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LTM4633 Scheda tecnica(PDF) 21 Page - Linear Technology |
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LTM4633 Scheda tecnica(HTML) 21 Page - Linear Technology |
21 / 32 page LTM4633 21 4633f For more information www.linear.com/LTM4633 applicaTions inForMaTion If we take this equation an differentiate it with respect temperature T, then: dVD dt = – VGO – VD T This dVD/dT change as a function of temperature is the typical ~–2.0mV/°C. This equation is simplified for the first order derivation. Solving for T, T = –(VG0–VD)/dVDprovidethetemperature. 1st Example: Figure 4 for 27°C, or 300°C Kelvin the diode voltage is 0.598V, thus, 300°C = –(1200mV – 598mV)/ –2.0 mV/°C) 2nd Example: Figure 4 for 75°C, or 350°C Kelvin the diode voltage is 0.50V, thus, 350°C = –(1200mV – 500mV)/ –2.0mV/°C) Converting the Kelvin scale to Celsius is simply taking the Kelvin temp and subtracting –273°C Kelvin from it. A typical forward voltage is measured and placed in the electrical characteristics section of the data sheet, and Figure 4 is the plot of this forward voltage. Measure this forward voltage at 27°C to establish a reference point. Then use the above expression while measuring the forward voltage over temperature will provide a general temperature monitor. The diode connected PNP transistor at the TEMP pins can beusedtomonitortheinternaltemperatureoftheLTM4633. A general temperature monitor can be implemented by connecting a resistor between TEMP and VIN to set the current to 100µA, and then monitoring the diode voltage drop with temperature. See Figure 19 for an example. Safety Considerations The LTM4633 module does not provide galvanic isolation from VIN to any of the three VOUTs. There is no internal fuse. If required, a slow blow fuse with a rating higher than the maximum input current can be used to protect the unit in case of a catastrophic failure. An inline circuit breaker function can also be used instead of a fuse. The fuse or circuit breaker should be selected to limit the current to the regulator during overvoltage in case of an internal top MOSFET fault. If the internal top MOSFET fails, then turning it off will not resolve the overvoltage, thus the internal bottom MOSFET will turn on indefinitely trying to protect the load. Under this fault condition, the input voltage will source very large currents to ground through the failed internal top MOSFET and enabled internal bot- tom MOSFET. This can cause excessive heat and board damage depending on how much power the input voltage can deliver to this system. A fuse or circuit breaker can be used as a secondary fault protector in this situation. Layout Checklist/Example The high integration of LTM4633 makes the PCB board layoutverysimpleandeasy.However,tooptimizeitselectri- cal and thermal performance, some layout considerations are still necessary. Use large PCB copper areas for high current paths, in- cluding VIN, GND, VOUT1, VOUT2, and VOUT3. It helps to minimize the PCB conduction loss and thermal stress. Place high frequency ceramic input and output capacitors next to the VIN, GND and the VOUT pins to minimize high frequency noise. Place a dedicated power ground layer underneath the unit. To minimize the via conduction loss and reduce module thermal stress, use multiple vias for interconnection be- tween top layer and other power layers. Donotputviasdirectlyonthepads,unlesstheyarecapped or plated over. Use a separated SGND ground copper area for components connected to signal pins. Connect the SGND to GND underneath the unit. Bring out test points on the signal pins for monitoring. Figure 6 gives a good example of the recommended layout. |
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