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AMS1503CT-15 Datasheet(PDF) 6 Page - Advanced Monolithic Systems |
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AMS1503CT-15 Datasheet(HTML) 6 Page - Advanced Monolithic Systems |
6 / 9 page Advanced Monolithic Systems 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140 AMS1503 APPLICATION HINTS Protection Diodes Unlike older regulators, the AMS1503 family does not need any protection diodes between the adjustment pin and the output and from the output to the input to prevent die over-stress. Internal resistors are limiting the internal current paths on the AMS1503 adjustment pin, therefore even with bypass capacitors on the adjust pin no protection diode is needed to ensure device safety under short-circuit conditions. The Adjust pin can be driven on a transient basis ±7V with respect to the output without any device degradation. Diodes between the Output pin and VPOWER pin are not usually needed. Microsecond surge currents of 25A to50A can be handled by the internal diode between the Output pin and VPOWER pin of the device. In normal operations it is difficult to get those values of surge currents even with the use of large output capacitances. If high value output capacitors are used, such as 1000 µF to 5000µF and the VPOWER pin is instantaneously shorted to ground, damage can occur. A diode from output to input is recommended, when a crowbar circuit at the input of the AMS1503 is used (Figure 6). Normal power supply cycling or even plugging and unplugging in the system will not generate current large enough to do any damage. CONTROL POWER OUTPUT AMS1503 SENSE ADJ R1 R2 V OUT V POWER + + + D2* D1* V CONTROL Figure 6. Optional Clamp Diodes Protect Against Input Crowbar Circuits If the AMS1503 is connected as a single supply device with the control and power input pins shorted together the internal diode between the output and the power input pin will protect the control input pin. As with any IC regulator, none the protection circuitry will be functional and the internal transistors will break down if the maximum input to output voltage differential is exceeded. Thermal Considerations The AMS1503 series have internal power and thermal limiting circuitry designed to protect the device under overload conditions. However maximum junction temperature ratings should not be exceeded under continuous normal load conditions. Careful consideration must be given to all sources of thermal resistance from junction to ambient, including junction-to-case, case-to-heat sink interface and heat sink resistance itself. Thermal resistance specification for both the Control Section and the Power Transistor are given in the electrical characteristics. The thermal resistance of the Control section is given as 0.65 °C/W and junction temperature of the Control section can run up to 125 °C. The thermal resistance of the Power section is given as 2.7 °C/W and junction temperature of the Power section can run up to 150 °C. Due to the thermal gradients between the power transistor and the control circuitry there is a significant difference in thermal resistance between the Control and Power sections. Virtually all the power dissipated by the device is dissipated in the power transistor. The temperature rise in the power transistor will be greater than the temperature rise in the Control section making the thermal resistance lower in the Control section. At power levels below 12W the temperature gradient will be less than 25 °C and the maximum ambient temperature will be determined by the junction temperature of the Control section. This is due to the lower maximum junction temperature in the Control section. At power levels above 12W the temperature gradient will be greater than 25 °C and the maximum ambient temperature will be determined by the Power section. In both cases the junction temperature is determined by the total power dissipated in the device. For most low dropout applications the power dissipation will be less than 12W. The power in the device is made up of two components: the power in the output transistor and the power in the drive circuit. The power in the control circuit is negligible. The power in the drive circuit is equal to: PDRIVE = (VCONTROL - VOUT)(ICONTROL) where ICONTROL is equal to between IOUT/100(typ) and IOUT/58(max). The power in the output transistor is equal to: POUTPUT = (VPOWER -VOUT)(IOUT) The total power is equal to: PTOTAL = PDRIVE + POUTPUT Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is the lowest resistance path for the heat flow. In order to ensure the best possible thermal flow from this area of the package to the heat sink proper mounting is required. Thermal compound at the case-to-heat sink interface is recommended. A thermally conductive spacer can be used, if the case of the device must be electrically isolated, but its added contribution to thermal resistance has to be considered. |
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