[Design View / Design Solution]
Powering The Signal Path
Using an integrated flyback IC along with post filtering will deliver a high-performance split-rail supply.
Power delivered to sensitive analog circuitry must be treated differently than power for digital circuitry. All circuits are affected by noise delivered through the power supply, yet analog loads tend to be more sensitive. The actual type of circuitry and application will determine the tolerable noise limits. Powering digital circuitry today is a fairly straightforward task and can be handled with available power design tools such as National Semiconductor’s WEBENCH tools (www.national.com/webench).
Sensitive analog designs such as those found within medical and video systems often require lower-noise power sources. System designers must pay special attention to ensure that the power-supply rails don’t contain noise above amplitude limits within specific areas of concern. Noise may need to be limited to specific amplitudes over frequency ranges that the system is dealing with (or harmonics of these frequencies).
This article will describe the issues that emerge when designing the power section of an analog system. Focus will be on generating power-supply rails for analog systems and discuss some specific power-supply options for these systems. Another topic is the noise associated with common power circuits and methods of reducing noise. Further, the article will offer tips on good practice when it comes to measuring power-supply noise.
Split-Rail Powering of Analog Circuits When biasing sensitive analog circuitry, designers often turn to split-rail power circuits to get optimal performance. Analog-system designers utilize differential power rails centered around a system ground to maintain a low-impedance analog reference. Creating an analog reference above system ground (often called a virtual ground) can result in subsequent signal-distortion problems.
A virtual ground can be created with resistors or an active circuit that sets a reference voltage for analog operations (amplifiers, integrators, comparators, etc.). When employing a virtual ground, a designer must deal with the fact that it will vary with ground currents passing through this node (which will always be the case).
As ground currents vary, the virtual ground will also vary by the product of the ground current and the impedance of the node, resulting in unwanted distortion in the signal path. When a true ground is used for the analog ground, this distortion doesn’t exist because the impedance of the ground itself is 0 (or close to it). Therefore, currents in/out of the node won’t cause any voltage change.
Another advantage of designing with a true ground reference is the ability to drive analog signals biased above and below earth ground. This provides clear advantages when it comes to driving differential signals between different earth ground potentials, as seen when driving signals between systems on different power grids.
No matter what tolerances and differences exist between the driving circuit and receiving circuit, the receiver knows where the analog signal can be found. Also, the analog signal biased around earth ground can expect lower leakage currents as compared to a signal that’s always at a potential above (or below) earth ground. Leakage current can result in higher distortion and attenuation of the analog signal being carried. True analog grounds also reduce the risk of signal problems during power up or power down of a circuit. Audio designs often need to deal with clicks and pops generated during powering up or down a circuit when using a virtual ground.
Isolated Power Different physical locations are often at different ground potentials. When power and/or a signal (analog or digital) is connected to a remote location, a designer must be aware that the ground differences between locations could cause system-related problems. Grounds connected together across cables may create unwanted ground currents that increase noise, negatively affecting system performance.
Designers often need to isolate analog signals using coupling capacitors or transformers. For dc signals, this may require special signal coding to ensure spectral density for proper energy transfer. Isolating a power supply makes it possible to float an entire circuit, thus eliminating potential noise issues. In addition, floating the power supply and associated circuitry can create better noise immunity, especially when it comes to induced noise such as ESD or EMI. The circuit discussed later in this article provides the option of isolating the ground and analog rails being generated.
Power-Supply Noise Noise can enter an analog system from many paths, including the power supply itself. Though a properly designed circuit will reject certain amounts of power-supply noise (often specified at an IC level as power-supply rejection ratio, or PSRR), the lower the power-supply noise, the lower the signal distortion caused by such noise.
Ideally a power supply provides a pure dc signal, yet this is unrealistic. All circuits generate noise and regulators generate many types, including thermal, shot, and in the case of switching regulators, switching noise.
In general, from a noise perspective, linear voltage regulators are desirable over switching regulators when powering sensitive analog circuits. Yet, because of the power-efficiency advantage, system designers are being forced to employ switching power supplies and power-supply subsystems (dc-dc converters). The move to switching power in sensitive analog systems will continue to increase as designers are forced to be more conscious of power-efficient design. Today, IC and system designers try to get the most functionality for the lowest amount of power (see www.national.com/powerwise for more information on power-efficient designs).
Switching-regulator noise is generated from multiple sources, with the highest contributions related to the “switching” of current flow. Switching of the current flow transfers energy between passive storage elements (inductors and capacitors) utilizing diodes and transistors to perform the switching. The devices charge during some cycles and discharge during others, resulting in ripple currents and ripple voltages. Switching ripple on power-supply output voltages may be intolerable when biasing analog circuitry (yet often acceptable with digital circuits).
When current flowing through switches (FETs, diodes, etc.) changes direction, as with a switching regulator, the current may instantaneously flow into a high impedance, resulting in high transient voltages. These transient voltages can be somewhat controlled with circuit design (often called snubbers), yet these transitions always result in unwanted conducted and/or coupled noise.
Later we will discuss switching noise (ripple and transient noise), methods of filtering noise, and methods of measuring noise.
Frequency Locking The ability to establish a fixed frequency in a switching regulator(s) provides the designer with the ability to move the switching noise to a frequency that doesn’t cause unwanted interference. Locking a regulator’s oscillator to a fixed (or variable) frequency can provide significant performance advantages in analog systems.
System designers can lock the regulator frequency to an available clock or to a software-controlled counter/timer and, as needed, employ software to dynamically move the switching frequency. As an example, this technique has been utilized in radio designs to ensure the regulator doesn’t cause interference while adjusting the radio frequency.
Negative Rail Options Numerous approaches exist to create a split-rail power supply, all which have tradeoffs. Design techniques include switch capacitor, buck-boost, Cuk, and flyback architectures.
Switch-cap converters: Switched-capacitor regulation is often used to create a negative rail from an existing positive rail. The basic concept is to charge a capacitor followed by reversing the capacitor’s polarity and connecting it to a second output capacitor. The faster the switching takes place, the lower the switching ripple (noise). The higher the load, the higher the ripple for a fixed-frequency and capacitor value, resulting in fairly large capacitors required for high-output currents.
For low-power analog rail generation, a switched-capacitor approach may be found acceptable. For sensitive analog designs, switched-cap approaches may carry more noise than can be tolerated and/or there may be limitations on the amount of deliverable current.
Buck-boost: A buck-boost converter is a switching topology that creates a negative rail from a positive voltage. This type of converter simply steers current through an inductor, followed by steering the inductor current backwards into an output capacitor (Fig. 1).
Using a buck regulator for a positive rail along with a separate buck-boost converter can create a positive and negative split-rail system. This approach requires two separate integrated circuits and two inductors. Both regulators can be synchronized if at least one of the switching controllers provide an external sync input, yet the inherent switching noise should be further filtered when powering most analog loads.
Such an approach is fairly straightforward, yet can be more costly and have higher noise than other approaches. It also doesn’t offer the option of isolating the input from the output.
Cuk: A Cuk regulator architecture utilizes two inductor elements to create the negative rail (Fig. 2). The Cuk converter inverts and can step up or step down the input voltage. The design uses two stages coupled by a storage capacitor. The first inductor stage acts like a switch-mode boost regulator, while the second inductor stage acts like an inverted buck regulator where the inversion of the current flow takes place by steering current on both sides of a storage capacitor (CCUK).
By using inductors on both the input and output, the Cuk converter produces less input and output current ripple compared to other inverting topologies, such as the buck-boost and flyback architectures. The lower noise of the Cuk converter may still be higher than can be tolerated for sensitive analog designs, resulting in the need for additional output filtering.
The Cuk converter’s operating states are shown in (Figure 2. During the first cycle, the transistor switch is closed and the diode is open. L1 is charged by the source and L2 is charged by CCUK, while the output current is provided by L2. In the second cycle, L1 charges CCUK and L2 discharges through the load. By applying the volt-second balance to either of the inductors, the relationship of VOUT to the duty cycle (D) is found to be:
VOUT = –(VIN x D)/(1 – D)
While the Cuk regulator is attractive for creating a negative rail for some analog designs, it does require two inductors and does not provide the ability to fully isolate the output from the input.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
