AnalogTalk

As computer speed increases, supply voltage goes down, and supply current goes up, designers have many design challenges for today’s, high current, and fast transient response processors. In less than two years, the peak output current has risen from ~14A to ~120A. As processor DC RMS and peak current requirements rise, the need for more phases increases. Typically 15 to 20 A must be handled per phase. Designers are required to design DC-DC converters in about the same amount of space as previous designs. DC/DC step-down voltage conversion is almost exclusively based on the synchronous buck topology. The multi-phase buck topology has inherent advantages over the single-phase buck topology and is used in applications where lower supply voltages and greater load-current are required. Multiphase converters distribute the total current across phase-shifted PWM channels, output MOSFETs, and inductors. Multiphase spread heat and lower stress on components.  Multiphase also operates at higher frequencies that allow the use of miniature passives reducing PCB area and cost. Reduced input and output ripple current is also achieved by phase shifting the PWM channels. This reduces the ripple RMS current requirement on input capacitors and output voltage ripple. Ceramic capacitors can be used for output filtering thus reducing the output inductance allowing for faster transient response .

With processor demands approaching 100A at low voltages (~1V), the need for using 3, and more phases is necessary.  Multiphase PCB interleaved layouts are used for PC and server applications where high output current and fast transient response is important. Interleaving multi-layer VCC and ground planes layers will minimize trace inductance. Two copper traces one inch long with a spacing of 20 mils has a total trace inductance of approximately 640 pH. Four interleaved layers have a trace inductance of approximately 210 pH. Interleaving can drastically reduce trace inductance and increase transient response performance. Not using interleaving and using a single layer will result in a trace inductance of approximately 10 nH. This approach may also be used in lower current applications where transient and thermal performance is important. Single-Phase Synchronous Buck converter has the efficiency versus switching frequency trade-off. The multi-phase interleaved buck topology can solve this problem.

The MCP1640 regulator’s operating voltage of down to 0.35V and start-up voltage of 0.65V allows use with even a single, completely drained Alkaline, NiMH or NiCd battery cell. A PWM/PFM option enables the device’s low quiescent and shutdown currents, and provides up to 96% efficiency, allowing for longer battery run times. The regulator’s two integrated FET transistors reduce component count, resulting in smaller overall designs.

The key here is the low operating voltages: 0.35V operating and 0.65V start up.  This means you can get more from a battery cell as it’s drained.   The MCP1640 can boost low voltages to keep the product running where previous designs had to accept that a cell was no good when ~0.9V was reached.

Other Info:

Quiescent current as low as 19 uA and output currents up to 350 mA, the 500 kHz MCP1640 regulator enables compact, longer-lasting battery applications in the consumer electronics market (e.g., electric razors, toothbrushes, GPS devices and portable music players), among others.

Related Links

MCP1640

Today’s blog lists common problems associated with using an op amp with a power supply and an input signal on a PC Board. It is divided into four categories: General Suggestions, Input State Problems, Bandwidth Issues, and Single Supply Pitfalls. We would like to hear from you, if you have any other inputs from experience.

In General

1. Be careful of the supply pins. Don’t make them too high per the amplifier specification sheet and don’t make them too low. High supplies will damage the part. In contrast, low supplies won’t bias the internal transistors and the amplifier won’t work or it may not operate properly.

2. Make sure the negative supply (usually ground) is in fact tied to a low impedance potential. Additionally, make sure the positive supply is the voltage you expect when it is referenced to the negative supply pin of the op amp. Placing a volt meter across the negative and positive supply pins will verify that you have the right relationship between the pins.

3. Ground can’t be trusted, especially in digital circuits. Plan your grounding scheme carefully. If the circuit has a lot of digital circuitry, consider separate ground and power planes. It is very difficult, if not impossible, to remove digital switching noise from an analog signal.

4. Decouple the amplifier power supplies with by-pass capacitors as close to the amplifier as possible. For CMOS amplifiers, a 0.1ìF capacitor is usually recommended. Also decouple the power supply with a 10ìF capacitor.

5. Use short lead lengths to the inputs of the amplifier. If you have a tendency to use the white perf. boards for prototyping, be aware that they can cause noise and oscillation. There is a good chance that these problems won’t be a problem with the PCB implementation of the circuit.

6. Amplifiers are static sensitive! If they are damaged, they may fail immediately or exhibit a soft error (like offset voltage or input bias current changes) that will get worse over time.

Input Stage Problems

1. Know what input range is required from your amplifier. If either inputs of the amplifier go beyond the specified input range, the output will typically be driven to one of the power supply rails.

2. If you have a high gain circuit, be aware of the offset voltage of the amplifier. That offset is gained with the rest of your signal and it might dominate the results at the output of the amplifier.

3. Don’t use rail-to-rail input stage amplifiers unless it is necessary. By the way, they are only needed when a buffer amplifier circuit is used or possibly an instrumentation amplifier configuration. Any circuit with gain will drive the output of the amplifier into the rail before the input has a problem.

Do You Have the Bandwidth?

1. Account for the bandwidth of the amplifier when sending signals through the circuit. You may have designed an amplifier for a gain of 10 and find that the AC output signal is much lower than expected. If this is the case, you may have to look for an amplifier with a wider bandwidth.

2. Instability problems can usually be solved by adding a capacitor in parallel with the feedback resistor around the amplifier. This does mean typically and not always. If an amplifier circuit is unstable, a quick stability analysis will show the problem and probably the solution.

Single Supply Rail-to-Rail

1. Operational Amplifier output drivers are capable of driving a limited amount of current to the load.

2. Capacitive loading an amplifier is risky business. Make sure the amplifier is specified to handle any loads that you may have.

3. It is very rare that a single supply amplifier will truly swing rail-to-rail. In reality, the output of most of these amplifiers can only come within 50 to 200mV from each rail. Check the product data sheets of your amplifier.

Related Links

Using Single Supply Operational Amplifiers in Embedded Systems