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LTM4636 Arkusz danych(PDF) 19 Page - Linear Technology |
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LTM4636 Arkusz danych(HTML) 19 Page - Linear Technology |
19 / 36 page LTM4636 19 4636f For more information www.linear.com/LTM4636 applicaTions inForMaTion Overtemperature Protection The LTM4636 has an overtemperature enhanced protec- tion features that can be used to detect overtemperature. The overtemperature feature uses the TMON pin voltage to monitor temperature. This pin varies from 0.994V at 25°C to 1.494 at 150°C, and will tripoff at ≥ 150°C. Tying TMON to ground disable this feature. RUNP and RUNC Enable The RUNP pin is used to enable the 5V PVCC supply that powers the power driver stage and enables the power stage ~1ms later. The RUNC pin is used to enable the control section that drives the power stage. The RUNP needs to be enabled first, and then RUNC. RUNP has a 0.85V threshold and can be connected to the input volt- age and RUNC has a 1.35V threshold and a 10k resistor to ground. See the Block Diagram for details. A 0.1µF capacitor from the RUNC pin to ground is used to set the delay for RUNC enable. INTVCC and PVCC Regulators The LTM4636 has an internal low dropout regulator from VIN called INTVCC. This regulator output has a 4.7μF ceramic capacitor internal. This regulator powers the control section. The PVCC 5V regulator supplies power to the power MOSFET driver stage. An additional 50mA can be used from this 5V PVCC supply for other needs. The input supply source resistance needs to be very low in order to minimize IR drops when operating from a 5V inputsource.Dependingontheoutputvoltageandcurrent, the input supply can source large current,and PVCC 5V regulator needs a minimum 4.70V supply. Additional input capacitance maybe needed for 5V inputs to limit the input droop. Stability Compensation The LTM4636 has already been internally compensated when COMPB is tied to COMPA for all output voltages. Table 5 is provided for most application requirements. For specific optimized requirements, disconnect COMPB from COMPA, and use LTpowerCAD to perform specific control loop optimization. Then select the desired external compensation and output capacitance for the desired optimized response. SW Pins The SW pins are generally for testing purposes by moni- toring these pins. These pins can also be used to dampen out switch node ringing caused by LC parasitic in the switched current paths. Usually a series R-C combina- tion is used called a snubber circuit. The resistor will dampen the resonance and the capacitor is chosen to only affect the high frequency ringing across the resistor. If the stray inductance or capacitance can be measured or approximated then a somewhat analytical technique can be used to select the snubber values. The inductance is usually easier to predict. It combines the power path board inductance in combination with the MOSFET interconnect bond wire inductance. First the SW pin can be monitored with a wide bandwidth scope with a high frequency scope probe. The ring fre- quency can be measured for its value. The impedance Z can be calculated: Z(L) = 2πfL, where f is the resonant frequency of the ring, and L is the total parasitic inductance in the switch path. If a resistor is selected that is equal to Z, then the ringing should be dampened. The snubber capacitor value is chosen so that its impedance is equal to the resistor at the ring frequency. Calculated by: Z(C) = 1/(2πfC). These values are a good place to start with. Modification to these components should be made to attenuate the ringing with the least amount of power loss. A recommended value of 2.2Ω in series with 2200pF to ground should work for most ap- plications. See Figure 19 for guideline. The 2.2Ω resistor should be an 0805 size. Thermal Considerations and Output Current Derating The thermal resistances reported in the Pin Configuration section of the data sheet are consistent with those param- eters defined by JESD51-12 and are intended for use with finite element analysis (FEA) software modeling tools that leverage the outcome of thermal modeling, simulation, and correlation to hardware evaluation performed on a µModule package mounted to a hardware test board. The motivation for providing these thermal coefficients in found in JESD51-12 (“Guidelines for Reporting and Using Electronic Package Thermal Information”). |
Podobny numer części - LTM4636 |
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Podobny opis - LTM4636 |
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