Datasheet LT1580, LT1580-2.5 (Analog Devices) - 10

LT1580/LT1580-2.5
U U W U APPLICATIONS INFORMATION
A protection diode between the V If the LT1580 is connected as a single supply device with OUT pin and the VPOWER pin is usually not needed. An internal diode between the the VCONTROL and VPOWER input pins shorted together the V internal diode between the V OUT pin and the VPOWER pin on the LT1580 can handle OUT and the VPOWER input pin microsecond surge currents of 50A to 100A. Even with will protect the VCONTROL input pin. large value output capacitors it is difficult to obtain those Like any other regulator exceeding the maximum input to values of surge currents in normal operation. Only with output differential can cause the internal transistors to large values of output capacitance, such as 1000µF to break down and none of the internal protection circuitry is 5000µF, and with the VPOWER pin instantaneously shorted then functional. to ground can damage occur. A crowbar circuit at the power input can generate those levels of current, and a
Thermal Considerations
diode from output to power input is then recommended. The LT1580 has internal current and thermal limiting This is shown in Figure 6. Normal power supply cycling or designed to protect the device under overload conditions. system “hot plugging and unplugging” will not do any For continuous normal load conditions maximum junction damage. temperature ratings must not be exceeded. It is important to give careful consideration to all sources of thermal VCONTROL resistance from junction to ambient. This includes junc- tion-to-case, case-to-heat sink interface and heat sink + resistance itself. Thermal resistance specifications are D1* D2* given in the electrical characteristics for both the Control VCONTROL section and the Power section of the device. The thermal V V V POWER POWER OUT VOUT + + resistance of the Control section is given as 0.65°C/W and LT1580 junction temperature of the Control section is allowed to SENSE run at up to 125°C. The thermal resistance of the Power ADJ R1 section is given as 2.7°C/W and the junction temperature of the Power section is allowed to run at up to 150°C. The *OPTIONAL DIODES: 1N4002 R2 difference in thermal resistances between Control and 1580 F06 Power sections is due to thermal gradients between the power transistor and the control circuitry.
Figure 6. Optional Clamp Diodes Protect Against Input Crowbar Circuits
Virtually all of the power dissipated by the device is dissipated in the power transistor. The temperature rise in A protection diode between the V the power transistor will be greater than the temperature OUT pin and the VCONTROL pin is usually not needed. An internal diode between the rise in the Control section so the effective thermal resis- V tance, temperature rise per watt dissipated, will be lower OUT pin and the VCONTROL pin on the LT1580 can handle microsecond surge currents of 1A to 10A. This can only in the Control section. At power levels below 12W the occur if the V temperature gradient will be less than 25 CONTROL pin is instantaneously shorted to °C and the ground with a crowbar circuit with large value output maximum ambient temperature will be determined by the capacitors. Since the V junction temperature of the Control section. This is due to CONTROL pin is usually a low current supply, this condition is unlikely. A protection diode from the lower maximum junction temperature in the Control the V section. At power levels greater than 12W the temperature OUT pin to the VCONTROL pin is recommended if the V gradient will be greater than 25 CONTROL pin can be instantaneously shorted to ground. °C and the maximum This is shown in Figure 6. Normal power supply cycling or ambient temperature will be determined by the Power system “hot plugging and unplugging” will not do any section. For both cases the junction temperature is deter- damage. mined by the total power dissipated in the device. For most 10