How to Generate Negative Voltage from a Single 5V or 3.3V Supply

Introduction

In modern electronic circuits, the common power supply voltage is often positive 5V or 3.3V, but some applications require negative voltage to drive amplifiers, level conversion, or reference circuits. For example, certain high-precision analog amplifiers must operate in a bipolar power supply environment (± voltage), or some sensors may need a negative reference voltage. As a result, generating a stable negative voltage under a single positive supply has become an important issue in circuit design.

This blog post will focus on how to use a 5V or 3.3V power supply to generate negative voltage, covering the common methods and corresponding circuit analysis.

The Simplest Solution: Charge Pumps

A charge pump is a circuit structure that uses switches and capacitors to transfer charge, obtaining a voltage with opposite polarity to the input at the output terminal through alternating charging and discharging. Popular charge pump inverters such as ICL7660, MAX1044, or LTC1046 can convert 5V to -5V (ideally) or 3.3V to -3.3V.

Basic Principle:

• Under the control of the switch (MOSFET or internal integrated switch), the positive voltage at the input is first stored on a capacitor.
• This stored charge is then flipped to the output, resulting in polarity reversal.

Typical Circuit Components:

• Two switches (MOSFETs or internal switches)
• Two capacitors:
  • A flying capacitor (first charged, then switched to output)
  • An output filter/storage capacitor (ensures a stable negative voltage)

Charge pumps are simple and compact, making them ideal when current demand is only a few milliamperes to tens of milliamperes. However, they typically have lower efficiency and limited output current capability.

Inverting DC-DC Converters

For higher current requirements, consider the inverting topology of a switching regulator. Inverting DC-DC converters can provide stable negative voltage at larger currents. Common PWM controllers (e.g., LM2577, LM2587, TPS5430) can produce outputs like -5V or -12V from a 5V supply when configured in an inverting topology.

How It Works:

• Similar to buck or boost circuits, but the feedback and transformer/inductor connections differ.
• In a single-inductor design, the diode and inductor store and release energy, generating an output voltage with the opposite polarity to the input.

Detailed Analysis:

• When the inductor and diode alternate conduction at the PWM switching frequency, the voltage across the inductor reverses polarity.
• Assume the input is +5V. When the switch (transistor) is on, the inductor is charged from the input while the output is disconnected from ground.
• Once the switch turns off, the inductor tries to maintain the current flow, pulling the output below ground, creating a negative voltage.
• This negative voltage is then rectified and filtered, providing a stable negative voltage to the load.

Compared to charge pumps, inverting DC-DC solutions deliver higher power and better efficiency at the cost of additional components (inductors, diodes) and a more complex PCB layout.

Key Design Considerations

1. Load Current Size
   • If the load current is only a few mA to around ten mA, a charge pump may suffice.
   • Higher current demands favor an inverting DC-DC converter.
2. Ripple and Noise
   • Negative voltage stability is crucial for precision analog circuits.
   • Charge pumps switch capacitors frequently, causing relatively high ripple. If ripple is critical, add filtering (capacitor or LC filter).
   • Switching inverters also produce switching noise—layout and filtering measures are necessary.
3. Efficiency and Heat Generation
   • Due to their operation principle, charge pumps typically have lower efficiency and are only suitable for low-power scenarios.
   • Inverting DC-DC converters can be more efficient but require inductors and a complex PCB layout, and designers must address heat dissipation and EMI.
4. Layout and Device Selection
   • Pay attention to voltage ratings, package size, and reliability of components.
   • Keep high-frequency switching nodes short to reduce parasitic inductance and capacitance. This helps limit radiation and crosstalk.

Specific Circuit Implementations

Implementation details will vary based on chip selection:

• Charge Pump Inverter (e.g., ICL7660)
  Requires only two external capacitors (one flying capacitor, one output filter).
• Inverting DC-DC Converter (e.g., LM2587)
  Requires at least one inductor and one diode. The chip usually integrates PWM control logic and MOSFET power switches internally.

Conclusion

In general, the common methods of generating a negative voltage from a single 5V or 3.3V supply fall into two main categories:

1. Charge Pump Inverter
2. Switch-Type Inverting DC-DC Converter

The former is suitable for low current, simple applications, while the latter caters to higher power and efficiency needs. When designing such a circuit, remember to evaluate load requirements, EMC considerations, cost, and reliability. It’s also essential to conduct thorough circuit simulations and experimental verification to achieve a stable and efficient negative voltage in real-world applications.