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PT5814N Datasheet(PDF) 5 Page - Texas Instruments |
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PT5814N Datasheet(HTML) 5 Page - Texas Instruments |
5 / 11 page For technical support and more information, see inside back cover or visit www.ti.com Application Notes Startup of the PT5800 & PT5810 Series ISRs with Back-Feeding Source (Pre-Bias Capability) In complex digital systems an external voltage can sometimes be present at the output of the regulator during power up. For example, this voltage may be backfed through a dual-supply logic component such as an FPGA or ASIC. Another path might be via a clamp diode (to a lower supply voltage) as part of a power-up sequencing implementation. Although the PT5800 (5-V input) and PT5810 (3.3-V input) series of regulators will sink current under steady- state operating conditions, they will not do so during startup 1 as long as certain conditions are maintained 2. This feature allows these regulators to start up while an external voltage is simultaneously applied to the output. Figure 2-1 is an application schematic that demonstrates this capability. The waveforms in Figure 2-2 show the behavior of the circuit as input power is applied. Note that the plot of the regulator output current (Io) is approxi- mately zero up to the timestamp ‘A’, even though a voltage is initially backfed to the output via the 3.3-V input supply and diodes D1 & D2. The regulator sources current 3 when it begins raising the output above the back-fed voltage to its nominal regulation value. Notes 1. Startup includes both the application of a valid input source voltage, or the removal of a ground signal from the Inhibit* control (pin 2) with a valid input source applied. The output of the regulator is effectively off (tri-state), during the period that the Inhibit* control is held low. Vin (1V/Div) Vo (1V/Div) Io (5A/Div) HORIZ SCALE 5ms/Div A Figure 2-2; Power-up Waveforms with Back-Feeding Source Figure 2-1; Schematic Demonstrating Startup into Pre-Bias Capability PT5812 14–17 9–13 5–7 V IN V OUT GND C out C IN (3.3 VDC) COM COM V IN V O (2.5 V) 8 18 Sense(+) Sense(–) D 2 I o D 1 2. To ensure that the regulator does not sink current, the input voltage must always be greater or equal to the output voltage throughout the power-up and power-down sequence. 3. If an external source backfeeding the regulator’s output is greater than the nominal regulation voltage, the output will begin sinking current at the end of its soft-start power-up sequence. If this current exceeds the rated output, the module could be overstressed. PT5800 & PT5810 Series |
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