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LT1019CS8-10 Datasheet(PDF) 7 Page - Linear Technology |
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LT1019CS8-10 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1019 1019fd the thermal regulation specification. Example: a 10V device with a nominal input voltage of 15V and load current of 5mA. Find the effect of an input voltage change of 1V and a load current change of 2mA. ∆P (line change) = (∆VIN)(ILOAD) = (1V)(5mA) = 5mW ∆VOUT = (0.5ppm/mW)(5mW) = 2.5ppm ∆P (load change) = (∆ILOAD)(VIN – VOUT) = (2mA)(5V) = 10mW ∆VOUT = (0.5ppm/mW)(10mW) = 5ppm Even though these effects are small, they should be taken into account in critical applications, especially where input voltage or load current is high. The second thermal effect is overall die temperature change. The magnitude of this change is the product of change in power dissipation times the thermal resistance (θJA) of the IC package ≅ (100°C/W to 150°C/W). The effect on the reference output is calculated by multiplying die temperature change by the temperature drift specifica- tion of the reference. Example: same conditions as above with θJA = 150°C/W and an LT1019 with 20ppm/°C drift specification. ∆P (line change) = 5mW ∆VOUT = (5mW)(150°C/W)(20ppm/°C) = 15ppm ∆P (load change) = 10mW ∆VOUT = (10mW)(150°C/W)(20ppm/°C) = 30ppm These calculations show that thermally induced output voltage variations can easily exceed the electrical effects. In critical applications where shifts in power dissipation are expected, a small clip-on heat sink can significantly improve these effects by reducing overall die temperature change. Alternately, an LT1019A can be used with four times lower TC. If warm-up drift is of concern, these measures will also help. With warm-up drift, total device power dissipation must be considered. In the example given, warm-up drift (worst case) is equal to: Warm-up drift = [(VIN)(IQ) + (VIN – VOUT)(ILOAD)] [(θJA)(TC)] with IQ (quiescent current) = 0.6mA, Warm-up drift = [(15V)(0.6mA) + (5V)(5mA)] [(150°C/W)(25ppm/°C)] = 127.5ppm Note that 74% of the warm-up drift is due to load current times input/output differential. This emphasizes the importance of keeping both these numbers low in critical applications. Note that line regulation is now affected by reference output impedance. R1 should have a wattage rating high enough to withstand full input voltage if output shorts must be tolerated. Even with load currents below 10mA, R1 can be used to reduce power dissipation in the LT1019 for lower warm-up drift, etc. Output Trimming Output voltage trimming on the LT1019 is nominally accomplished with a potentiometer connected from out- put to ground with the wiper tied to the trim pin. The LT1019 was made compatible with existing references, so the trim range is large: + 6%, – 6% for the LT1019-2.5, + 5%, – 13% for the LT1019-5, and + 5%, – 27% for the LT1019-10. This large trim range makes precision trim- ming rather difficult. One solution is to insert resistors in series with both ends of the potentiometer. This has the disadvantage of potentially poor tracking between the fixed resistors and the potentiometer. A second method of reducing trim range is to insert a resistor in series with the wiper of the potentiometer. This works well only for very small trim range because of the mismatch in TCs between the series resistor and the internal thin film resistors. These film resistors can have a TC as high as 500ppm/°C. That same TC is then transferred to the change in output voltage: a 1% shift in output voltage causes a (500ppm)(1%) = 5ppm/°C change in output voltage drift. APPLICATIO S I FOR ATIO |
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