The Layout of a Dual Power Supply: Functional Characteristics

The layout of a power supply

As I recall the first major electronic training system I worked on while in the military, my thoughts immediately focus on power supplies and their importance. As my training progressed, the focus on maintaining these power supplies was thoroughly emphasized. The need for power, in this case, was a matter of life or death since the system itself was a Radar Guided Missile system.

The importance of power supplies to the functionality of all electronic devices remains the same. If you doubt that, then forget to charge your smartphone. Without a power supply, electronic devices, regardless of complexity, cease to function. However, power supplies are not a one-size-fits-all scenario. Some electronic devices require a more specific type of power supply to enable proper functionality. One such power supply that fits this unique criterion is a Dual Power supply.

Dual Power Supply Characteristics

The name, dual power supply, can be a little misleading. While it does provide two separate voltages, one positive and one negative, the power supply itself is still a singular source. The difference here, for example, is you have access to both +12 volts and -12 volts simultaneously from a singular source.

This unique power supply configuration can be advantageous and makes certain device functionality possible. The need for dual power supplies is a staple in devices like laptops and televisions.

Many electronic analog circuits require a dual power supply for proper functionality, and this is especially true of operational amplifier circuits. A negative voltage supply is a requirement of various digital systems as well, such as analog-to-digital converters and comparators. However, for these devices, the current demand will be minimal and generating such a small negative power supply as a stand-alone is neither cost-effective nor efficient. This is primarily due to the necessity of significant quantities of integrated-circuits and discrete components to achieve the desired power level.

Dual Power Supply and Operational Amplifiers

Operational amplifiers require dual power supply rails for proper functionality. Their typical range of input signals that produce output voltage can be either negative or positive and therefore require the need to shift. 

A standard signal from an operational amplifier is relative to ground and this is typically between the two power supply rails. Therefore, the positive supply and negative supply is generally symmetrical in reference to ground, but not every time. Without a dual power supply, the output signal from an operational amplifier would clip at the ground potential.

While observing the above connection diagram of an Operational amplifier, it is evident that there is no ground pin or terminal.

Operational amplifiers only require the voltage between the positive rail and the negative rail to work. It is possible to supply its voltage needs with a single power supply that is equal to the sum. However, since these circuits usually necessitate a ground reference, we describe Operational amplifiers as possessing dual power supplies.

The Layout of a Dual Power Supply

Since many of today's devices require lower voltages, we will examine the process of converting a single voltage into a +5 volts and -5 volts DC dual power supply. There are various methods available that afford you the ability to split a single voltage, but they do not provide a constant virtual ground potential.

For example, we could use a pair of batteries to achieve our goal. However, regardless of the effectiveness of our circuit design, inevitably, one battery will deplete more rapidly than the other. This, of course, leads to balancing issues with our dual-polarity voltages.

Virtual Ground, when referring to Operational amplifiers, indicates that the voltage at a particular node is nearly equivalent to the ground voltage or 0 volts. However, there is no physical connection to ground. The described concept is beneficial in the analysis of Operational amplifier circuits, and it simplifies many of the necessary calculations.

Then again, we could also use a resistor potential divider. However, in that scenario, a portion of the power will dissipate in the form of heat, and the dual power supply voltage will suffer from stability issues. Unless, of course, we incorporate CMOS switched-capacitor voltage converters into our design, such as the ICL7660. These monolithic CMOS switched-capacitor voltage converters are capable of doubling, inverting, dividing, and even multiplying a positive input voltage.

For our layout, we will utilize the ICL7660 and ICL7660A to create our dual power supply. Also, in terms of specifications, these monolithic CMOS voltage converters can convert input voltages ranging from +1.5 volts to +10 volts, with an output voltage range from -1.5 volts to -10 volts. Furthermore, both the ICL7660 and the ICL7660A already contain all of the necessary circuitry to complete a negative voltage converter, except for the two required external capacitors.

The above circuit diagram is a representation of the completed ±5-volt dual power supply utilizing the ICL760. Also, with this design, you can obtain the necessary +5-volt input voltage from any USB port, whether it is a wall charger or a computer's USB port.

Other ICL7660 Application Considerations

• You should place the C2 capacitor near the IC2 to inhibit device latch-up. Also, do not apply more than 12 volts for ICL7660A and 10 volts for ICL7660.

• Avoid connecting the LV terminal to ground when utilizing supply voltages over 3.5 volts.

• When utilizing polarized capacitors, you should connect the [+] terminal of C2 to ground, and you should also connect the [+] terminal of C1 to pin 2 of ICL7660 as well as ICL7660A.

• As a recommendation, you should use low-value ESR (Equivalent Series Resistance) capacitors in place of C1 and C2, for best performance. "above diagram"

• You should connect a buffer capacitor across the input supply if the wire length is long between the USB and the circuit.

• There is an output circuit limitation of 40 milliamps for this circuit design. If your dual power supply requirement is higher, you can use the IC MAX660 instead of U1. "above diagram"

With proper care for power supplies, your printed circuit board will be produced. 

The dual power supply is an advantageous circuit and, in some cases, an absolute requirement. Although it can be a simple circuit as a whole, it does not marginalize its importance in the functionality of devices like Operational amplifiers. Dual power supplies are an essential component in many of today's electronic devices. 

When working through the layout of a power supply, use the suite of design and analysis tools from Cadence. Working through the layout of your circuits is easy with Allegro PCB Designer and its circuit analysis tools will ensure they get to production reliably. 

If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts and us.