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LM22677QTJE-ADJ датащи(PDF) 11 Page - National Semiconductor (TI) |
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LM22677QTJE-ADJ датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page 30074123 FIGURE 5. Resistive Feedback Divider A maximum value of 10 k Ω is recommended for the sum of R1 and R2 to keep high output voltage accuracy for the –ADJ option. A maximum of 2 k Ω is recommended for the -5.0 out- put voltage option. For the 5V fixed output voltage option, the total internal divider resistance is typically 9.93 k Ω. At loads less than 5 mA, the boot capacitor will not hold enough charge to power the internal high side driver. The output voltage may droop until the boot capacitor is recharged. Selecting a total feedback resistance to be below 3 k Ω will provide some minimal load and can keep the output voltage from collapsing in such low load conditions. Catch Diode A Schottky type re-circulating diode is required for all LM22677 applications. Ultra-fast diodes which are not Schot- tky diodes are not recommended and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery characteristics and low forward volt- age drop of Schottky diodes are particularly important diode characteristics for high input voltage and low output voltage applications common to the LM22677. The reverse recovery characteristic determines how long the current surge lasts each cycle when the N-channel MOSFET is turned on. The reverse recovery characteristics of Schottky diodes mini- mizes the peak instantaneous power in the switch occurring during turn-on for each cycle. The resulting switching losses are significantly reduced when using a Schottky diode. The reverse breakdown rating should be selected for the maxi- mum V IN, plus some safety margin. A rule of thumb is to select a diode with the reverse voltage rating of 1.3 times the max- imum input voltage. The forward voltage drop has a significant impact on the con- version efficiency, especially for applications with a low output voltage. ‘Rated’ current for diodes varies widely from various manufacturers. The worst case is to assume a short circuit load condition. In this case the diode will carry the output cur- rent almost continuously. For the LM22677 this current can be as high as 7.1A (typical). Assuming a worst case 1V drop across the diode, the maximum diode power dissipation can be as high as 7.1W. 11 www.national.com |
Аналогичный номер детали - LM22677QTJE-ADJ |
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Аналогичное описание - LM22677QTJE-ADJ |
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