I am still in the process of tweaking the tube hybrid amp and decided to play around with the BUF These little buffers are very popular in the headamp DIY arena, but they can be quite expensive. It appears to be a CMOY type with the buf in the feedback loop. So being curious I decided to etch the amp up.

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It can be used inside the feedback loop of op amps to increase output current, eliminate thermal feedback and improve capacitive load drive.

For low power applications, the BUF operates on 1. Output circuitry is fully protected by internal current limit and thermal shut-down making it rugged and easy to use.

International Airport Industrial Park? Street Address: S. Tucson Blvd. Tel: ? Telex: ? FAX: ? Immediate Product Info: ? The power dissipation of this product will cause some parameters to shift when warmed up. See typical performance curves for over-temperature performance. See Output voltage specifications. See text for recommendations. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party.

Continuous Operating Temperature Burr-Brown recommends that all integrated circuits be handled with appropriate precautions.

Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet published specifications.

For many applications, however, the continuous output current will be limited by thermal effects. Junction temperature is reduced in the DDPAK surface-mount power package because it is soldered directly to the circuit board. The TO package used with a good heat sink further reduces junction temperature, allowing maximum possible output swing. BW V— Signal path indicated in bold. Note: 1 Stage currents are set by I1.

Simplified Circuit Diagram. Figure 2 shows the BUF connected as an open-loop buffer. The source impedance and optional input resistor, RS, influence frequency response—see typical curves.

Power supplies should be bypassed with capacitors connected close to the device pins. Capacitor values as low as 0.

F will assure stable operation in most applications, but high output current and fast output slewing can demand large current transients from the power supplies. Solid tantalum 10? F capacitors are recommended. F V— Optional connection for wide bandwidth — see text. Buffer Connections.

Power dissipated in the BUF will cause the junction temperature to rise. When the thermal protection is activated, the output stage is disabled, allowing the device to cool. Quiescent current is approximately 6mA during thermal shutdown. This can cause the protection circuit to cycle on and off with a period ranging from a fraction of a second to several minutes or more, depending on package type, signal, load and thermal environment.

The thermal protection circuit is designed to prevent damage during abnormal conditions. Any tendency to activate the thermal protection circuit during normal operation is a sign of an inadequate heat sink or excessive power dissipation for the package type. TO package provides the best thermal performance. When the TO is used with a properly sized heat sink, output is not limited by thermal performance. See Application Bulletin AB for details on heat sink calculations. Its mounting tab should be soldered to a circuit board copper area for good heat dissipation.

Figure 3 shows typical thermal resistance from junction to ambient as a function of the copper area. The DIP and SO-8 surface-mount packages are excellent for applications requiring high output current with low average power dissipation.

To achieve the best possible thermal performance with the DIP or SO-8 packages, solder the device directly to a circuit board. Since much of the heat is dissipated by conduction through the package pins, sockets will degrade thermal performance. Use wide circuit board traces on all the device pins, including pins that are not connected. With the DIP package, use traces on both sides of the printed circuit board if possible. With DC signals, power dissipation is equal to the product of output current times the voltage across the conducting output transistor, VS — VO.

Power dissipation can be minimized by using the lowest possible power supply voltage necessary to assure the required output voltage swing. For resistive loads, the maximum power dissipation occurs at a DC output voltage of one-half the power supply voltage. Dissipation with AC signals is lower. Application Bulletin AB explains how to calculate or measure power dissipation with unusual signals and loads.

Any tendency to activate the thermal protection circuit indicates excessive power dissipation or an inadequate heat sink.

To estimate the margin of safety in a complete design, increase the ambient temperature until the thermal protection is triggered. If the output is unable to follow the input within approximately 3V such as with an output short-circuit , the input will conduct increased current from the input source. This is limited by the internal ? If the input source can be damaged by this increase in load current, an additional resistor can be connected in series with the input.

To select this mode, leave the bandwidth control pin open no connection. Bandwidth can be extended to approximately MHz by connecting the bandwidth control pin to V—. This increases the quiescent current to approximately 15mA. Intermediate bandwidths can be set by connecting a resistor in series with the bandwidth control pin—see typical curve "Quiescent Current vs Resistance" for resistor selection. Characteristics of the bandwidth control pin can be seen in the simplified circuit diagram, Figure 1.

The rated output current and slew rate are not affected by the bandwidth control, but the current limit value changes slightly. Output voltage swing is somewhat improved in the wide bandwidth mode.

The increased quiescent current when in wide bandwidth mode produces greater power dissipation during low output current conditions. Phase shift is affected by various operating conditions that may affect stability of the op amp—see typical Gain and Phase curves. Most general-purpose or precision op amps remain unitygain stable with the BUF connected inside the feedback loop as shown. Large capacitive loads may require the BUF to be connected for wide bandwidth for stable operation.

High speed or fast-settling op amps generally require the wide bandwidth mode to remain stable and to assure good dynamic performance. To check for stability with an op amp, look for oscillations or excessive ringing on signal pulses with the intended load and worst case conditions that affect phase response of the buffer.

When operated open-loop, circuit board layout and bypassing technique can affect dynamic performance. For best results, use a ground plane type circuit board layout and bypass the power supplies with 0. F ceramic chip capacitors at the device pins in parallel with solid tantalum 10? F capacitors. Source resistance will affect high-frequency peaking and step response overshoot and ringing. Best response is usually achieved with a series input resistor of 25? Response with some loads especially capacitive can be improved with a resistor of 10?

Use with unity-gain stable high speed op amps. Boosting Op Amp Output Current. Increasing CL may cause excessive ringing or instability. Use Wide BW mode. NOTE: 1 Single, dual, and quad versions. VIN 1? F OPA k? High Performance Headphone Driver.

F 10k? BUF 10k? Pseudo-Ground Driver. Current-Output Valve Driver. Bridge-Connected Motor Driver. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. Samples may or may not be available. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.


Class A headphone amplifier with BUF634



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