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PA10A Datasheet(PDF) 4 Page - Cirrus Logic |
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PA10A Datasheet(HTML) 4 Page - Cirrus Logic |
4 / 5 page 4 PA10U PA10 • PA10A P r o d u c t I n n o v a t i o n F r o m OPERATING CONSIDERATIONS GENERAL Please read Application Note 1 "General Operating Con- siderations" which covers stability, supplies, heat sinking, mounting, current limit, SOA interpretation, and specification interpretation. Visit www.apexmicrotech.com for design tools that help automate tasks such as calculations for stability, internal power dissipation, current limit; heat sink selection; Apex’s complete Application Notes library; Technical Seminar Workbook; and Evaluation Kits. SAFEOPERATINGAREA(SOA) The output stage of most power amplifiers has three distinct limitations: 1. The current handling capability of the transistor geometry and the wire bonds. 2. The second breakdown effect which occurs whenever the simultaneous collector current and collector-emitter voltage exceeds specified limits. 3. The junction temperature of the output transistors. The SOA curves combine the effect of these limits. For a given application, the direction and magnitude of the output current should be calculated or measured and checked against the SOA curves. This is simple for resistive loads but more complex for reactive and EMF generating loads. 1. For DC outputs, especially those resulting from fault condi- tions, check worst case stress levels against the new SOA graph. For sine wave outputs, use Power Design1 to plot a load line. Make sure the load line does not cross the 0.5ms limit and that excursions beyond any other second breakdown line do not exceed the time label, and have a duty cycle of no more than 10%. 1 Note 1. Power Design is a self-extracting Excel spread- sheet available free from www.apexmicrotech.com For other waveform outputs, manual load line plotting is recommended. Applications Note 22, SOA AND LOAD LINES, will be helpful. A Spice type analysis can be very useful in that a hardware setup often calls for instruments or amplifiers with wide common mode rejection ranges. 2. The amplifier can handle any EMF generating or reactive load and short circuits to the supply rail or shorts to com- mon if the current limits are set as follows at T C = 85°C: SHORT TO ±V S SHORT TO ±V S C, L, OR EMF LOAD COMMON 50V .21A .61A 40V .3A .87A 35V .36A 1.0A 30V .46A 1.4A 25V .61A 1.7A 20V .87A 2.2A 15V 1.4A 2.9A CURRENTLIMITING Refer to Application Note 9, "Current Limiting", for details of both fixed and foldover current limit operation. Visit the Apex web site at www.apexmicrotech.com for a copy of the Power Design spreadsheet (Excel) which plots current limits vs. steady state SOA. Beware that current limit should be thought of as a +/–20% function initially and varies about 2:1 over the range of –55°C to 125°C. For fixed current limit, leave pin 7 open and use equations 1 and 2. R CL = 0.65/LCL (1) I CL = 0.65/RCL (2) Where: I CL is the current limit in amperes. R CL is the current limit resistor in ohms. For certain applications, foldover current limit adds a slope to the current limit which allows more power to be delivered to the load without violating the SOA. For maximum foldover slope, ground pin 7 and use equations 3 and 4. 0.65 + (Vo * 0.014) I CL = (3) R CL 0.65 + (Vo * 0.014) R CL = (4) I CL Where: Vo is the output voltage in volts. Most designers start with either equation 1 to set R CL for the desired current at 0v out, or with equation 4 to set R CL at the maximum output voltage. Equation 3 should then be used to plot the resulting foldover limits on the SOA graph. If equa- tion 3 results in a negative current limit, foldover slope must be reduced. This can happen when the output voltage is the opposite polarity of the supply conducting the current. In applications where a reduced foldover slope is desired, this can be achieved by adding a resistor (R FO) between pin 7 and ground. Use equations 4 and 5 with this new resistor in the circuit. 0.65 + Vo * 0.14 10.14 + R FO I CL = (5) R CL 0.65 + Vo * 0.14 10.14 + R FO R CL = (6) I CL Where: R FO is in K ohms. |
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