AD713: Output short circuit specifications

Q I have got some questions about the output short circuit capability of AD713JR-16. In the datasheet (Rev. C) it is not clear said if the outputs are short circuit protected. The absolute maximum ratings say, output short circuit duration for one amplifier is indefinite. 1. Does this mean, there is no time limit, if only one output is shorted to ground? 2. What happens, if all four outputs are shorted to ground at the same time? Is the device destroyed, does it depend on the thermal characteristics of the SOIC package? 3. Is there a thermal overload protection or thermal shutdown? In the specifications the output current at short circuit condition is typical 25mA. 4. Is this the typical value of the short circuit current limit? 5. What is the (aproximate) maximum value of the short circuit current limit? A 1. We only guarantee indefinite short circuit capability if one output is shorted. Permanently shorting 2 or more outputs may damage or destroy the AD713. 2. If all four outputs are shorted to ground , then the device will dissipate power P = 4 x 25mA x Vs = 100mA x Vs = 100mA x 15V = 1.5W. This will cause a temperature rise of around 150 degrees above ambient, if the device has no heat sink attached, which will almost certainly destroy the device at temperatures above 0degC. 3. Figure 8 of the datasheet shows how the short circuit current limit varies with temperature. It automatically reduces the short circuit current limit when the junction temperature rises, as shown. 4. & 5. See figure 3 on the datasheet. At +/-15V supplies and high output impedance (>2k) the voltage swing is more or less constant with load resistance. As one moves from right to left, in the direction of decreasing the load resistance, the curve becomes an exponential shape. This corresponds to a region of CONSTANT CURRENT. If the x axis was linear then it would be a straight line, but since the x-axis is logarithmic, the constant current as R decreases is an exponential. The constant current is around 25mA, but this can vary with temperature, supply voltage and each device will have a different value. It's not possible to guarantee a maximum short circuit current limit, since only a typical is given in the datasheet.

Q I am currently working on a design requirement that needs to create an output of +10V and -10V at up to 20mA continuous (implies a 500ohm load). The device I have been considering for this application is the AD713. TPC3 on the datasheet implies that the AD713 should meet this requirement (500ohm load). However, TPC3 also states that the curve applies for +/-15V supplies, where my application will be working from +/-12V supplies (could go as low as +/-11.6V). How would TPC3 be modified to reflect this? A Operating from +/-11.6V minimum supply, you are going to run into output head room problems even with the output unloaded. Referring to page 2, the output swing from a +/-15V supply with a 2k load is +13V to –12.5V at 25degC and +/-12V over temperature. This implies output head room of between 2 and 3 volts to either supply. Output headroom is fairly constant as the supply voltage is reduced as you can see from TPC 2. Assuming +/-11.6V supply and a 2k load, the output will swing close to +10V but falls short of –10V. If you then factor in a 500ohm load and variations over temperature, you can be fairly confident that the output will not swing +/-10V. Is there any possibility of increasing the supply voltages? I presume you have selected the AD713 for availability in military grade and it’s biFET input (low voltage offset and input bias current). If you really need the output to swing +/-10V from +/-11.6V supply, I would suggest using the AD713 as the input stage and using a second opamp with rail to rail output stage to provide the required output swing. IF you close the feedback loop from the output of the rail to rail back to the input of the AD713, you get a compound amplifier with the input characteristics of the AD713 and a rail to rail output ( by rail to rail output, I simply mean output head room of less than 100mV unloaded).

AD713: Overvoltage at the inputs

Q Has the AD713 internal protective diodes that allows voltages as high as 0.7 V beyond the supplies to be applied at the input of either terminal without causing damage. A In answer to your Question- No the part cannot protect itself against an overvoltage of 0.7volts without external components.Kindly see the following advise: In general we suggest you add external Schottky diodes and current limiting resistor to any input which is likely to exceed the abs max ratings. Note the diodes need to be mounted physically close to the AD713. Ideally, you should design your system to ensure that the AD713 is powered up fully before any analog or digital signals are applied. I've also included a few general notes on overvoltage. Any other semiconductor IC has basic ESD protection diodes which protect the device from possible ESD hits due to handling and production. It is the designers responsibility to provide external protection circuitry if the input is likely to exceed the supplies at any time. These ESD diodes can protect the IC from ESD hits up to about 1.5kV. These ESD protection diodes will act to clamp the voltage at any pin to within 0.5V of the supplies. (So that's the problem solved right? No not quite.) ESD protection diodes can carry quite high currents but only for a short period of time so they can protect the IC from large pulses of short duration (the total energy is still quite low). The maximum DC current which these protection diodes can carry is 10mA. Therefore unless you can guarantee that the current into in pin will me less then 10mA you need some kind of external protection. External protection could be as simple as a series resistor to limit the current into a pin. For example if the maximum overvoltage voltage applied to a pin will be 5V you need to add a 500Ohm series resistor in each line to limit the current to <10mA. The higher you can make this series resistance the better. A high series resistance in a IO line can cause other problems such as slowing the rise and fall time of high speed signals. You might also want to protect against higher overvoltages but you don't want to add any more series resistance, so what can you do? Well, you can do this by adding external Schottky diodes between each input line and the supply lines. A schottky diode will clamp applied voltages to within ~0.3V of the supply so the majority of the current will be diverted via the external diodes (which can carry higher current) and not through the internal ESD protection diodes. There are other protection techniques which include use of spark gaps, large capacitors to earth ground, small choke inductors and more. One the best structures I have seen for protecting against both overvoltage and ESD is a small series resistor followed by Schottky diodes to the supplies followed by another small series resistor. You can see that designing suitable protection circuitry is not a trivial matter. You need to decide how much protection you need, how much abuse you expect the card to be subjected to, how much board space and component cost you can allow, and what test levels you need to meet. Check app notes AN-202 and AN-397 on Analog Devices website for more information on overvoltage protection.

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