107 lines
5.6 KiB
Markdown
107 lines
5.6 KiB
Markdown
🇬🇧 English | 🇩🇪 [Deutsch](README.de.md)
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# Design: 12V to 3.3V Step-Down with SY8120IABC
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This document describes the design of a high-efficiency, synchronous DC/DC step-down converter. It serves as a modern alternative to older designs like the TPS5430.
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## 1. Design Goals
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* **Input Voltage ($V_{in}$):** 12V (Range 10.5V - 14V)
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* **Output Voltage ($V_{out}$):** 3.3V
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* **Output Current ($I_{out}$):** Designed for approx. 0.5A, with reserves up to 2A.
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* **Core Component:** Silergy SY8120IABC (Synchronous, 1MHz, up to 18V, 2A)
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* **Objectives:** High efficiency, compact layout, minimal number of external components (no external diode).
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## 2. Component Design
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The calculation of the external components is based on the datasheet of the SY8120IABC.
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### a) Feedback Resistors (R1, R2) - Optimized for "Basic Parts"
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To use common and cost-effective "Basic Parts" from JLCPCB, resistors from the E24 series are chosen. The goal is to match the required voltage ratio for approx. 3.3V as closely as possible.
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The reference voltage ($V_{FB}$) of the SY8120IABC is **0.6V**.
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An excellent combination available as "Basic Parts" is:
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* **`R1` (upper resistor) = 120 kΩ**
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* **`R2` (lower resistor) = 27 kΩ**
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The resulting output voltage is calculated as follows:
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$V_{out} = 0.6V \times (1 + \frac{R1}{R2}) = 0.6V \times (1 + \frac{120kΩ}{27kΩ}) \approx \mathbf{3.27V}$
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This output voltage is ideal and completely safe for all target components.
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### b) Inductor (L1)
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Due to the high switching frequency of 1 MHz, a physically small inductor with a lower inductance value can be used. The datasheet recommends values in the range of 2.2µH to 10µH.
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* We choose a value of **`L1 = 4.7 µH`**.
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* **Important:** The saturation current ($I_{sat}$) must be higher than the IC's current limit (approx. 3A). An inductor with **$I_{sat} > 3A$** is selected.
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### c) Capacitors (C1, C2, C3)
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* **Input Capacitor (C1):** A **`10µF / 25V`** ceramic capacitor (0805) is sufficient to stabilize the input voltage.
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* **Output Capacitor (C2):** A **`22µF / 10V`** ceramic capacitor (0805) is recommended for a stable output voltage with low ripple.
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* **Bootstrap Capacitor (C3):** As specified in the datasheet, a **`100nF`** capacitor is connected between the BOOT and SW pins.
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*(Note: As discussed later, C1 and C2 can also be replaced by the same `22µF / 25V` capacitor for a simplified Bill of Materials.)*
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## 3. Final Bill of Materials (BOM) for LCSC/JLCPCB
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This list has been checked for availability (as of June 24, 2025).
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| Component (Ref) | Value | LCSC Part # | Package | JLCPCB Status | Note |
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| :--- | :--- | :--- | :--- | :--- | :--- |
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| **IC1** | **SY8120IABC**| `C479076` | SOT23-6 | Extended Part | Synchronous Step-Down Regulator. |
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| **L1** | **4.7 µH** | `C9400` | 12.3x12.3mm | Extended Part | sxn SMDRI127-330MT. Isat=5.5A, DCR=65mΩ. (User's choice) |
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| **C1, C2** | **22 µF, 25V** | `C45783` | 0805 | Basic Part | Samsung CL21A226MAQNNNE. (Used for both input & output) |
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| **C3 (Bootstrap)**| **100 nF, 25V** | `C1760` | 0402 | Basic Part | Samsung CL05B104KO5NNNC. (User's choice) |
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| **R1** | **120 kΩ, 1%**| `C25821` | 0402 | Basic Part | UNI-ROYAL 0402WGF1203TCE. (Feedback Resistor, upper) |
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| **R2** | **27 kΩ, 1%**| `C25890` | 0402 | Basic Part | UNI-ROYAL 0402WGF2702TCE. (Feedback Resistor, lower) |
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## 4. Important Layout Notes
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For a high-frequency converter (1MHz), a good layout is even more critical than usual.
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1. **Critical Loop (Input):** The path from the input capacitor `C1` to the `VIN` pin of the IC, and from the `GND` pin of the IC back to `C1`, must be absolutely minimal. Short, wide traces are mandatory here.
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2. **SW (Switching Node):** The `SW` pin carries the high-frequency switched voltage. The trace from this pin to the inductor `L1` should be short and wide, but kept away from sensitive signals (like the feedback line).
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3. **Feedback Path:** The resistors `R1` and `R2` should be placed close to the `FB` pin of the IC. The trace from the `FB` pin to the resistor divider should be short and should not be routed parallel to noisy traces (like SW).
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4. **Grounding:** A continuous ground plane underneath the components is the best solution to keep return paths short and improve thermal performance.
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## 5. Efficiency Calculation (SY8120IABC Design)
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This chapter analyzes the power loss and overall efficiency of the circuit for different load cases.
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### 5.1 Power Loss of the Feedback Network
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The feedback network for setting the output voltage has a permanent but very low power consumption.
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* **Resistors:** $R_{total} = R1 + R2 = 120kΩ + 27kΩ = 147kΩ$
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* **Power:** $P = V^2 / R = (3.27V)^2 / 147000Ω \approx 0.0000727W$
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The static loss in the feedback network is therefore only about **73 µW** and is negligible for the overall efficiency.
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### 5.2 Overall Efficiency
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The calculation is based on typical values from the SY8120IABC datasheet and the selected external components.
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**Parameters for Calculation:**
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* Input Voltage ($V_{in}$): 12 V
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* Output Voltage ($V_{out}$): 3.27 V
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* Duty Cycle ($D$): $3.27V / 12V \approx 0.273$
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* $R_{DS(on)}$ High-Side MOSFET: 130 mΩ (0.13 Ω)
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* $R_{DS(on)}$ Low-Side MOSFET: 105 mΩ (0.105 Ω)
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* Inductor Resistance (DCR): 65 mΩ (0.065 Ω) (for the `SMDRI127-330MT`)
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* Quiescent Current ($I_Q$): approx. 200 µA (0.0002 A)
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---
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#### Case 1: Load Current $I_{out}$ = 100 mA (0.1 A)
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* **Output Power ($P_{out}$):** $3.27V \times 0.1A = \mathbf{327\ mW}$
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* **Main Losses ($P_{loss}$):**
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* IC Conduction Loss (High-Side): $(0.1A)^2 \times 0.13Ω \times 0.273 \approx 0.36\ mW$
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* IC Conduction Loss (Low-Side): $(0.1A)^2 \times 0.105Ω \times (1-0.273) \approx 0.76\ mW$
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* Inductor Loss (DCR): $(0.1A)^2 \times 0.065Ω = 0.65\ mW$
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