Datasheet LT1185 (Analog Devices) - 8

LT1185
U U W U APPLICATIO S I FOR ATIO
factor is that local ambient temperature may be somewhat
Ground Pin Current
higher because of the point source of heat. The conse- Ground pin current for the LT1185 is approximately 2mA quences of excess junction temperature include poor plus I reliability, especially for plastic packages, and the possi- OUT/40. At IOUT = 3A, ground pin current is typically 2mA + 3/40 = 77mA. Worst case guarantees on the ratio of bility of thermal shutdown or degraded electrical charac- I teristics. The final design should be checked in situ with a OUT to ground pin current are contained in the Electrical Specifications. thermocouple attached to the regulator case under worst- case conditions of high ambient, high input voltage and Ground pin current can be important for two reasons. It full load. adds to power dissipation in the regulator and it can affect load/line regulation if a long line is run from the ground pin
What About Overloads?
to load ground. The additional power dissipation is found by multiplying ground pin current by input voltage. In a IC regulators with thermal shutdown, like the LT1185, typical example, with V allow heat sink designs which concentrate on worst-case IN = 8V, VOUT = 5V and IOUT = 2A, the LT1185 will dissipate (8V – 5V)(2A) = 6W in the pass “normal” conditions and ignore “fault” conditions. An transistor and (2A/40)(8V) = 0.4W in the internal drive output overload or short may force the regulator to exceed circuitry. This is only a 1.5% efficiency loss, and a 6.7% its maximum junction temperature rating, but thermal increase in regulator power dissipation, but these values shutdown is designed to prevent regulator failure under will increase at higher output voltages. these conditions. A word of caution however; thermal shutdown temperatures are typically 175°C in the control Ground pin current can affect regulation as shown in portion of the die and 180°C to 225°C in the power Figure 2. Parasitic resistance in the ground pin lead will transistor section. Extended operation at these tempera- create a voltage drop which increases output voltage as tures can cause permanent degradation of plastic encap- load current is increased. Similarly, output voltage can sulation. Designs which may be subjected to extended decrease as input voltage increases because the “IOUT/40” periods of overload should either use the hermetic TO-3 component of ground pin current drops significantly at package or increase heat sink size. Foldback current higher input-output differentials. These effects are small limiting can be implemented to minimize power levels enough to be ignored for local regulation applications, but under fault conditions. + +
External Current Limit
PARASITIC LEAD RESISTANCES The LT1185 requires a resistor to set current limit. The r – r a b + value of this resistor is 15k divided by the desired current R I LIM GND VIN limit (in amps). The resistor for 2A current limit would be R1* LOAD 15k/2A = 7.5k. Tolerance over temperature is ±10%, so 2.37k REF GND VOUT FB current limit is normally set 15% above maximum load R2 current. Foldback limiting can be employed if short-circuit – VIN LT1185 current must be lower than full load current (see Typical Applications). VOUT – LT1185 • F02 The LT1185 has internal current limiting which will over- ride external current limit if power in the pass transistor *R1 SHOULD BE CONNECTED DIRECTLY TO GROUND LEAD, NOT TO THE LOAD, SO THAT ra ≈ 0Ω. THIS LIMITS THE OUTPUT VOLTAGE ERROR TO (IGND)(rb). is excessive. The internal limit is ≈ 3.6A with a foldback ERRORS CREATED BY ra ARE MULTIPLIED BY (1 + R2/R1). NOTE THAT VOUT INCREASES WITH INCREASING GROUND PIN CURRENT. R2 SHOULD BE CONNECTED characteristic which is dependent on input-output volt- DIRECTLY TO LOAD FOR REMOTE SENSING age, not output voltage per se (see Typical Performace Characteristics).
Figure 2. Proper Connection of Positive Sense Lead
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