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UCC28700DBVR Datasheet(PDF) 9 Page - Texas Instruments |
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UCC28700DBVR Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 31 page  LC S1 CS D PA LC P K R R T N R L ´ ´ ´ ´ = UCC28700, UCC28701 UCC28702, UCC28703 www.ti.com SLUSB41 – JULY 2012 CS (Current Sense): The current-sense pin is connected through a series resistor (RLC) to the current-sense resistor (RCS). The current-sense threshold is 0.75 V for IPP(max) and 0.25 V for IPP(min). The series resistor RLC provides the function of feed-forward line compensation to eliminate change in IPP due to change in di/dt and the propagation delay of the internal comparator and MOSFET turn-off time. There is an internal leading-edge blanking time of 235 ns to eliminate sensitivity to the MOSFET turn-on current spike. It should not be necessary to place a bypass capacitor on the CS pin. The value of RCS is determined by the target output current in constant-current (CC) regulation. The values of RCS and RLC can be determined by the equations below. The term ηXFMR is intended to account for the energy stored in the transformer but not delivered to the secondary. This includes transformer resistance and core loss, bias power, and primary-to-secondary leakage ratio. Example: With a transformer core and winding loss of 5%, primary-to-secondary leakage inductance of 3.5%, and bias power to output power ratio of 1.5%. The ηXFMR value is approximately: 1 - 0.05 - 0.035 - 0.015 = 0.9. where • VCCR is a current regulation constant (see ELECTRICAL CHARACTERISTICS), • NPS is the transformer primary-to-secondary turns ratio (a ratio of 13 to 15 is recommended for 5-V output), • IOCC is the target output current in constant-current regulation, • ηXFMR is the transformer efficiency. (3) where • RS1 is the VS pin high-side resistor value, • RCS is the current-sense resistor value, • TD is the current-sense delay including MOSFET turn-off delay, add ~50 ns to MOSFET delay, • NPA is the transformer primary-to-auxiliary turns ratio, • LP is the transformer primary inductance, • KLC is a current-scaling constant (see ELECTRICAL CHARACTERISTICS). (4) CBC (Cable Compensation), Pin 1 UCC28700: The cable compensation pin is connected to a resistor to ground to program the amount of output voltage compensation to offset cable resistance. The cable compensation block provides a 0-V to 3-V voltage level on the CBC pin corresponding to 0 to IOCC output current. The resistance selected on the CBC pin programs a current mirror that is summed into the VS feedback divider therefore increasing the output voltage as IOUT increases. There is an internal series resistance of 28 kΩ to the CBC pin which sets a maximum cable compensation of a 5-V output to 400 mV when CBC is shorted to ground. The CBC resistance value can be determined by the equation below. where • VO is the output voltage, • VF is the diode forward voltage, • VOCBC is the target cable compensation voltage at the output terminals, • VCBC(max) is the maximum voltage at the cable compensation pin at the maximum converter output current (see ELECTRICAL CHARACTERISTICS), • VVSR is the CV regulating level at the VS input (see ELECTRICAL CHARACTERISTICS). (5) NTC (NTC Thermistor Shut-down), Pin 1 UCC28701/2/3: These versions of the UCC28700 family utilize pin 1 for an external NTC thermistor to allow user-programmable external thermal shut-down. The shut-down threshold is 0.95 V with an internal 105-µA current source which results in a 9.05-k Ω thermistor shut-down threshold. These controllers have either zero or fixed internal cable compensation. Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703 |
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