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BUF634U-2K5 Datasheet(PDF) 7 Page - Burr-Brown (TI) |
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BUF634U-2K5 Datasheet(HTML) 7 Page - Burr-Brown (TI) |
7 / 20 page ® BUF634 7 APPLICATION INFORMATION Figure 1 is a simplified circuit diagram of the BUF634 showing its open-loop complementary follower design. FIGURE 2. Buffer Connections. V– 10µF 10µF V O BUF634 Optional connection for wide bandwidth — see text. R S 3 V+ 6 R L 4 7 1 V IN DIP/SO-8 Pinout shown OUTPUT CURRENT The BUF634 can deliver up to ±250mA continuous output current. Internal circuitry limits output current to approxi- mately ±350mA—see typical performance curve “Short Circuit Current vs Temperature”. For many applications, however, the continuous output current will be limited by thermal effects. The output voltage swing capability varies with junction temperature and output current—see typical curves “Output Voltage Swing vs Output Current.” Although all four pack- age types are tested for the same output performance using a high speed test, the higher junction temperatures with the DIP and SO-8 package types will often provide less output voltage swing. Junction temperature is reduced in the DDPAK surface-mount power package because it is soldered directly to the circuit board. The TO-220 package used with a good heat sink further reduces junction temperature, allowing maximum possible output swing. THERMAL PROTECTION Power dissipated in the BUF634 will cause the junction temperature to rise. A thermal protection circuit in the BUF634 will disable the output when the junction tempera- ture reaches approximately 175 °C. When the thermal pro- tection is activated, the output stage is disabled, allowing the device to cool. Quiescent current is approximately 6mA during thermal shutdown. When the junction temperature cools to approximately 165 °C the output circuitry is again enabled. 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-220 package provides the best thermal performance. When the TO-220 is used with a properly sized heat sink, output is not limited by thermal performance. See Applica- tion Bulletin AB-037 for details on heat sink calculations. The DDPAK also has excellent thermal characteristics. 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 mounting tab of the TO-220 and DDPAK packages is electrically connected to the V– power supply. 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. Figure 2 shows the BUF634 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 0.1 µF 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 µF capacitors are recommended. High frequency open-loop applications may benefit from special bypassing and layout considerations—see “High Frequency Applications” at end of applications discussion. FIGURE 1. Simplified Circuit Diagram. 200 Ω I 1 (1) V+ V O BW V– 150 Ω 4k Ω Signal path indicated in bold. Note: (1) Stage currents are set by I 1. Thermal Shutdown V IN |
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