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LTM4676A датащи(PDF) 53 Page - Linear Technology |
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LTM4676A датащи(HTML) 53 Page - Linear Technology |
53 / 136 page LTM4676A 53 4676afa For more information www.linear.com/LTM4676A applicaTions inForMaTion The relative phasing of all active channels in a PolyPhase rail should be optimally phased. The relative phasing of each rail is 360°/n, where n is the number of phases in the rail. MFR_PWM_CONFIG[2:0] configures channel relative phasing with respect to the SYNC pin. Phase relationship values are indicated with 0° corresponding to the falling edge of SYNC being coincident with the turn-on of the top MOSFETs, MTn. The MFR_PWM_CONFIG command can be altered via I2C commands, but only when switching action is disengaged, i.e., the module’s outputs are turned off. The MFR_PWM_CONFIG command takes on the value stored in NVM at SVIN power-up, but is overridden according to aresistorpin-strapappliedbetweentheFSWPHCFGpinand SGND only if the module is configured to respect resistor pin-strap settings (MFR_CONFIG_ALL[6] = 0b). Table 4 highlights available resistor pin-strap and corresponding MFR_PWM_CONFIG[2:0] settings. Some combinations of FREQUENCY_SWITCH and MFR_PWM_CONFIG[2:0] are not available by resistor pin-strapping the FSWPHCFG pin. All combinations of supported values for FREQUENCY_SWITCH and MFR_PWM_CONFIG[2:0] can be configured by NVM programming—or, I2C transactions, provided switching action is disengaged, i.e., the module’s outputs are turned off. Care must be taken to minimize capacitance on SYNC to assure that the pull-up resistor versus the capacitor load has a low enough time constant for the application to form a “clean” clock. (See “Open-Drain Pins”, later in this section.) When an LTM4676A is configured as a sync slave, it is permissible for external circuitry to drive the SYNC pin from a current-limited source (less than 10mA), rather than using a pull-up resistor. Any external circuitry must not drive high with arbitrarily low impedance at SVIN power-up,becausetheSYNCoutputcanbelowimpedance until NVM contents have been downloaded to RAM. Recommended LTM4676A switching frequencies of operation for many common VIN-to-VOUT applications are indicated in Table 7. When the two channels of an LTM4676A are stepping input voltage(s) down to output voltages whose recommended switching frequencies in Table7aresignificantlydifferent,operationatthehigherof thetworecommendedswitchingfrequenciesispreferable, butminimumon-timemustbeconsidered.(SeeMinimum On-Time Considerations section.) For example, consider an application in which it is desired for an LTM4676A to step-down 24VIN to 1.5VOUT on Channel 0, and 24VIN to 3.3VOUT on Channel 1: according to Table 7, the recommendedswitchingfrequencyis425kHzand750kHz, respectively. However, the switching frequency setting of the LTM4676A is common to both channels. Based on the aforementioned guidance, operation at 750kHz would be preferred—in order to keep inductor ripple currents reasonable—however, it is then realized that the on-time for a 24VIN to 1.5VOUT condition at 750kHz is 83ns, shy of the 90ns guardband recommendation. Therefore, for this particular example, the recommended switching frequency becomes 650kHz. Table 7. Recommended Switching Frequency for Various VIN-to-VOUT Step-Down Scenarios 5VIN 8VIN 12VIN 24VIN 0.9VOUT 350kHz 350kHz 350kHz 250kHz 1.0VOUT 350kHz 350kHz 350kHz 250kHz 1.2VOUT 350kHz 350kHz 350kHz 350kHz 1.5VOUT 350kHz 350kHz 425kHz 425kHz 1.8VOUT 425kHz 425kHz 500kHz 500kHz 2.5VOUT 425kHz 500kHz 575kHz 650kHz 3.3VOUT 425kHz 575kHz 650kHz 750kHz 5.0VOUT N/A 500kHz 750kHz 1MHz The current drawn by the SVIN pin of the LTM4676A is not digitized or computed. A value representing the estimated SVINcurrentislocatedintheMFR_IIN_OFFSETncommand, and is used in the computations of input current readback telemetry, namely READ_IIN and and MFR_READ_IINn. The recommended setting of MFR_IIN_OFFSETn is found in Table 8. The same value should be used for MFR_IIN_OFFSET0 and MFR_IIN_OFFSET1 (i.e., Pages 0x00 and 0x01). |
Аналогичный номер детали - LTM4676A |
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Аналогичное описание - LTM4676A |
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