LMR36510 FB pin draws only 2.1 nA typical. With R48 = 24.9 kΩ, the divider current is 1.0 V / 24.9 kΩ = 40 µA. Using a 10 kΩ bottom would push that to 100 µA — wasted on a battery-monitor product whose entire idle target is sub-mA. Higher impedance is also fine because the FB pin is high impedance; the only consideration is noise pickup, mitigated by the 0603 placement directly under the IC.
3.3V Stage (TLV62569DBV)¶
The 3.3 V rail is generated by U4, a TLV62569DBV — TI's 2.5 V–5.5 V input, 2 A synchronous buck in SOT-23-5. Cascaded after the 5 V stage rather than running directly off the battery because:
- Smaller, cheaper, more efficient at 5 V → 3.3 V than any wide-Vin buck would be at 12–48 V → 3.3 V.
- Sequencing is automatic: the 3.3 V rail cannot come up before 5 V is good, so the ESP32-S3 sees a clean, ordered power-up.
| Component | Function |
|---|---|
| U4 — TLV62569DBV | 2.5–5.5 V input, 2 A synchronous buck. fSW = 1.5 MHz typical. VFB = 0.6 V typical. |
| L1 — 2.2 µH | Switching inductor sized for ~50 % ripple at 1 A load. |
| R25 — 100 kΩ | Pull-up from EN (pin 1) to the 5 V rail. Enables the converter whenever 5 V is present. No software control of EN. |
| R31 / R39 | Feedback divider — R31 = 453 kΩ (top, VOUT → FB), R39 = 100 kΩ (bottom, FB → GND). VOUT = 0.6 × (1 + 453 / 100) = 3.32 V (≈ 3.3 V nominal). |
| C22 (4.7 µF) + C23 (1 µF) | Input filtering from the 5 V system rail. |
| C24 — 22 µF | Output filter on the 3.3 V rail. |
Feedback divider choice: Total bottom-leg current = 0.6 V / 100 kΩ = 6 µA. Same logic as the LMR36510 — high impedance is safe at this Vref because FB current is sub-nA, and the savings matter on a continuous battery monitor.
Capacitor Specifications¶
As-built ratings from BOM audit. See BOM for full component list.
| Designator | Nominal Value | Location / Function | As-Built Rating |
|---|---|---|---|
| C55 | 100 nF | LMR36510 bootstrap | 100V 0603 |
| C56, C57, C58 | 22 µF each | LMR36510 output filter | 16V 0603 |
| C47 | 2.2 µF | LMR36510 VIN decoupling | 100V 0805 |
| C50 | 220 nF | LMR36510 VIN decoupling | 100V 0603 |
| C22 | 4.7 µF | TLV62569 VIN | 16V 0603 |
| C23 | 1 µF | TLV62569 VIN | 50V 0603 |
| C24 | 22 µF | TLV62569 output | 16V 0603 |
| C42 | 22 µF | MT3608 VIN (5 V rail) | 16V 0603 |
| C43, C63 | 22 µF each | MT3608 output (12 V boost rail — see alternator_field_drive.md) | 16V 0603 — see Future Plans |
| LM2907 decoupling | 1 µF | LM2907 pins 5/6 (COL/V+) | TBD |
| SN74LVC1T45 decoupling | 1 µF | VCCA (pin 1) | TBD |
| TPS2553 decoupling | 100 nF | TPS2553 VIN | TBD |
Efficiency Performance¶
5V Rail (12V Input)¶
| Load Current | Output Power | Efficiency | Power Loss |
|---|---|---|---|
| 1 mA | 5 mW | 80% | 1 mW |
| 10 mA | 50 mW | 89% | 6 mW |
| 100 mA | 0.5 W | 92% | 43 mW |
| 300 mA | 1.5 W | 94% | 96 mW |
| 500 mA | 2.5 W | 94% | 0.16 W |
| 1 A | 5.0 W | 92% | 0.43 W |
3.3V Rail (5V Input)¶
| Load Current | Output Power | Efficiency | Power Loss |
|---|---|---|---|
| 1 mA | 3 mW | 90% | 0 mW |
| 10 mA | 33 mW | 94.5% | 2 mW |
| 100 mA | 0.33 W | 95% | 17 mW |
| 500 mA | 1.65 W | 95.5% | 78 mW |
| 1 A | 3.30 W | 95% | 0.17 W |
| 2 A | 6.60 W | 92% | 0.57 W |
Current Limitations¶
Maximum Continuous Current¶
| Rail | Thermal Limit | Safe Continuous | Failure Point |
|---|---|---|---|
| 5V | 1A (datasheet) | 1A | >1A (overcurrent protection) |
| 3.3V | 2A (datasheet) | 2A | >2A (overcurrent protection) |
Practical Operating Limits¶
- 5V rail: Limited to 1A by LMR36510ADDA current capability
- 3.3V rail: Limited to 2A by TLV62569DBV current capability
- Combined power: 11.6W maximum (5W + 6.6W)
- Input current at 12V: ~1A maximum at full load
System Efficiency Analysis¶
Overall Efficiency (12V → 3.3V)¶
When both converters operate in series:
| 3.3V Load | 5V Stage | 3.3V Stage | Overall |
|---|---|---|---|
| 100 mA | 92% | 95% | 87.4% |
| 500 mA | 94% | 95.5% | 89.8% |
| 1 A | 92% | 95% | 87.4% |
Power Budget at Maximum Load¶
- 3.3V rail: 6.6W @ 2A
- 5V rail power required: 7.17W (including 3.3V stage losses)
- 12V input power: 8.2W (including 5V stage losses)
- Total system efficiency: 80.5% at maximum 3.3V load
Control and Enable Logic¶
- 3.3V Enable: Controlled via 5V rail through R25 (100kΩ)
- Enable threshold: ~1.4V (typical for TLV62569)
- Soft-start: Integrated in both converters
- Shutdown current: <10µA when disabled
Performance Goals¶
- High efficiency: >90% at typical operating loads
- Wide input range: Support 12V-48V automotive/industrial systems
- Clean regulation: <50mV ripple on both rails
- Fast transient response: <50µs settling time
- Low quiescent current: Minimize idle power consumption
Thermal Considerations¶
- 5V stage: 0.43W dissipation at 1A requires adequate copper area
- 3.3V stage: 0.57W dissipation at 2A may require thermal vias
- PCB design: 4-layer board recommended for thermal management
- Component spacing: Allow airflow around inductors and ICs
Notes¶
- Both converters include integrated synchronous rectification for high efficiency
- Output capacitor ESR affects stability and ripple performance
- Input filtering critical for EMI compliance
- Enable sequencing: 5V must be stable before 3.3V rail activates
Nominal Current Budget in our use case¶
| Component | Active Mode | Sleep Mode | Rail |
|---|---|---|---|
| ESP32 (WiFi off) | 80 mA | 10 µA | 3.3V |
| ADS1115 ADC | 150 µA | 150 µA | 3.3V |
| INA228 Current Monitor | 1 mA | 1 mA | 3.3V |
| BMP390 Pressure Sensor | 3 µA | 3 µA | 3.3V |
| LM2907 Frequency Converter | 5 mA | 5 mA | 5V |
| Voltage Dividers | 20 mA | 20 mA | Various |
| System Total | 106 mA | 26 mA | Mixed |
Operating Point Efficiency¶
At typical load currents, the system operates in the high-efficiency region:
| Operating Mode | Load Current | 5V Stage Efficiency | 3.3V Stage Efficiency | Overall Efficiency |
|---|---|---|---|---|
| Active (ESP32 on) | 106 mA | 92% | 95% | 87.4% |
| Sleep (ESP32 off) | 26 mA | 90% | 95% | 85.5% |
Power Consumption Summary¶
| Mode | 3.3V Power | 5V Power | 12V Input Power | Efficiency |
|---|---|---|---|---|
| Active | 350 mW | 25 mW | 430 mW | 87.4% |
| Sleep | 86 mW | 25 mW | 130 mW | 85.5% |
Design Margin¶
- Current headroom: 10-20× safety margin on both rails
- Thermal margin: Minimal power dissipation (<100mW total)
- Voltage regulation: Excellent for precision analog circuits
- EMI performance: Switching converters with integrated synchronous rectification
This load profile demonstrates optimal utilization of the switching supply's capabilities, operating in the peak efficiency region while maintaining substantial design margin for future expansion.
5V System Rail — Power Mux and Load Distribution¶
The system 5V rail is the output of a TPS2116DRL power multiplexer. The TPS2116 selects between two inputs — the LMR36510 output (battery-derived) and USB 5V — and routes the active source to the system 5V rail. Priority and switchover behavior are governed by the TPS2116's internal logic.
| TPS2116 Decoupling | Status |
|---|---|
| C_in (each input pin) | Not populated (see Future Plans) |
| C_out (system 5V output) | Not populated (see Future Plans) |
The USB-C 5V input has no bulk capacitance in the current design (see Future Plans).
5V System Rail Loads¶
All devices below are supplied from the TPS2116 output (system 5V rail):
| Device | Function | Local Decoupling |
|---|---|---|
| TLV62569DBV | 3.3V buck converter | C22 (4.7 µF) + C23 (1 µF) at VIN |
| MT3608 | Boost converter for alternator gate driver (12V) | C42 (22 µF) at VIN |
| LM2907 | Frequency-to-voltage converter | 1 µF at pins 5/6 (COL/V+) |
| SN74LVC1T45 | 1-bit level shifter (5V side) | 1 µF at VCCA (pin 1) |
| TPS2553 | Load switch (buzzer control circuit) | 0.1 µF at VIN |
| FDN340P | P-channel MOSFET | None |
| BC847 | NPN transistor | None |
The MT3608 generates a 12V boost rail from the 5V system rail. This rail is used exclusively by the alternator field drive gate driver (LM5109A). Output decoupling: C43 + C63 (22 µF each — see Capacitor Specifications). Full circuit documentation: see alternator_field_drive.md, Block 2.
3.3V System Rail Loads¶
The 3.3V rail is the output of the TLV62569DBV step-down converter, fed from the 5V system rail.
| Device | Function | Local Decoupling |
|---|---|---|
| ESP32-S3 | Microcontroller | TBD |
| ADS1115 | 16-bit ADC | TBD |
| INA228 | Current/power monitor | TBD |
| BMP390 | Pressure/temperature sensor | TBD |
| TLV9154IDR | Quad op-amp | TBD |
Future Plans¶
-
Upgrade C43 and C63 to 25 V rating: Currently 22 µF / 16 V X5R on the MT3608 12 V boost output. Operating voltage is 75% of rated voltage — minimal margin against boost converter startup overshoot, transient response, and DC bias derating (effective capacitance approximately 9–11 µF at 12 V). Replace with 22 µF / 25 V 0603 X5R (e.g., Murata GRM188C61E226ME01D). Drop-in same footprint. C42 on the MT3608 input (5 V rail) is fine at 16 V and does not need to change.
-
USB-C 5V input bulk capacitance: Add 10–22 µF + 100 nF near the USB-C connector. Cable inductance and transient load steps cause voltage droop without local bulk capacitance. Low risk; place physically close to the connector to be effective.
-
TPS2116 decoupling: Add 1 µF on each input and 4.7 µF on the output per TPS2116 datasheet recommendation. The TPS2116 currently has no local bypass capacitors.
-
Bulk capacitor on 5V system rail (optional, conservative sizing): A shared 10 µF on the TPS2116 output would improve transient response for load steps from ESP32 WiFi activity. Adding bulk capacitance increases energy available during a fault event — size conservatively. 10 µF is sufficient; do not use 22 µF or larger.