To size a dc dc converter schematic for a standard 12V input to 5V output at 3A (15W), assuming a 300 kHz switching frequency and 30% inductor current ripple, you need a 10 µH inductor (saturation current ≥4.5A), a 47 µF ceramic output capacitor, and a buck controller like the Texas Instruments TPS54331. The exact calculated inductance is 10.8 µH, which rounds to the standard 10 µH EIA value.
The Direct Answer: 12V to 5V, 3A Buck Component Values
The governing formula for the inductor in a continuous conduction mode (CCM) buck converter is:
L = (V_out × (V_in - V_out)) / (V_in × f_sw × ΔI_L)
Substituting our exact query values:
- V_in: 12V
- V_out: 5V
- f_sw (switching frequency): 300,000 Hz
- ΔI_L (ripple current): 30% of 3A = 0.9A
L = (5 × (12 - 5)) / (12 × 300,000 × 0.9) = 35 / 3,240,000 = 10.8 µH
We select the closest standard value of 10 µH. The peak inductor current is I_out + (ΔI_L / 2) = 3A + 0.45A = 3.45A. To prevent core saturation during transients, add a 30% margin, dictating an inductor with a saturation current rating of at least 4.5A. For the output capacitor, targeting a 50mV voltage ripple (1% of V_out) requires a minimum of 7.5 µF theoretically, but we specify 47 µF (e.g., two 22 µF X7R MLCCs in parallel) to handle equivalent series resistance (ESR) and load transient response.
Neighboring Values: ±20% Load and Voltage Variations
Real-world loads fluctuate. If your microcontroller or peripheral draws 20% more current, or if your 5V rail sags/tolerances shift, your component margins must hold. Here is how the required inductance and peak current shift across a ±20% range of your target parameters.
| V_out (V) | I_out (A) | Ripple (30%) | Calculated L (µH) | Standard L Pick | Min Sat Current (A) |
|---|---|---|---|---|---|
| 4.5V (-10%) | 2.4A (-20%) | 0.72A | 11.5 µH | 10 µH | 3.6A |
| 5.0V (Nom) | 3.0A (Nom) | 0.90A | 10.8 µH | 10 µH | 4.5A |
| 5.0V (Nom) | 3.6A (+20%) | 1.08A | 9.0 µH | 10 µH | 5.2A |
| 5.5V (+10%) | 3.0A (Nom) | 0.90A | 10.0 µH | 10 µH | 4.4A |
| 5.5V (+10%) | 3.6A (+20%) | 1.08A | 8.3 µH | 8.2 µH | 5.1A |
Note: A single 10 µH inductor rated for 5.5A saturation comfortably covers the entire ±20% envelope without saturating or dropping out of CCM.
How the Schematic Shifts: 12V vs 24V vs 48V Inputs
Just as AC mains designs shift drastically between 120V, 230V, and 3-phase, DC-DC schematics must adapt to standard DC bus voltages: 12V (automotive/battery), 24V (industrial/solar), and 48V (telecom/mild-hybrid). The assumption that fixes our 12V answer (a ~41% duty cycle) breaks down at higher voltages.
- 12V to 5V (Duty Cycle ~41%): Standard buck topology. The TPS54331 handles this easily with a standard 10 µH inductor.
- 24V to 5V (Duty Cycle ~20%): The on-time shrinks. To maintain the same 300 kHz frequency and 30% ripple, the inductor value must increase to 22 µH. Alternatively, you must increase the switching frequency to 600 kHz to keep the 10 µH inductor, which increases switching losses.
- 48V to 5V (Duty Cycle ~10%): A standard buck controller will fail here due to minimum on-time limitations. The schematic must shift to a synchronous buck with high-voltage rating (like the LM5164) or a flyback/isolated topology if safety isolation is required. The inductor value jumps to 47 µH to prevent massive ripple currents at low duty cycles.
For a deeper look at how input voltage impacts duty cycle limits, refer to the TI Basic Calculation of a Buck Converter application note.
Decision Tree: Selecting the Right Controller IC
Don't guess your IC. Use this decision path based on your finalized input voltage and maximum load current to terminate on a specific, purchasable part number.
| Condition (If...) | And...) | Then Pick This IC) | Package / Notes |
|---|---|---|---|
| V_in max ≤ 28V | I_out ≤ 3A | TPS54331 (TI) | SOIC-8, requires external boot diode |
| V_in max ≤ 28V | I_out ≤ 5A | TPS5450 (TI) | SOIC-8, integrated FETs, 500kHz fixed |
| V_in max ≤ 60V | I_out ≤ 5A | LM5164 (TI) | SOT-23-6, ultra-compact, COT control |
| V_in max ≤ 100V | I_out ≤ 10A | LM5170-Q1 (TI) | Requires external MOSFETs, bidirectional capable |
| V_in < V_out (Boost) | I_out ≤ 2A | TPS61232 (TI) | Boost topology, integrated 3A switch |
When This Conversion is Meaningless
The calculations above assume three fixed variables: Continuous Conduction Mode (CCM), a known switching frequency, and an assumed 85% efficiency for thermal headroom. This conversion becomes meaningless—and potentially dangerous to your hardware—if:
- Switching frequency is unknown or variable: If you use a hysteretic (ripple-based) controller with no fixed clock, the ΔI_L formula cannot be solved statically. The inductor value will dictate the frequency, not the other way around.
- Thermal constraints are ignored: If your enclosure is sealed and ambient temperature reaches 60°C, a 10 µH inductor with 40mΩ DCR will overheat and desolder itself from the pad, regardless of what the math says about ripple current.
- You are operating in Discontinuous Conduction Mode (DCM): At very light loads (e.g., < 300mA), the inductor current drops to zero before the next switching cycle. The CCM formulas overestimate the required inductance, leading to unnecessarily large, expensive magnetics.
Frequently Asked Questions
Can I use a 10 µH inductor with a lower current rating if my average load is only 1A?
No. The inductor must be rated for the peak current limit of the controller IC, not just your average load. If the TPS54331 enters current limit (typically ~5A) during a short circuit, a 2A-rated inductor will saturate, turning into a dead short and destroying the internal MOSFET.
How do I verify the schematic on the bench?
Use a current probe on your oscilloscope to measure the inductor ripple. You should see a triangle wave centered on your DC load current. If the peak-to-peak amplitude exceeds 1.5A (50% of your 3A target), your inductance is too low or your switching frequency has dropped. You can cross-reference physical inductor selections using the Coilcraft Power Inductor Finder to match DCR and saturation limits.
