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UCC28250RGPT Datasheet(PDF) 24 Page - Texas Instruments |
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UCC28250RGPT Datasheet(HTML) 24 Page - Texas Instruments |
24 / 53 page ( ) ( ) R F 2 R 0.7 V V V R 11 A V 0.7 V ´ - < m ´ - R 1 2 V 0.7 V R R 0.7 V - = ´ ( ) ( ) R F 2 R 3 R 0.7 V V V 11 A R V 0.7 V R 11 A V ´ - - m ´ ´ - = m ´ ( ) PTC1 PTC2 PTC1 PTC2 0.7 V R R 11 A R R 0 ´ - - m ´ ´ ³ 1 PTC1 R 3.7 R = ´ ( ) ( ) PTC1 PTC2 PTC1 PTC2 3 PTC1 PTC2 2.6 V 0.7 V R R 11 A R R R 11 A 2.6 V R 0.7 V R é ù ´ ´ - - m ´ ´ ë û = m ´ ´ + ´ VREF OVP UCC28250 + 0.7 V 11 mA R3 R1 R2 VIN or VOUT UCC28250 SLUSA29C – APRIL 2010 – REVISED JULY 2011 www.ti.com OVP/OTP (19/6) The OVP/OTP pin provides multiple fault protection functions. If the voltage on the OVP/OTP pin exceeds 0.7 V, a fault shutdown occurs. All outputs stop switching and stay off (low) during the shutdown, and the SS pin is pulled to ground internally. Once the fault condition is cleared (i.e. OVP/OTP voltage drops below 0.7 V), the UCC28250 enters hiccup mode. A soft-start cycle begins after the hiccup cycle is finished. An internal 11- µA switched current source is used to create hysteresis. If the external resistor divider runs from line voltage VIN, a line over voltage protection is implemented. If the external resistor divider runs from the output voltage, output over voltage fault protection is achieved. Figure 17 shows the over-voltage protection external configuration at the OVP/OTP pin. According to the protection threshold VR and recovery threshold VF, choose an arbitrary R2 value. To ensure a realistic solution, R2 needs to meet the following: (16) The other two resistors, R1 and R3 can be calculated. (17) (18) If the external resistor divider runs from 3.3-V VREF, and replaces R2 with a positive temperature coefficient (PTC) thermistor, an over temperature fault protection with programmable hysteresis is accomplished (Figure 18). Choose an arbitrary PTC value, which has a resistance as RPTC1 at protection temperature and resistance as RPTC2 at recovery temperature. Because of its positive temperature coefficient, RPTC1 is larger than RPTC2. To ensure an available solution, RPTC1 and RPTC2 need to meet the criteria. (19) And resistors R1 and R3 can be calculated as: (20) (21) Figure 17. Over Voltage Protection 24 Submit Documentation Feedback Copyright © 2010–2011, Texas Instruments Incorporated Product Folder Link(s): UCC28250 |
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