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LTM8055 Arkusz danych(PDF) 15 Page - Linear Technology |
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LTM8055 Arkusz danych(HTML) 15 Page - Linear Technology |
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15 / 26 page  LTM8055 15 8055fb For more information www.linear.com/LTM8055 Switching Mode TheMODEpinallowstheusertoselecteitherdiscontinuous mode or forced continuous mode switching operation. In forcedcontinuousmode,theLTM8055willnotskipcycles, evenwhentheinternalinductorcurrentfallstozerooreven reverses direction. This has the advantage of operating at the same fixed frequency for all load conditions, which can be useful when designing to EMI or output noise speci- fications. Forced continuous mode, however, uses more current at light loads, and allows current to flow from the load back into the input if the output is raised above the regulation point. This reverse current can raise the input voltage and be hazardous if the input is allowed to rise uncontrollably. Please refer to Input Precautions in this section for a discussion of this behavior. Forced continuous operation may provide improved output regulation when the LTM8055 transitions from buck, buck-boost or boost operating modes, especially at lighter loads. In such a case, it can be desirable to oper- ate in forced continuous mode except when the internal inductor current is about to reverse. If so, apply a current sense resistor between VOUT and IOUT and tie the LL and MODE pins together. The LL pin is low when the current through the output sense resistor is about one-tenth the full-scale maximum. When the output current falls to this level, the LL pin will pull the MODE pin down, putting the LTM8055 in discontinuous mode, preventing reverse cur- rent from flowing from the output to the input. In the case where MODE and LL are tied together, a small capacitor (~0.1µF) from these pins to GND may improve the light load transient response by delaying the transition from the discontinuous to forced continuous switching modes. MODE may be tied to GND for the purpose of blocking reverse current if no output current sense resistor is used. FB Resistor Divider and Load Regulation The LTM8055 regulates its FB pin to 1.2V, using a resistor divider to sense the output voltage. The location at which the output voltage is sensed affects the load regulation. If there is a current sense resistor between VOUT and IOUT, and the output is sensed at VOUT, the voltage at the load will drop by the value of the current sense resistor multiplied by the output current. If the output voltage can be sensed at IOUT, the load regulation may be improved. PCB Layout Most of the headaches associated with PCB layout have been alleviated or even eliminated by the high level of integration of the LTM8055. The LTM8055 is neverthe- less a switching power supply, and care must be taken to minimize EMI and ensure proper operation. Even with the high level of integration, you may fail to achieve specified operation with a haphazard or poor layout. See Figure 6 for a suggested layout. Ensure that the grounding and heat sinking are acceptable. A few rules to keep in mind are: 1. Place the RFB and RT resistors as close as possible to their respective pins. APPLICATIONS INFORMATION VIN ZENER DIODE R Q 8055 F05a LTM8055 LOAD CURRENT GND VOUT SOURCING LOAD VIN RUN 8055 F05b 10µF LTM8055 LOAD CURRENT GND VOUT SOURCING LOAD EXTERNAL REFERENCE VOLTAGE + – Figure 5a. The MOSFET Q Dissipates Momentary Energy to GND. The Zener Diode and Resistor Are Chosen to Ensure That the MOSFET Turns On Above the Maximum VIN Voltage Under Normal Operation Figure 5b. This Comparator Circuit Turns Off the LTM8055 if the Input Rises Above a Predetermined Threshold. When the LTM8055 Turns Off, the Energy Stored in the Internal Inductor Will Raise VIN a Small Amount Above the Threshold |
Podobny numer części - LTM8055 |
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Podobny opis - LTM8055 |
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