unavlab
151 lines
5.5 KiB
Markdown
151 lines
5.5 KiB
Markdown
# Power Supply Design Guide
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## Topology Selection
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| Topology | When to Use | Efficiency | Noise | Cost |
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|----------|-------------|------------|-------|------|
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| **LDO** | Vdrop < 1V, Iout < 500mA, noise-sensitive | Low (η ≈ Vout/Vin) | Very low | Low |
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| **Buck** | Vdrop > 1V, Iout > 200mA, efficiency matters | High (85-95%) | Medium | Medium |
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| **Buck + LDO chain** | High Vin, multiple rails, mixed noise needs | Good overall | Low on LDO rails | Medium |
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| **Boost** | Vout > Vin (battery → 5V, etc.) | Medium (80-90%) | Medium-High | Medium |
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| **Buck-Boost** | Vin can be above or below Vout | Medium | High | High |
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| **Charge Pump** | Small current (<100mA), voltage doubling/inversion | Medium | High | Low |
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### Decision Tree
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```
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Is Vin always > Vout?
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├─ Yes → Is (Vin - Vout) < 1V AND Iout < 500mA?
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│ ├─ Yes → LDO
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│ └─ No → Is noise critical on this rail?
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│ ├─ Yes → Buck + LDO (buck for bulk, LDO for clean rail)
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│ └─ No → Buck
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└─ No → Is Vin sometimes < Vout?
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├─ Yes → Buck-Boost (or SEPIC)
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└─ No → Boost
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```
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## Input Protection
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Every power input should have:
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1. **Reverse polarity protection**: P-MOSFET (preferred) or Schottky diode (simpler, lossy)
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2. **Overvoltage / transient protection**: TVS diode rated above max operating voltage
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3. **Inrush current limiting**: NTC thermistor or soft-start IC for high-capacitance designs
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4. **Fuse**: Resettable PTC or standard fuse, rated for max expected current × 1.5
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```python
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# circuit-synth pattern: input protection block
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@circuit(name="input_protection")
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def input_protection(vin_raw, vin_protected, gnd):
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fuse = Component(symbol="Device:Fuse_Small", ref="F", value="2A",
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footprint="Fuse:Fuse_1206_3216Metric")
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tvs = Component(symbol="Diode:TVS", ref="D_TVS", value="SMBJ15A",
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footprint="Diode_SMD:D_SMA")
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cin = Component(symbol="Device:C", ref="C_IN", value="10uF",
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footprint="Capacitor_SMD:C_1206_3216Metric")
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vin_raw += fuse[1]
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fuse[2] += tvs["A"] # TVS anode to fused input
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tvs["K"] += gnd # TVS cathode to ground (unidirectional)
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vin_protected += fuse[2], cin[1]
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gnd += cin[2]
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```
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## Decoupling Strategy
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### Per-IC decoupling (mandatory)
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- **100nF ceramic (X7R/X5R, 0402 or 0603)** on every VDD pin, placed as close as possible.
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- For MCUs with VDDA (analog supply): separate 100nF + 1µF, with ferrite bead isolation.
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- For MCUs with VCAP (internal regulator output): capacitor per datasheet (typically 2.2µF).
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### Per-rail bulk capacitors
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- **10µF ceramic** per power rail (at the regulator output).
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- **100µF electrolytic/tantalum** for high-current rails (>500mA).
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### High-frequency bypass
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- **100nF + 10nF** pair for noise-sensitive analog circuits.
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- Place smaller cap closest to IC pin.
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### Layout rules
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- Capacitor → IC pin via is shorter than capacitor → pour via.
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- Ground vias under decoupling caps (direct path to ground plane).
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- Never route high-speed signals under/near switching regulators.
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## Common Regulator Reference Circuits
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### LDO: AMS1117-3.3 (SOT-223)
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```python
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@circuit(name="ldo_3v3")
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def ldo_3v3(vin, vout_3v3, gnd):
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reg = Component(symbol="Regulator_Linear:AMS1117-3.3", ref="U_LDO",
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footprint="Package_TO_SOT_SMD:SOT-223-3_TabPin2")
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cin = Component(symbol="Device:C", ref="C_LDO_IN", value="10uF",
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footprint="Capacitor_SMD:C_0805_2012Metric")
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cout = Component(symbol="Device:C", ref="C_LDO_OUT", value="22uF",
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footprint="Capacitor_SMD:C_0805_2012Metric")
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vin += reg["VI"], cin[1]
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vout_3v3 += reg["VO"], cout[1]
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gnd += reg["GND"], cin[2], cout[2]
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```
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### Buck: TPS54331 (SOIC-8) — 3A, 3.5-28V input
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```python
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@circuit(name="buck_5v")
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def buck_5v(vin, vout_5v, gnd):
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buck = Component(symbol="Regulator_Switching:TPS54331", ref="U_BUCK",
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footprint="Package_SO:SOIC-8_3.9x4.9mm_P1.27mm")
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inductor = Component(symbol="Device:L", ref="L_BUCK", value="15uH",
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footprint="Inductor_SMD:L_1210_3225Metric")
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diode = Component(symbol="Diode:SS34", ref="D_BUCK",
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footprint="Diode_SMD:D_SMA")
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# Feedback resistors, bootstrap cap, input/output caps...
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# (Complete circuit depends on exact target voltage)
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vin += buck["VIN"]
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buck["GND"] += gnd
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buck["PH"] += inductor[1], diode["K"]
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inductor[2] += vout_5v
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diode["A"] += gnd
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```
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## Multi-Rail Design Patterns
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### Pattern: 12V → 5V (Buck) → 3.3V (LDO) → 1.8V (LDO)
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Good for: general-purpose MCU boards, moderate current.
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```
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VIN_12V ──→ [Buck] ──→ 5V rail (1A)
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├──→ [LDO] ──→ 3.3V rail (500mA)
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│ └──→ MCU VDD, peripherals
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└──→ [LDO] ──→ 1.8V rail (200mA)
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└──→ MCU VCORE (if separate)
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```
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### Pattern: USB 5V → 3.3V (LDO)
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Good for: simple USB-powered devices.
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```
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USB_VBUS ──→ [Fuse 500mA] ──→ [LDO] ──→ 3.3V
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```
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### Pattern: Battery (3.0-4.2V) → 3.3V (Buck-Boost or LDO)
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Good for: battery-powered IoT. LDO if battery always > 3.5V; buck-boost otherwise.
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## Thermal Considerations
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- Calculate power dissipation: `P = (Vin - Vout) × Iout` for LDO
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- Check junction temperature: `Tj = Ta + P × θja`
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- SOT-223 θja ≈ 65°C/W; SOIC-8 ≈ 125°C/W; QFN exposed pad ≈ 35°C/W
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- If Tj > 125°C at max load → need larger package or switch to buck
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