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4 / 5 page PA12 • PA12A 4 PA12U load and short circuits to the supply rail or common if the current limits are set as follows at T C = 25°C: SHORT TO ±V S SHORT TO ±V S C, L, OR EMF LOAD COMMON 50V .30A 2.4A 40V .58A 2.9A 35V .87A 3.7A 30V 1.5A 4.1A 25V 2.4A 4.9A 20V 2.9A 6.3A 15V 4.2A 8.0A These simplified limits may be exceeded with further analysis using the operating conditions for a specific application. CURRENTLIMITING Refer to Application Note 9, "Current Limiting", for details of both fixed and foldover current limit operation. Visit the Apex Microtechnology web site at www.apexanalog.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. Vo * 0.14 0.65 + 10.14 + R FO I CL = (5) R CL Vo * 0.14 0.65 + 10.14 + R FO R CL = (6) I CL Where: R FO is in K ohms. 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.apexanalog.com for design tools that help automate tasks such as calculations for stability, internal power dissipation, current limit; heat sink selection; Apex Mi- crotechnology’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 all limits for this Power Op Amp. 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. However, the following guidelines may save extensive analytical efforts. 1. Capacitive and dynamic* inductive loads up to the following maximum are safe with the current limits set as specified. CAPACITIVE LOAD INDUCTIVE LOAD ±V S I LIM = 5A I LIM = 10A I LIM = 5A I LIM = 10A 50V 200µF 125µF 5mH 2.0mH 40V 500µF 350µF 15mH 3.0mH 35V 2.0mF 850µF 50mH 5.0mH 30V 7.0mF 2.5mF 150mH 10mH 25V 25mF 10mF 500mH 20mH 20V 60mF 20mF 1,000mH 30mH 15V 150mF 60mF 2,500mH 50mH *If the inductive load is driven near steady state conditions, allowing the output voltage to drop more than 8V below the supply rail with I LIM = 15A or 25V below the supply rail with ILIM = 5A while the amplifier is current limiting, the inductor must be capacitively coupled or the current limit must be lowered to meet SOA criteria. 2. The amplifier can handle any EMF generating or reactive T C = 25°C T C = 125°C T C = 85°C THERMAL SECOND BREAKDOWN SOA 20 50 70 100 40 30 10 SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, V S - VO (V) 15 .6 .4 10 6.0 4.0 2.0 3.0 .3 1.0 |
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