Inverting Buck Boost Converter

An inverting buck-boost converter takes a positive DC input voltage and produces a negative output voltage whose magnitude can be either higher or lower than the input. Key component values and operating parameters for continuous conduction mode (CCM) follow from the input voltage, desired output magnitude, load current, and switching frequency. All numeric fields accept decimals, fractions (3/4), and mixed numbers (5 1/2).

Input voltage is the positive DC supply feeding the converter; typical values range from 3.3 V for battery-powered circuits to 48 V for industrial applications. Output voltage magnitude is the absolute value of the desired negative output, so entering 5 targets a -5 V rail; the topology inherently inverts polarity, so the actual output is the negative of the entered value. Output current is the maximum load current the converter must supply. Switching frequency trades component size for loss: higher frequencies allow smaller inductors and capacitors but increase switching losses, and common values fall between 50 kHz and 500 kHz. Inductor current ripple and output voltage ripple, set as percentages, define acceptable component stress and output noise; estimated efficiency accounts for real-world losses when computing input power and current.

Duty cycle: D = Vout / (Vin + Vout), where Vout is the output voltage magnitude. Minimum inductance: L = (Vin x D) / (delta_IL x fsw), which keeps the converter in CCM at the specified ripple. Minimum output capacitance: C = (Iout x D) / (delta_Vout x fsw).

A 3.7 V lithium cell stepped down to a -5 V rail gives a duty cycle of D = Vout / (Vin + Vout) = 5 / (3.7 + 5) = 5 / 8.7, or approximately 0.575, about 57.5%. Minimum inductance and minimum capacitance follow the same duty cycle through L = (Vin x D) / (delta_IL x fsw) and C = (Iout x D) / (delta_Vout x fsw), scaled by whatever ripple percentages and switching frequency the design settles on.

• Results recalculate automatically after a short delay following any field change, alongside a manual calculate action.
• A step-by-step derivation, enabled in settings, shows the symbolic formulas, substituted values, and intermediate calculations behind each result.
• Reset restores all fields to their default values and clears any saved state.

Inverting buck-boost converters appear across a wide range of electronic systems wherever a negative supply rail is required from a positive source. The scenarios below illustrate common applications and the parameters involved.

• Analog Signal Processing: op-amp circuits and data acquisition systems often require symmetric positive and negative supply rails; a -12 V rail generated from a single +12 V input, at 200 mA output current and 150 kHz switching frequency, sizes the inductor and capacitor for low-noise analog performance.
• Battery-Powered Instruments: portable test equipment running from a 3.7 V lithium cell may need -5 V for sensor biasing, and a 3.7 V input, 5 V output magnitude, and 50 mA load current reveal the duty cycle and inductor value needed to maintain regulation across the battery discharge curve.
• Audio Amplifier Bias: Class AB audio amplifiers require negative bias voltages for the output stage; converting a 24 V main supply to -15 V at 500 mA with a tight 0.5% output ripple keeps audio reproduction clean of supply-induced distortion.
• Industrial Control Systems: legacy 4-20 mA current loop interfaces and certain PLC analog input modules expect -10 V reference rails; a 48 V bus stepped down to -10 V at 100 mA with a high switching frequency minimizes inductor size in space-constrained DIN rail enclosures.
• LED Driver Circuits: certain LED configurations in automotive and architectural lighting use negative voltage rails for ground-referenced current sinking; sizing the converter for -9 V at 2 A from a 12 V automotive supply with 30% inductor ripple balances efficiency against magnetics volume.
• Telecommunications Equipment: negative 48 V distribution is standard in telecom infrastructure; a -48 V intermediate rail generated from a +24 V rectified input at moderate current lets designers verify inductor saturation margins and capacitor ripple current ratings before selecting specific components.
• Research and Prototyping: lab prototypes frequently need adjustable negative voltages during development; sweeping the output voltage from -3.3 V to -24 V while observing how duty cycle and inductor requirements shift guides initial component sourcing before board layout begins.