| ??? 03/03/07 02:22 Modified: 03/03/07 02:33 Read: times |
#134179 - Datasheet is a bit incomplete Responding to: ???'s previous message |
Suresh said:
But if my load is resistive and its value is 150 Ohms as shown below, then refering to figure 9, could i say that the drop accross the load would be 5V. Is it correct? Yes. But please don't try to use this circuitry to decrease the output voltage to a certain wished level. That's no good idea for some reasons: 1. 5V across 150Ohm means a current of 33mA, which is huge in an OPamp circuit. 2. 33mA causes a lot of heat, namely 33mA^2 x 150Ohm = 160mW at the 150R resistor and within the OPamp. 3. The data of figure 9 only shows typical values. So, what causes an output voltage of 5V with the one OPamp can cause 2V or 7V with an other. 4. This current limiting is extremely temperature sensitive. So, the fabricated output voltage of 5V will drift all the time. Suresh said:
and i am also interested to know about the maximum source and sink current rating of TL072 since iam unable to find using figure 9. This data isn't specified for the TL072. The reason might be, that the current limiting circuitry is different from that of the LF412. But you can use figure 9 to estimate the expected currents:  At about 600R load maximum output voltage is 10V, yielding a maximum output current of about 17mA at 10V output voltage. At about 250R load maximum output voltage is 7.5V, yielding a maximum output current of about 30mA at 7.5V output voltage. At about 150R load maximum output voltage is 5V, yielding a maximum output current of about 33mA at 5V output voltage. At about 110R load maximum output voltage is 2.5V, yielding a maximum output current of about 23mA at 2.5V output voltage. So, you see, the maximum output current isn't a constant value like with the LF412, but depends somewhat on the output voltage. The maximum short circuit current of TL072 (at 0V output voltage) is about 22mA. This isn't specified anywhere in the datasheet, but is a value I know from experience, since I worked a lot with the TL072 in the past. To find a suited R2 for your LED driver when using the TL072, you can take figure 9 to make an estimation: 10mA is flowing at an output voltage of roughly 11.5V. This means an internal voltage drop of 15V - 11.5V = 3.5V. With a +-10V supply the output voltage at 10mA is about 6.5V, then. So, R2 = (6.5V - Uled) / 10mA = 470Ohm is a good start. Take note, that this is only a rough estimation, not a calculation. It's the best you can draw from an incomplete datasheet. If I were to design such a LED driver, I wouldn't use this scheme, just because the output voltage isn't specified and can drastically differ and drift. I would use an additional transistor stage to turn-on the LED. Kai |
