Datasheet LT1581, LT1581-2.5 (Analog Devices) - 9

LT1581/LT1581-2.5
U U W U APPLICATIONS INFORMATION
1000µF to 5000µF, and with the V
Thermal Considerations
POWER pin instanta- neously shorted to ground can damage occur. A crowbar The LT1581 has internal current and thermal limiting circuit at the power input can generate those levels of designed to protect the device under overload conditions. current and a diode from output to power input is then For continuous normal load conditions maximum junction recommended. This is shown in Figure 6. Normal power temperature ratings must not be exceeded. It is important supply cycling or system “hot-plugging and unplugging” to give careful consideration to all sources of thermal will not do any damage. resistance from junction to ambient. This includes junc- A protection diode between the OUTPUT pin and the tion-to-case, case-to-heat sink interface and heat sink V resistance itself. Thermal resistance specifications are CONTROL pin is usually not needed. An internal diode between the OUTPUT pin and the V given in the electrical characteristics for both the Control CONTROL pin on the LT1581 can handle microsecond surge currents of 1A to section and the Power section of the device. The thermal 10A. This can only occur if the V resistance of the Control section is given as 0.65 CONTROL pin is instanta- °C/W and neously shorted to ground with a crowbar circuit with junction temperature of the Control section is allowed to large value output capacitors. Since the V run at up to 125 CONTROL pin is °C. The thermal resistance of the Power usually a low current supply, this condition is unlikely. A section is given as 2.5°C/W and the junction temperature protection diode from the OUTPUT pin to the V of the Power section is allowed to run at up to 150 CONTROL pin °C. The is recommended if the V difference in thermal resistances between Control and CONTROL pin can be instanta- neously shorted to ground. This is shown in Figure 6. Power sections is due to thermal gradients between the Normal power supply cycling or system “hot-plugging power transistor and the control circuitry. and unplugging” will not do any damage. Virtually all of the power dissipated by the device is V dissipated in the power transistor. The temperature rise in CONTROL the power transistor will be greater than the temperature rise in the Control section so the effective thermal resis- + D1* D2* tance, temperature rise per watt dissipated, will be lower CONTROL in the Control section. At power levels below 12W the V POWER OUTPUT POWER VOUT + + temperature gradient will be less than 25°C and the LT1581 maximum ambient temperature will be determined by the SENSE junction temperature of the Control section. This is due to ADJ R1 the lower maximum junction temperature in the Control section. At power levels greater than 12W the temperature *OPTIONAL DIODES: 1N4002 R2 gradient will be greater than 25°C and the maximum ambient temperature will be determined by the Power 1581 F06 section. For both cases the junction temperature is deter-
Figure 6. Optional Clamp Diodes Protect Against
mined by the total power dissipated in the device. For most
Input Crowbar Circuits
low dropout applications the power dissipation will be less than 12W. If the LT1581 is connected as a single supply device with the control and power input pins shorted together, the The power in the device is made up of two main compo- internal diode between the output and the power input pins nents: the power in the output transistor and the power in will protect the control input pin. the drive circuit. The additional power in the control circuit is negligible. Like any other regulator exceeding the maximum input-to- output differential can cause the internal transistors to The power in the drive circuit will be equal to: break down and none of the internal protection circuitry is PDRIVE = (VCONTROL – VOUT)(ICONTROL) then functional. 9