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UCC28251 Datasheet(PDF) 29 Page - Texas Instruments |
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UCC28251 Datasheet(HTML) 29 Page - Texas Instruments |
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29 / 55 page 
UCC28251 www.ti.com SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014 9 Application and Implementation NOTE Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. 9.1 Application Information The UCC28251 is a high performance PWM controller with advanced synchronous rectifier outputs and is ideally suited for regulated half-bridge, full-bridge and push-pull converters. A dedicated internal pre-biased start up control loop working in conjunction with a primary-side voltage loop achieves monotonic pre-biased start up for either primary-side or secondary-side control applications. The UCC28251 architecture allows either voltage mode or current mode control. Input voltage feedforward can be implemented, allowing PWM ramp generator to improve the converter line transient response. Advanced cycle-by-cycle current limit achieves volt-second balancing even during fault conditions. The hiccup timer helps the system to stay within a safe operation range under over load conditions. With a multifunction OVP/OTP pin, combinations of input voltage protection, output voltage protection and over temperature protection can be implemented. The UCC28251 allows individual programming of dead time between primary-side switch and secondary-side SRs, in order to allow optimal power stage design. Dead time can also be reduced to zero, and this allows optimal system configuration considering the delays on the gate driver stage. The UCC28251 also provides complete system level protection functions, including UVLO, thermal shut down and over voltage, over current protection. 9.1.1 Error Amplifier and PWM Generation The UCC28251 includes a high performance internal error amplifier with low input offset, high source/sink current capability and high gain bandwidth (typical 3.5 MHz). The reference of the error amplifier (REF/EA+ pin) is set externally to support flexible trimming of the voltage loop, and to make the controller flexible for both primary side, as well as secondary-side control. The extra positive input for the error amplifier is the SS pin which is used to externally program the soft-start time of the converter’s output. During steady state operation, the primary switch duty cycle, D, is generated based on the external ramp on RAMP/CS pin and the COMP pin voltage. A higher COMP pin voltage results in a larger duty cycle. The secondary-side SR duty cycle is SR_D = (1-D), complementary to the primary-side duty cycle, without considering the dead time between primary-side switch and secondary-side SR. The primary outputs begin to switch when COMP pin voltage is above the 420 mV internal offset. The synchronous rectifier outputs, SRA and SRB, follow after the primary outputs and have a minimum 50% duty cycle during startup. SRA and SRB continue to be active and will only be disabled when UCC28251 is disabled (either through UVLO, EN shut down, OVP and etc). According to the internal logic, the minimum pulse width for the primary-side OUTA and OUTB is typically 100 ns. During soft start, the primary-side switch duty cycle is generated based on the external ramp on RAMP/CS pin and the COMP pin voltage. However, the duty cycle of secondary-side SR is generated based on an internal ramp and the COMP pin voltage. When the converter is controlled on the primary side, an internal ramp is a fixed ramp with 3-V peak voltage. When the converter is controlled on secondary side, an internal ramp is generated based on the internal pre-biased start-up loop. An internal pre-biased start-up loop modifies the SR duty cycle during soft start to achieve the optimal pre-biased start-up performance. After the SS pin reaches 2.9 V, the pre-biased start-up control loop is disabled. The secondary-side SR instantaneously changes into its steady state value as complementary to the primary-side duty cycle. 9.1.2 Prebiased Start Up With the internal error amplifier, UCC28251 supports both primary-side control and secondary-side control. For different control methods, the controller is configured accordingly and so is the pre-biased start-up control. During soft start, both the primary-side switches’ duty cycle and secondary-side SRs’ duty cycle are increased. This gradually increases the output voltage until steady state operation is reached, thereby reducing surge current. Copyright © 2013–2014, Texas Instruments Incorporated Submit Documentation Feedback 29 Product Folder Links: UCC28251 |
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