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Unlike other devices, the error amplifier in the UCC family is a true, low output-impedance, 2MHz operational amplifier. As such, the COMP terminal can both source and sink current. CS: CS is the input to the current sense comparators. The UCC family contains digital current sense filter- ing, which disconnects the CS terminal from the current sense comparator during the ns interval immediately following the rising edge of the OUT pin.
This digital filter- ing, also called leading-edge blanking, means that in most applications, no analog filtering RC filter is re- quired on CS. The over-current comparator is only intended for fault sensing, and exceeding the over-current threshold will cause a soft start cycle.
FB: FB is the inverting input of the error amplifier. For best stability, keep FB lead length as short as possible and FB stray capacitance as small as possible. The output stage also provides a very low impedance to overshoot and undershoot. This means that in many cases, external schottky clamp diodes are not required. RC: RC is the oscillator timing pin. For fixed frequency operation, set timing capacitor charging current by con- necting a resistor from REF to RC.
For best performance, keep the timing capacitor lead to GND as short and direct as possible. If possible, use separate ground traces for the timing capacitor and all other func- tions.
The frequency of oscillation can be estimated with the following equations:.
The control for many of these switchmode supplies was revolutionized with two significant introductions; an advance technique known as current mode control, and a novel PWM solution, the UC controller. This IC contained several innovative features for general purpose current mode controlled applications. Included were high speed circuitry, undervoltage lockout, an op-amp type error amplifier, fast overcurrrent protection, a precision reference and a high current totem-pole output. The popular UC control circuit architecture has been recently improved upon to deliver even higher levels of protection and performance. Advanced circuitry such as leading edge blanking of the current sense signal, soft-start and full cycle restart have been built-in to minimize external parts count. Additionally, these integrated circuits have been developed on a BiCMOS wafer fabrication process geared to virtually eliminate supply power and propagation delays in comparison to the bipolar UC devices. This application note will highlight the features incoporated into this new generation of PWM controllers in addition to realizeable enhancements in typical applications.
UCC3804N - Schaltregler-IC DIL-8, UCC3804, Texas Instruments
UCC3804N - Switching controller IC DIL-8, UCC3804, Texas Instruments