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# Design: 12V to 3.3V Step-Down with SY8120IABC

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.

## 1. Design Goals

* **Input Voltage ($V_{in}$):** 12V (Range 10.5V - 14V)
* **Output Voltage ($V_{out}$):** 3.3V
* **Output Current ($I_{out}$):** Designed for approx. 0.5A, with reserves up to 2A.
* **Core Component:** Silergy SY8120IABC (Synchronous, 1MHz, up to 18V, 2A)
* **Objectives:** High efficiency, compact layout, minimal number of external components (no external diode).

## 2. Component Design

The calculation of the external components is based on the datasheet of the SY8120IABC.

### a) Feedback Resistors (R1, R2) - Optimized for "Basic Parts"

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.

The reference voltage ($V_{FB}$) of the SY8120IABC is **0.6V**.

An excellent combination available as "Basic Parts" is:
* **`R1` (upper resistor) = 120 kΩ**
* **`R2` (lower resistor) = 27 kΩ**

The resulting output voltage is calculated as follows:
$V_{out} = 0.6V \times (1 + \frac{R1}{R2}) = 0.6V \times (1 + \frac{120kΩ}{27kΩ}) \approx \mathbf{3.27V}$

This output voltage is ideal and completely safe for all target components.

### b) Inductor (L1)

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.

* We choose a value of **`L1 = 4.7 µH`**.
* **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.

### c) Capacitors (C1, C2, C3)

* **Input Capacitor (C1):** A **`10µF / 25V`** ceramic capacitor (0805) is sufficient to stabilize the input voltage.
* **Output Capacitor (C2):** A **`22µF / 10V`** ceramic capacitor (0805) is recommended for a stable output voltage with low ripple.
* **Bootstrap Capacitor (C3):** As specified in the datasheet, a **`100nF`** capacitor is connected between the BOOT and SW pins.

*(Note: As discussed later, C1 and C2 can also be replaced by the same `22µF / 25V` capacitor for a simplified Bill of Materials.)*

## 3. Final Bill of Materials (BOM) for LCSC/JLCPCB

This list has been checked for availability (as of June 24, 2025).

| Component (Ref) | Value | LCSC Part # | Package | JLCPCB Status | Note |
| :--- | :--- | :--- | :--- | :--- | :--- |
| **IC1** | **SY8120IABC**| `C479076` | SOT23-6 | Extended Part | Synchronous Step-Down Regulator. |
| **L1** | **4.7 µH** | `C9400` | 12.3x12.3mm | Extended Part | sxn SMDRI127-330MT. Isat=5.5A, DCR=65mΩ. (User's choice) |
| **C1, C2** | **22 µF, 25V** | `C45783` | 0805 | Basic Part | Samsung CL21A226MAQNNNE. (Used for both input & output) |
| **C3 (Bootstrap)**| **100 nF, 25V** | `C1760` | 0402 | Basic Part | Samsung CL05B104KO5NNNC. (User's choice) |
| **R1** | **120 kΩ, 1%**| `C25821` | 0402 | Basic Part | UNI-ROYAL 0402WGF1203TCE. (Feedback Resistor, upper) |
| **R2** | **27 kΩ, 1%**| `C25890` | 0402 | Basic Part | UNI-ROYAL 0402WGF2702TCE. (Feedback Resistor, lower) |

## 4. Important Layout Notes

For a high-frequency converter (1MHz), a good layout is even more critical than usual.

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.
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).
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).
4.  **Grounding:** A continuous ground plane underneath the components is the best solution to keep return paths short and improve thermal performance.

## 5. Efficiency Calculation (SY8120IABC Design)

This chapter analyzes the power loss and overall efficiency of the circuit for different load cases.

### 5.1 Power Loss of the Feedback Network

The feedback network for setting the output voltage has a permanent but very low power consumption.

* **Resistors:** $R_{total} = R1 + R2 = 120kΩ + 27kΩ = 147kΩ$
* **Power:** $P = V^2 / R = (3.27V)^2 / 147000Ω \approx 0.0000727W$

The static loss in the feedback network is therefore only about **73 µW** and is negligible for the overall efficiency.

### 5.2 Overall Efficiency

The calculation is based on typical values from the SY8120IABC datasheet and the selected external components.

**Parameters for Calculation:**

* Input Voltage ($V_{in}$): 12 V
* Output Voltage ($V_{out}$): 3.27 V
* Duty Cycle ($D$): $3.27V / 12V \approx 0.273$
* $R_{DS(on)}$ High-Side MOSFET: 130 mΩ (0.13 Ω)
* $R_{DS(on)}$ Low-Side MOSFET: 105 mΩ (0.105 Ω)
* Inductor Resistance (DCR): 65 mΩ (0.065 Ω) (for the `SMDRI127-330MT`)
* Quiescent Current ($I_Q$): approx. 200 µA (0.0002 A)

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#### Case 1: Load Current $I_{out}$ = 100 mA (0.1 A)

* **Output Power ($P_{out}$):** $3.27V \times 0.1A = \mathbf{327\ mW}$
* **Main Losses ($P_{loss}$):**
    * IC Conduction Loss (High-Side): $(0.1A)^2 \times 0.13Ω \times 0.273 \approx 0.36\ mW$
    * IC Conduction Loss (Low-Side): $(0.1A)^2 \times 0.105Ω \times (1-0.273) \approx 0.76\ mW$
    * Inductor Loss (DCR): $(0.1A)^2 \times 0.065Ω = 0.65\ mW$
