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PTB48580BAZ Datasheet(PDF) 8 Page - Texas Instruments |
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PTB48580BAZ Datasheet(HTML) 8 Page - Texas Instruments |
8 / 13 page Application Notes For technical support and further information visit http://power.ti.com Configuring the PTB48580 Series of DC/DC Converters for DSL Applications When operated as a pair, the PTB4850x and PTB48580 converters are specifically designed to provide all the required supply voltages for powering xDSL chipsets. The PTB4850x produces two logic voltages. They include a 3.3-V source for logic and I/O, and a low-voltage for powering a digital signal processor core. The PTB48580 produces a balanced pair of complementary supply voltages that is required for the xDSL transceiver ICs. When used together in these types of applications, the PTB4850x and PTB48580 may be configured for power-up sequencing, and also synchronized to a common switch conversion frequency. Figure 2-1 shows the required cross-connects between the two converters to enable these two features. Switching Frequency Synchronization Unsynchronized, the difference in switch frequency introduces a beat frequency into the input and output AC ripple components from the converters. The beat frequency can vary considerably with any slight variation in either converter’s switch frequency. This results in a variable and undefined frequency spectrum for the ripple waveforms, which would normally require separate filters at the input of each converter. When the switch frequency of the converters are synchronized, the ripple components are constrained to the fundamental and higher. This simplifies the design of the output filters, and allows a common filter to be specified for the treatment of input ripple. Figure 2-1; Example of PTB4850x & PTB48580A Modules Configured for DSL Applications –VTCVR PTB4850xA +VI Enable –VI VO1 VO2 COM Sync Out EN Out VO2 Adj PTB48580A +VI Enable –VI +VO –VO COM Sync In ±VO Adj VCCIO VCORE +VTCVR –48 V RTN –48 V Input Filter + – Power-Up Sequencing The desired power-up sequence for the AC7 supply volt- ages requires that the two logic-level voltages from the PTB4850x converter rise to regulation prior to the two complementary voltages that power the transceiver ICs. This sequence cannot be guaranteed if the PTB4850x and PTB48580 are allowed to power up independently, especially if the 48-V input voltage rises relatively slowly. To ensure the desired power-up sequence, the “EN Out” pin of the PTB4850x is directly connected to the active- low “Enable” input of the PTB48580 (see Figure 2-1). This allows the PTB4850x to momentarily hold off the outputs from the PTB48580 until the logic-level voltages have risen first. Figure 2-2 shows the power-up wave- forms of all four supply voltages from the schematic of Figure 2-1. Figure 2-2; Power-Up Sequencing Waveforms PTB48580 Series V (1 V/Div) V (1 V/Div) CORE CCIO +V (5 V/Div) –V (5 V/Div) TCVR TCVR HORIZ SCALE: 10 ms/Div Not Recommended for New Designs |
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