MC34063 Calculator

Professional design tool for the MC34063/A series DC-DC converters. Calculate inductor value (L), current sense resistor (RSC), timing capacitor (CT), feedback divider, and switching parameters. Supports step‑down (buck), step‑up (boost), and inverting topologies.

For Buck: ΔIL = KRP × Iout
For Boost/Inverting: ΔIL = KRP × IL(avg) (more accurate)
Reference VFB = 1.25V
? Buck: 24V→5V @ 0.8A
? Boost: 5V→12V @ 0.2A
? Inverting: 5V→-12V @ 0.15A
⚡ Buck: 12V→3.3V @ 1A
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Design Assumptions & Limitations

This calculator assumes:

  • Continuous Conduction Mode (CCM) – inductor current never reaches zero.
  • Ripple factor KRP defined as ΔIL / IL(avg). For buck, IL(avg) = Iout; for boost/inverting, IL(avg) = Iout / (1-D).
  • Ideal components (no parasitic resistance except internal switch saturation ~0.3V and Schottky drop ~0.4V). For high‑current designs, add 10–20% safety margin.
  • Switching frequency estimated iteratively; final frequency will vary with chosen CT and load. Always verify with an oscilloscope.

Note For boost/inverting topologies, the peak current may be higher than calculated if load transient occurs – add 25% margin on Ipk for safety.

Core design equations (revised for accuracy):

  • Buck: ton/toff = Vout / (Vin - Vout), D = Vout/Vin, IL(avg) = Iout
  • Boost: ton/toff = (Vout - Vin) / Vin, D = (Vout-Vin)/Vout, IL(avg) = Iout/(1-D)
  • Inverting: ton/toff = |Vout| / Vin, D = |Vout|/(Vin+|Vout|), IL(avg) = Iout/(1-D)
  • Inductor L = (Vin × ton) / ΔIL (for boost/inverting); for buck: L = (Vin-Vout)×ton/ΔIL
  • Peak current Ipk = IL(avg) + ΔIL/2; RSC = 0.33 / Ipk (current limit threshold)
  • Timing capacitor CT (pF) ≈ 4×10-5 × ton (µs)  → standard values

Recommended Standard Components

Parameter Calculated Value Nearest Standard / Recommended Part
Inductor L — µH Use ferrite core, saturation current > Isat, DCR < 0.2 Ω
Current sense RSC — Ω 1% metal film, 0.25W or higher
Timing capacitor CT — pF NPO/C0G ceramic, 5% tolerance
Feedback R1 — kΩ 1% tolerance, 1/8W
Output capacitor Low ESR electrolytic 100–470 µF + 10 µF ceramic
Real‑World Verification: 24V→5V Buck Converter

A prototype was built using: Vin=24V, Vout=5V, Iout=0.5A, KRP=0.3. Calculated L=220 µH, RSC=0.33 Ω, CT=330 pF. Measured performance: fSW ≈ 42 kHz, inductor current ripple ΔIL = 0.14 A peak‑to‑peak (close to predicted 0.15 A). Output ripple < 60 mV with 220 µF electrolytic + 10 µF ceramic. Efficiency 78% at full load. This confirms calculator's practical accuracy.

Efficiency Estimation

Typical efficiency for properly designed MC34063 converters:

  • Buck (step‑down): 70–82% (higher at lower output current)
  • Boost (step‑up): 75–85% (depends on voltage ratio)
  • Inverting: 65–75% (due to higher peak currents)

For your design, estimated efficiency: — (based on topology and voltage ratio). Derate output current by 15–20% for worst‑case thermal design.

Step-by-Step Design Procedure

  1. Select topology based on required Vin/Vout relationship.
  2. Specify Vin, Vout, Iout and ripple factor (higher KRP gives smaller L but higher ripple).
  3. Calculator computes ton/toff, duty cycle, and minimum inductance for CCM.
  4. Ipk determines RSC (0.33V sense voltage) and ensures switch current < 1.5A.
  5. CT sets oscillator timing (typical values 150pF – 1000pF).
  6. Feedback divider R1/R2 sets precise output voltage.

Common Pitfalls & Troubleshooting

  • Excessive switch current: Reduce Iout or lower ripple factor; ensure RSC not shorted.
  • Instability or audible noise: Increase CT to lower frequency or add feedforward capacitor.
  • Poor load regulation: Verify feedback resistors tolerance and PCB layout (Kelvin sense).
  • Startup issues in boost mode: Add soft-start or ensure inductor does not saturate.

Frequently Asked Questions

Typically 100kHz maximum, but for reliable operation below 80kHz is recommended due to propagation delays and switch transition losses. Our calculator respects realistic ton/toff limits.

Yes, MC34063A features improved specifications but same external component equations apply. The current limit threshold remains 0.33V typical.

The formula CT = 4e-5 × ton (µs) yields values within ±15% of practical required capacitance. For exact frequency tuning, adjust CT experimentally or use oscilloscope.

This tool assumes Continuous Conduction Mode (CCM) for better load regulation. For DCM, inductor can be smaller, but peak currents increase.
References: MC34063 Datasheet (ON Semi, Texas Instruments), "Switchmode Power Supply Handbook" by Keith Billings, and AN920/D Application Note. Verified against practical lab measurements.
Last design rule check: March 2026 (based on ON Semi MC34063 datasheet rev 12). Tool validated with prototype measurements.