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LTM4651 Folha de dados(PDF) 17 Page - Linear Technology |
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LTM4651 Folha de dados(HTML) 17 Page - Linear Technology |
17 / 34 page LTM4651 17 4651f For more information www.linear.com/LTM4651 Power Module Protection TheLTM4651’scurrentmodecontrolarchitectureprovides fast cycle-by-cycle current limit in an overcurrent condi- tion, as shown in the Typical Performance Characteristics section. If the output voltage collapses sufficiently due to an overload or short-circuit condition, minimum on-time will be violated (EQ. 3) and the internal oscillator will then fold-back automatically to one-fifth of the LTM4651’s programmed switching frequency—hereby reducing the output current and affording the load a chance to recover. The LTM4651 features input overvoltage shutdown protection: when VIN+|VOUT–| > 68V, switching action ceases (with 4V of hysteresis)—however, be advised that this protection is only active outside the LTM4651’s safe operating area (see Note 1 and Note 4 of the Electrical Characteristics table). The LTM4651 ceases switching action if internal tempera- tures exceed 165°C. The control IC resumes operation after a 10°C cool-down hysteresis. Note that these typical parameters are based on measurements in a lab oven and arenotproductiontested.Thisovertemperatureprotection is intended to protect the device during momentary over- loadconditions.Themaximumratedjunctiontemperature will be exceeded when this overtemperature protection is active. Continuous operation above the specified absolute maximum operating junction temperature may impair device reliability or permanently damage the device. The LTM4651 does not feature any specialized output overvoltage protection beyond what is inherent to the control loop’s servo mechanism. RUN Pin Enable The RUN pin is used to enable the power module or se- quence the power module. The threshold is 1.2V. The RUN pincanbeusedtoprovideanundervoltagelockout(UVLO) function by connecting a resistor divider from the input supply to the RUN pin, as shown in Figure 3. Undervoltage lockout keeps the LTM4651 in shutdown until the supply input voltage is above a certain voltage programmed by the user. The RUN pin hysteresis voltage prevents noise from falsely tripping UVLO. Resistors are chosen by first selecting RB (refer to Figure 3). Then: RA =RB • VIN(ON) 1.2V –1 (15) where VIN(ON) is the input voltage at which the undervolt- age lockout is overcome and the supply turns on. RA may be replaced with a hardwired connection from VD to RUN. The VIN turn-off voltage, VIN(OFF) is given by: VIN(OFF) =1.07V • RA RB +1 (16) IfUVLOisnotneeded,RUNcanbeconnectedtoLTM4651’s VD or VIN pins. When RUN is below its threshold, UVLO is engaged, MT and MB are turned off, INTVCC ceases to be regulated, and ISETa is discharged to SVOUT– by internal circuitry. Loop Compensation External loop compensation may be preferred for some applications and can be implemented easily, as follows: leave COMPb open circuit; connect a series-RC network (RTH and CTH) from COMPa to SVOUT–; in some instances, connect a capacitor (CTHP) from COMPa to SVOUT– (par- alleling the RTH-CTH series-RC network). See Table 8 for suggested input and output capacitances for a variety of operatingconditions.Additionally,theLTpowerCADdesign tool is available for transient and stability analysis. APPLICATIONS INFORMATION Figure 3. Undervoltage Lockout Resistive Divider RUN PIN RA RB VSUPPLY 4651 F03 |
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Descrição semelhante - LTM4651 |
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