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UCC28250 Datasheet(PDF) 19 Page - Texas Instruments |
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UCC28250 Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 50 page  O O2 O1 REF / EA V R 1 R V + æ ö = - ´ ç ÷ è ø UCC28250 www.ti.com SLUSA29B – APRIL 2010 – REVISED OCTOBER 2010 FB/EA- (2/9) FB/EA- is the inverting input of the UCC28250’s internal error amplifier. When the UCC28250 is configured for secondary-side control, connect the output voltage sensing divider to this pin. The voltage divider can be selected according to the voltage on REF/EA+ pin. Referring to Figure 13, pick the lower resistor RO1 value arbitrarily, and choose the upper resistor RO2 value as: (8) Because the control loop gain is affected by voltage divider resistor values, choose an appropriate RO1 value so that the voltage loop DC gain is larger than 40 dB to prevent interference between the primary-side control loop and the SR control loop during start up. When the UCC28250 is sitting on the primary side, the error amplifier is connected as a voltage follower. Connect FB/EA- directly with COMP pin. The maximum voltage allowed on FB/EA- pin is 3.7 V. COMP (3/10) The COMP pin is the internal error amplifier’s output. The voltage range of COMP pin is 0 V to 3 V. Both the primary-side switches’ duty cycle and secondary-side SRs’ duty cycle is controlled by the COMP pin voltage. At steady state, a higher COMP pin voltage results in a larger duty cycle for the primary-side switches and a smaller duty cycle on the SRs. When the UCC28250 controller is set up for secondary-side control, connect the compensation network from the FB/EA- pin to the COMP pin. For primary-side control, the error amplifier is connected as a voltage follower. Directly connect the COMP pin to the FB/EA- pin. VSENSE (14/1) The VSENSE pin is used to directly sense the output voltage and to feed it into a transconductance error amplifier. The measured voltage allows the UCC28250 to achieve optimal pre-biased start up performance. When configured as a secondary-side controller, the output voltage is sensed and fed into the FB/EA- pin. The UCC28250 uses a conventional error amplifier approach to allow type III compensation. Therefore, the FB/EA- pin voltage always follows the REF/EA+ voltage. The FB/EA- pin does not reflect the true output voltage and therefore this dedicated VSENSE pin is required. The voltage divider connected to VSENSE is discussed in the Pre-Biased Start-Up Section. When UCC28250 is set up as primary-side control, connect VSENSE pin to VREF. Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 19 Product Folder Link(s): UCC28250 |
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