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UCC28250PWR Datasheet(PDF) 19 Page - Texas Instruments |
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UCC28250PWR Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 48 page  SS SS REF / EA 27 A T C V + m ´ = VSENSE VREF COMP FB/EA- REF/EA+ SS UCC28250 + + C SS C Z2 R Z2 C P1 C Z3 R Z3 R O2 R O1 R S2 R S1 VOUT UCC28250 www.ti.com SLUSA29A – APRIL 2010 – REVISED JUNE 2010 SS (Soft Start Programming Pin) (13/20) The soft-start circuit gradually increases the converter’s output voltage until steady state operation is reached. This reduces start-up stresses and current surge. When the UCC28250 reaches its valid operating threshold, the SS pin capacitor is charged with a 27-µA current source. The UCC28250’s internal error amplifier non-inverting terminal follows the SS pin voltage on REF/EA+ pin voltage depending on which one is lower. Hence, during soft start, the SS pin voltage is lower than REF/EA+. The internal error amplifier then uses the SS pin as its reference voltage, until the SS pin voltage rises above the REF/EA+ level. Once the SS pin voltage is above REF/EA+ voltage, soft-start time is considered finished. The soft-start implementation scheme and timing is different, depending on the location of the UCC28250 with respect to the isolation barrier. For secondary-side control, the internal error amplifier is used to achieve the voltage regulation. The REF/EA+ is connected to an external reference voltage, FB/EA- is connected to the voltage sensing divider, and the error amplifier’s output pin (COMP) is connected through a compensation filter back to the FB/EA- pin (Figure 13). In this case, the primary output’s start-up is a closed loop soft start (soft-start input reference of error amplifier). The output soft-start time is determined by the external capacitor connected at SS pin based on the internal 27-µA charging current and the voltage set at REF/EA+ pin. Based on the soft-start time TSS, choose soft start capacitor CSS value as: (9) Figure 13. Error Amplifier EAMP Connections for secondary-side Control Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 19 Product Folder Link(s): UCC28250 |
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