Power Hotspot

Video systems operating from single +5-V supplies use ac-coupled RGB and composite video signals that operate only within the useful dynamic range of the single-supply amplifiers in the signal chain. For example, if a single +5-V supply rail-to-rail amplifier has a useful dynamic range of down to a few millivolts of ground and up a few millivolts of +5 V, the incoming video signal plus sync must be constrained to that voltage range. Because the NTSC standard composite video...

Most small-size color LCD displays today use white LEDs for backlighting. These systems usually involve handheld devices with an LED drive circuit powered by a battery whose output voltage varies over time. Therefore, an optimum LED driver design requires a system approach covering: Battery type LCD characteristics System power requirements and efficiency LED driver IC and its external components PCB layout...

Switch-mode power supplies (SMPSs) traditionally are implemented using a basic analog control loop. But recent advances in digital signal controllers (DSCs) enable designs that begin to make fully digital control schemes practical and economical. Still, early adoption of this technology is expected to be in high-end applications, where the benefits of full digital control are the most immediate. Yet many analog power...

Many microcontrollers feature a pulse-width-modulated (PWM) output that can be low-pass filtered to produce a variable dc voltage. Without additional circuitry, however, this technique is limited to controlling very low-power loads. The circuit here illustrates a scheme that lets this dc voltage control a high-power load, such as a motor, actuator, or heating element (see the figure). Furthermore, the load voltage may be higher...

A switching power supply's main output usually will have a load regulation of better than 0.1%. However, what if it doesn't need regulation that tight on the main output, but instead needs better regulation on a second output? The averaging technique described here eliminates the need for adding linear regulators to achieve better regulation on the second output, reducing cost and parts count. Figure 1 shows a typical switching regulator....

For applications requiring only a few milliamps of supply current, an RS-232 transceiver IC (MAX202) and a few external components can generate bipolar supply rails up to ±30 V. Such capability is useful ±15-V op-amp circuits that supply (for example) meter drive, LCD bias, and gasdetector bias. In the application for which it was designed, the internal charge pump of this IC generates ±10 V to drive an RS-232 line. ...

Most common dc-dc power application circuits are designed for a positive buck or a boost circuit. However, certain designs may require a negative buck or boost topology. The problem usually arises because most board designers aren't accustomed to thinking in terms of negative voltages. The transfer function for a positive buck is VOUT/VIN = D, and interestingly, the transfer function for a negative buck is –VOUT/ ...

BUDGE ING, Maxim Integrated Products Inc., Sunnyvale, Calif. Budge_Ing@maximhq.com Because digital systems often have only a single-polarity power supply, a common problem becomes translating an opposite-polarity pulse train into a positive-or negative-pulse output. Here are some simple circuits that can do the job easily and reliably. For positive-supply systems, the circuit of ...

In automotive applications, more and more on-board modules need to meet stringent requirements for ignition-off current, with many modules requiring less than 100 mA of current. A controller-area network (CAN) interface IC and a microprocessor in sleep mode can easily consume half of this current, leaving little operating leeway for the power supply. On top of this, the module's power supply must withstand automotive battery voltages of 24 V ...

Lots of modern monitors feature an indication of incoming video signals, even if the monitors are switched off. In most cases, an LED provides this indication by fading (slowly increasing and decreasing) its light intensity. This type of indication is very specific, and thus convenient, for use in other devices for the same purpose. Monitors typically stay connected to the power line, even when they're switched off. However, this doesn't apply for all video devices. Some may need...

The simplest link from a main supply and backup battery to a load is the diode-OR connection. But a diode-OR doesn't work when the backup battery's voltage exceeds the main supply voltage. The circuit in Figure 1 handles that condition. Its main switch-mode supply voltage ranges from 7 to 30 V, and the backup supply is a 9-V battery. IC1 is an ultra-low-power device that includes a comparator and a 1.182-V bandgap reference. During normal operation, the comparator output is low,...

For analog IC design engineers, using matched components in circuits is one of the keys to creating highly accurate circuits. I have worked on many types of power-management chips, and a small, yet very important, part of them would be the matched feedback resistors that contribute to output voltage accuracy. To optimize the matching of resistors on an IC, you would typically use multiple segments of the exact same size resistors. An advantage to this approach is that process...

The R-C twin-tee passive circuit supplies band-reject (notch) filtering to portable applications. It has a circuit Q (loaded) of 0.25. Satisfactory rejection can be achieved when the bridge is balanced (close tolerances of adjustable components) and the capacitor unloaded Qs (a function of capacitor ESR) are large compared to the circuit Q. A schematic of a symmetrical twin-tee circuit is shown in the figure. The design equations are as follows:...

Communication systems often require large ac output coupling to remove dc voltage on the transmission line and to isolate ground connections between transmit and receive systems. Generally, a feedback network is used to minimize the output capacitance. Yet once the feedback's tradeoffs are understood, the circuit can be reorganized and improved. The final feedback network presented here yields a more compact solution with equivalent performance. The traditional feedback network...

Many applications require a minimum number of conductors in the link between two areas of a device—for example, a flip-style cell phone. While a Mobile Pixel Link (MPL) device offers a convenient way to transmit high-speed RGB video data across three wires, the LCD display may still need a synchronous serial interface for configuration and control. In addition, the backlight can consist of multiple LEDs that must be driven with a higher voltage, and the voltage...

A friend bought an electric jacket to wear on his motorcycle. But it was on all the time when it was plugged in, which made him sweaty at certain temperature ranges. He was looking for a way to control the heater inside the jacket. He tried the simple route and put a potentiometer in-line with the jacket. But he found that it got so hot, it began to smoke and melted his gloves. (The heater circuit draws upwards of 5 to 10 A.) To solve the problem, I designed ...

The resistive voltage divider—a basic circuit taught in every introductory electronics course—may cause some problems during its implementation. If you've ever stood in front of a resistor kit while punching values into your calculator, this article is for you. Typical kits don't include every value. So, finding an elusive ratio for which both values are commonly available can be a challenge. The small spreadsheet described below not only provides a...

Designers frequently use special circuits to reduce the power required to hold a relay in the latched position. Analog circuits such as the "brute force" resistor-capacitor solution or the time-variable current sink tend to be very bulky or waste energy in the pass-transistor. The simple and small digital circuit shown in the figure energizes a relay coil at full power for the time required for reliable latch. It then automatically reduces the coil current by 50% to maintain the...

The circuit of Figure 1 provides a "diode-OR" function for applications that must switch automatically between the main and backup supply voltages. Such applications include battery-backed memory supplies and any battery-operated device with a wall-adapter connection. For example, a battery-backed SRAM circuit (nonvolatile memory module) requires at least two power sources: a high-current active path for the SRAM memory...

Illuminating more than one very bright white LED requires a choice of configuration, either series or parallel. Of course, each configuration has design tradeoffs. A parallel connection requires a lower voltage across each LED, yet ballast resistors or current sources are needed to accomplish matched light uniformity. Different levels of bias current, and thus light coming from each LED, create a disrupting light source. However, light matching with ballast resistors or current sources...