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LT1185M Datasheet(PDF) 8 Page - Linear Technology |
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LT1185M Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1185 APPLICATIO S I FOR ATIO factor is that local ambient temperature may be somewhat higher because of the point source of heat. The conse- quences of excess junction temperature include poor reliability, especially for plastic packages, and the possi- bility of thermal shutdown or degraded electrical charac- teristics. The final design should be checked in situ with a thermocouple attached to the regulator case under worst- case conditions of high ambient, high input voltage and full load. What About Overloads? IC regulators with thermal shutdown, like the LT1185, allow heat sink designs which concentrate on worst-case “normal” conditions and ignore “fault” conditions. An output overload or short may force the regulator to exceed its maximum junction temperature rating, but thermal shutdown is designed to prevent regulator failure under these conditions. A word of caution however; thermal shutdown temperatures are typically 175 °C in the control portion of the die and 180 °C to 225°C in the power transistor section. Extended operation at these tempera- tures can cause permanent degradation of plastic encap- sulation. Designs which may be subjected to extended periods of overload should either use the hermetic TO-3 package or increase heat sink size. Foldback current limiting can be implemented to minimize power levels under fault conditions. External Current Limit The LT1185 requires a resistor to set current limit. The value of this resistor is 15k divided by the desired current limit (in amps). The resistor for 2A current limit would be 15k/2A = 7.5k. Tolerance over temperature is ±10%, so current limit is normally set 15% above maximum load current. Foldback limiting can be employed if short-circuit current must be lower than full load current (see Typical Applications). The LT1185 has internal current limiting which will over- ride external current limit if power in the pass transistor is excessive. The internal limit is ≈3.6A with a foldback characteristic which is dependent on input-output voltage, not output voltage per se (see Typical Performace Charac- teristics). Ground Pin Current Ground pin current for the LT1185 is approximately 2mA plus IOUT/40. At IOUT = 3A, ground pin current is typically 2mA + 3/40 = 77mA. Worst case guarantees on the ratio of IOUT to ground pin current are contained in the Electrical Specifications. Ground pin current can be important for two reasons. It adds to power dissipation in the regulator and it can affect load/line regulation if a long line is run from the ground pin to load ground. The additional power dissipation is found by multiplying ground pin current by input voltage. In a typical example, with VIN = 8V, VOUT = 5V and IOUT = 2A, the LT1185 will dissipate (8V – 5V)(2A) = 6W in the pass transistor and (2A/40)(8V) = 0.4W in the internal drive circuitry. This is only a 1.5% efficiency loss, and a 6.7% increase in regulator power dissipation, but these values will increase at higher output voltages. Ground pin current can affect regulation as shown in Figure 2. Parasitic resistance in the ground pin lead will create a voltage drop which increases output voltage as load current is increased. Similarly, output voltage can decrease as input voltage increases because the “IOUT/40” component of ground pin current drops significantly at higher input-output differentials. These effects are small enough to be ignored for local regulation applications, but for remote sense applications, they may need to be consid- ered. Ground lead resistance of 0.4 Ω would cause an output voltage error of up to (3A/40)(0.4 Ω) = 30mV, or 0.6% at VOUT = 5V. Note that if the sense leads are connected as shown in Figure 2, with ra ≈ 0Ω, this error is a fixed number of millivolts, and does not increase as a function of DC output voltage. |
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