Datasheet LT1019 (Analog Devices) - 7

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