Abstract

It is common for Internet of Things (IoT) devices, industrial sensors, meters, precision, and medical equipment to require both a positive and negative voltage. Often, these voltages must be symmetrical and sourced from a single power supply. This article explains the market trends, technical requirements, and a comparative analysis of solutions, aiming to equip the sales team with the insights needed to effectively promote the products.

Definition of Terms

Converter: Power management integrated circuit with or without integrated switches inside an IC

Regulator: A converter with integrated switches

Controller: A converter with external switches

The Market

Various electronic designs require one or more negative voltages in the power supply, often coming together with a symmetrical positive voltage. Some typical application examples are:

• In gate drives for the charger and traction inverter of electric vehicles (to drive gallium nitride (GaN) FETs and isolated-gate bipolar transistors (IGBTs) for example).
• In high performance ADC and DAC and rail-to-rail operational amplifiers (op amps) for industrial and medical applications.
• In LCD displays for consumer products.
• In driving (avalanche) photodiodes.
• In medical applications like X-rays.

The following details two typical block diagrams of such designs.

Gate Drivers

For high power switch-mode power supplies and motor drives, a negative driving voltage is often required, that is due to:

• Systems may not have a tightly placed and coupled PCB layout, its circuit ground usually couples with noises from all around the system and may fluctuate around ground level.
• The main power devices such as IGBTs, silicon carbide (SiC), or GaN FETs are often placed up to centimeters away from the gate control circuitry unless they are all housed inside a module. Hence, the signal coming out of the gate drivers may be distorted as they reach the power devices, the additional safety margin is desired.
• Advanced power devices such as GaN FETs often have a low turn-on threshold, making them more sensitive to gate voltage ringing. Some high voltage GaN FETs may have high CGD or wide process variation, which may cause a Miller effect-induced turn-on. In this case, the end customers are suggested to apply a negative gate voltage to ensure the device maintains its off status. For certain types of IGBTs, a negative voltage is required to completely turn off.

One example is using an isolated driver, ADuM4120. In such applications, the power devices are driven from positive voltage as in V1 and negative voltage as in V2, as seen in Figure 1.

Rail-to-Ramp Op Amps

For various signal conditioning applications, rail-to-rail op amps are often used where the output needs to have a wide span close to supply, the input swings around the reference, or when the highest precision is required. A typical example of a phono preamplifier system is shown in Figure 2. This design requires one positive 15 V and one negative 15 V.

The Requirements

The IC topologies to generate a positive voltage from the main supply are often well understood, which include low dropout (LDO) regulator, buck, boost, buckboost, etc. However, the selections and compromises in creating negative voltage were not discussed in depth in prior publications. Let us look at some of the requirements and design challenges.

Isolation

Sometimes, the ± voltages need to be isolated from the supply, mainly due to safety reasons or no common ground. For example, in electric vehicle powertrain, a 12 V control bus is mainly supplied from the auxiliary 12 V battery. It must be isolated to control the high voltage battery, so any low voltage fault does not lead to safety hazards. Such 12 V is often converted to ±5 V, or ±15 V with galvanic isolation, to supply multiple signal chain and driver ICs in traction inverters or chargers. Other industrial inverters like photovoltaic inverters or motor drives may require isolation as well.

Compact Size

For certain applications such as medical patient monitors, miniaturization is a key design target. Such devices need to read and amplify various sensor signals through multiple high precision converters. A tiny solution to generate ± voltages to power such converters is highly desirable.

Efficiency

Improving efficiency is often a target for any new designs. For example, a common trend in op amp applications is to use lower rail voltages, if no obvious distortion is found at the output, and it is more efficient if it takes less power to generate these rail voltages.

Timing and Symmetry

For special applications like medical X-ray, the ± voltages may not require high accuracy, but they must be symmetrical with minimal difference in absolute value, hence it’s best to have accurate regulation and timing control of both voltages.

The Solutions

The solutions are listed in the order of complexity and general performance, the pros and cons are also shown for comparison.

Zener Diode

A simple way to generate ± voltages without an IC is to use a Zener diode, as shown in Figure 3. In this solution, the output of the V3 source is split by Dz and Rz. If V3 is 9 V and Dz is a 5 V Zener diode, then the gate will be driven by +5 V and –4 V. This method provides a low cost solution as it does not require additional ICs. However, this solution is highly inefficient and is not suitable for applications that require tens of milliamps and a well-regulated output voltage. Hence, this topology is not often used.

Charge Pump

Using a charge pump is a convenient method to invert the positive input since no magnetic component is required. There are many charge pump ICs to achieve such a function and are preferred under different situations.

For low power needs, Analog Devices offers a number of regulated and unregulated charge pumps, like the LTC1983 in Figure 4 for example. While this solution is very simple with a small form factor, the drawback is on efficiency, and possible high electromagnetic interference (EMI). This device category is limited on load current and is generally used in applications that need less than 100 mA.

Alternatively, in the interest that low noise/low EMI is desired to avoid possible interference with other sensitive circuitry (especially for medical equipment, sensing, and communication applications), ADI offers products like the LTC3265 that integrates low noise LDO regulators to each of the dual charge pump outputs (Figure 5). While the output current is limited to 50 mA, this solution is much more EMI friendly and integrates both positive and negative output rails in just one IC. With very low output noise, it is quite helpful in precision instrumentation applications to drive low power op amps and data converters.

In an application where both a high load current positive voltage rail (for system power), and a smaller load current negative voltage rail (for a bias or reference) are needed, a discrete negative voltage charge pump can be applied to almost any buck or boost regulator without an additional IC. An example circuit is shown in the article “Generating Negative Output Voltage from Positive Input Voltage Using MAX17291 Boost Converter IC with Active Discharge Feature”, and uses the MAX17291 with external circuitry to create the charge pump. The drawback is in the charge pump’s load regulation and dynamic load response.

Inverting Converter

Charge pumps are relatively more useful with known input/output combinations without accurate regulation needs, and the related noise interference is taken care of by additional filtering. For applications with a wide range of input or output voltages with tight regulation needs, it is recommended to use inductor-based switch-mode topologies.

There are a few such topologies that can handle positive to negative conversion, often all are categorized as inverting topologies and may confuse engineers. While they can often perform the same power conversion task, there are design compromises. Below are three typical topologies. The first two are similar; however, using a buck IC provides more options even though they are not specifically designed to generate negative voltages.

• Inverting buck-boost converter using a buck IC
• Standalone inverting buck-boost converter
• Dual-inductor (CÜK) inverting buck-boost converter

Topology (1): Inverting Buck-Boost Converter using a Buck IC

When a typical synchronous buck converter’s output side is switched with circuit ground, an inverting buck-boost (IBB) converter is created, as shown in Figure 6. This approach is popular as there are many options for synchronous buck regulators or controllers available on the market. For noise-sensitive applications, ADI’s Silent Switcher^® monolithic buck regulators, such as the LT8624S using Silent Switcher 3 technology, can be configured as an IBB to generate a negative voltage rail with both excellent wideband and EMI noise performance. Figure 6 shows an example circuit of the LT8624S as an IBB, and can be found in the article “Fast-Transient Negative Voltage Rail for Noise-Sensitive Applications”. For further filtering, a low noise negative input LDO regulator can be added to the output. ADI has a variety of synchronous buck controller options available with external FETs if higher power is desired while using this topology.

The disadvantage here is the IC is referring to the buck converter ground, but not the system ground (which is the positive side of the output). If a microcontroller is needed to perform functions like enable, SYNC, or just receive a PGOOD signal, then an external level shifter circuit may be needed, which can be inconvenient. An example of such extra level shifter circuitry can be found in the article “Generating Negative Voltages—Why You Need Level Shifting in Buck-Boost Circuits”, and shown in Figure 6. If PMBus®/I²C communication is desired, then a level shifter may not work, and an external digital isolator IC may become necessary.

If a converter with no need for external sensing or control is used, then using a buck IC as an IBB is preferred, with a wider variety of options. All buck point-ofload converters at any voltage and current ratings can be configured in this way, but most will need external level shifters to be controlled externally.

Topology (2): Standalone Inverting Buck-Boost Converter

When external level shifters are not desired in the application, there are two solutions: use an asynchronous IBB, or integrate the level shifters into the buck IC. For example:

• Asynchronous IBB: An asynchronous IBB can be designed by using a PMOS as the primary switch and a diode instead of a synchronous switch. This allows the IC to be referenced to system ground without the need for level shifters. Here the positive side of the output load is tied to the input ground. The IC option here can be the LTC3863 as shown in Figure 7. It is often less efficient than using a buck IC because a PMOS and diode usually have more losses than an NMOS-based synchronous converter.





• Buck-based IBB with integrated level shifters: Instead of using external level shifters when using a buck IC as an IBB, each input and output signal can have its own level shifter integrated into the IC. This is convenient for designers. For example, the MAX17577/MAX17578 and MAX17579/MAX17580 are buck-based IBB converters that integrate level shifters at the EN and RESET pins.
  
  If high power and high efficiency are desired, then the LTC3896 is recommended. It is a more sophisticated, high performance synchronous switching controller with integrated level shifters. While it is a relatively large IC in a 38-lead TSSOP package, it is very energy efficient and supports NMOS for both switches. This device is recommended for power requirements greater than 100 W.

Topology (3): Dual-Inductor (CÜK) Inverting Buck-Boost Converter

When switching noise is a concern, a CÜK converter can generate a negative output voltage with less noise than an IBB converter. This topology is shown in Figure 8, with two inductors and one coupling capacitor. The advantage of this converter is in its simplicity, only a low-side switch is needed to invert the input, and it can be an NMOS so efficiency is high. For example, the LT8330 requires just 8 pins and is not difficult to design with. This IC is one of ADI’s regulators with two integrated error amplifiers that enable it to sense either positive or negative output voltage. Similar regulators such as the LT8331, LT8333, LT8334, LT8570, and LT8580 offer different ratings and features to cover a variety of common application requirements.

While this topology does need two inductors, if the two inductors are coupled as shown in Figure 8, the output ripple is significantly reduced, and may save on output capacitor size. Also, since one inductor sits on each input and output side, the currents are continuous, and the entire circuit can be less noisy than other topologies. If more power is desired, a controller IC with an external low-side FET, such as the LT3758 can be a good option.

Flyback Converter

If a transformer is required for isolation purposes (like in a flyback converter), then it is very easy to create ± output voltages by adding another winding on the output side. Here on the transformer, by setting multiple windings in different directions, together with blocking diodes, a positive or negative voltage can be generated, as in Figure 9. For example, the LT8306 doesn’t need an optocoupler for feedback, saving on bill of materials.

While convenient, the generated negative voltage is unregulated and if regulation is needed, it’s recommended to add another negative input LDO regulator at the output.

Special Dual-Multitopology Converter

Considering most applications that desire a negative output also require a complementary positive output, ADI has a variety of solutions that use the previously mentioned topologies and provide two or more ± voltages within one IC.

Such examples, as in:

• Dual 42 V_IN, 3 A boost/inverting regulator LT8582;
• Dual 50 V_IN, 2 A multitopology regulator LT8471;
• Dual 5.5 V_IN, 2 A/1.2 A boost/inverting regulator ADP5076;
• 3-channel 60 V isolated micropower management unit ADP1034

Power Module Solutions

For many engineers who desire ultrasmall solution size or an off-shelf fully integrated power solution, a micro power module can be considered.

For example, the LTM4655 is a 40 V_IN, dual 4 A inverting μModule^® regulator, with two fully independent output channels, each configurable for positive or negative output, and is already EN550222 Class B compliant for low EMI performance. It saves a lot of design and troubleshooting work.

The LTM8049 is another good option, with up to 20 V_IN, two outputs with up to +24 V or down to –24 V.

Conclusion

It is not convenient to add a negative voltage rail in the system—IC vendors are offering negative-voltage-free as a solution advantage. For example, the GaNFET manufacturers are convincing customers not to use a negative gate drive, and op amp makers are recommending single-supply op amps with better performance. However, the demand for creating negative voltages still exists in many high end applications.

Table 1 shows a comparison of some solutions mentioned in this article for reference. Since ADI manufactures thousands of applicable ICs of different topologies and different ratings, the recommended limits and general features can be subjective and differ by each part number. If you are reading this as a design engineer, besides running a search on analog.com, please feel free to contact your ADI local representative to consult for the best product that may work for your design.

References

Schnell, Ryan. “Driving a Unipolar Gate Driver in a Bipolar Way.” Analog Dialogue, Vol. 52. No. 10, October 2018.

“Generating Negative Output Voltage from Positive Input Voltage Using MAX17291 Boost Converter IC with Active Discharge Feature.” Analog Devices, Inc.

Dostal, Frederik. “Generating Negative Voltages—Why You Need Level Shifting in Buck-Boost Circuits.” Analog Dialogue, Vol. 57, No. 2, May 2023.

Internal Power Switch Boost Regulators. Analog Devices, Inc.

Keeping, Steven. “Using an Inverting Regulator for Buck/Boost DC-to-DC Voltage Conversion.” DigiKey, August 2015.

Schaeffner, Thomas. “The Best Way to Generate a Negative Voltage for your System.” Newelectronics, January 2018.

Dostal, Frederik. “The Art of Generating Negative Voltages.” Power Systems Design, January 2016.