Description The UC1846 /7 series control gear provides automatic feed-forward compensation of all necessary features to achieve a fixed programmable pulse-by-pulse current limiting frequency, current mode control scheme, while automatic symmetry correction in push-pull, maintaining a minimum external parts count. The technology has obvious superior performance in improving line regulation, enhancing load response characteristics, enhancing load response characteristics, and improving the simplicity and easy parallel operation capability of modular power supply design control loops. Topological advantages include the system's inherent pulse-by-pulse current limiting capability, push-pull common-mode range converters with wide-symmetry auto-correcting differential current sense amplifiers, and the ability to parallel power modules while maintaining equal current sharing. Includes built-in sub-voltage 500mA (peak) totem pole output lock and programmable current limit, in addition, bandgap referenced soft-start capability. Therefore, a shutdown feature is available under voltage lockout, which can initiate a full soft-start capable shutdown with auto-restart or latch power off. The UC1846 has a low output in the off state; a telecom power converter, while the UCX847 has a high output in the off state for an industrial power converter.

Overview

The UCX846/7 family of control devices provides the necessary functionality to implement off-line or DC-DC fixed frequency, current mode control schemes with a minimum number of external components. Internally implemented circuitry includes an undervoltage lockout with less than 1 mA start-up current, a precision reference trimmed for accuracy at the error amplifier input, logic to ensure lockout operation, a pwm comparator that also provides current limit control, and a pwm comparator designed for Totem pole output stages that source or sink high peak currents. The output stage is suitable for driving n-channel mosfet or bipolar transistor switches and is low in the off state.
Current Sense Amplifier

Current sense amplifiers can be used in a variety of ways to detect peak switch currents for comparison with error voltages. For reference, the internally regulated supply limits the maximum swing on the inverting input of the pwm comparator to about 3.5v. Therefore, for a fixed gain of 3, the maximum differential voltage at the current sense input must be kept below 1.2V. Functional Block Diagram Characterization (Continued)

In some applications, a small rc filter may be required to reduce switching transients.
Differential inputs allow long-range noise-free sensing.
Current Sense Amplifier Connections

Oscillator By using all NPN transistors to implement the oscillator, the UCX846/7 achieves excellent temperature stability and waveform clarity at frequencies in excess of 1MHz.
The external resistor RT is used to generate a constant current to the capacitor CT to generate a linear sawtooth wave. The oscillator frequency can be approximated by choosing rt and ct

Device functional mode

One of the most attractive features of current-limited current-mode converters is the ability to limit the pulse-by-pulse peak switching current by simply limiting the error voltage to a maximum value.
The shutdown shutdown circuit is designed to provide a fast acting general purpose shutdown port for implementing protection circuits and remote shutdown functions. The circuit can be divided into an input section consisting of a comparator with a 350 mV temperature compensated bias and an output section consisting of a three-transistor latch. Shutdown is accomplished by applying a signal greater than 350 mV to pin 16, triggering the output latch, and setting the PWM latch to provide an immediate signal to the output.
Application and Implementation Notes The information in the application section below is not part of the ti component specification and ti does not warrant its accuracy or completeness. TI's customers are responsible for determining the suitability of parts for their purpose. Customers should verify and test their design implementation to confirm system functionality.
Application Information
The UCX846/7 family of control devices provides all the necessary functions to implement a fixed frequency, current mode control scheme while maintaining a minimum external part count. The superior performance of this technology can be measured by improved line regulation, enhanced load response characteristics, and design of simple control loops. Topological advantages include inherent pulse-by-pulse current limiting capability, automatic symmetry correction for push-pull converters. Protection circuits include undervoltage lockout and programmable current limit, as well as soft-start capability. A shutdown function is also available, enabling a complete shutdown or power down with an automatic restart.
typical application

Power Supply Recommendations The vin supply terminal of the device requires placing low esr noise decoupling capacitors as directly as possible from the vin terminal to the gnd terminal. Ceramic capacitors with stable dielectric properties over temperature, such as X7R or better, are recommended.
The VC power terminal of the device needs to place the resistor as directly as possible from the VC terminal to the VIN terminal.
Layout guidelines place a low ESR and ESL decoupling capacitor, CREF, in the range of 1-µF to 2.2-µF, preferably ceramic, from the VREF pin to GND.
EA+ is the non-inverting input, EA- is the inverting input, and COMP is the output of the error amplifier. Place the resistor and capacitor series network between the ea+pin and the comp pin to minimize traces of the resistor and capacitor series network.
Place a low ESR and ESL capacitor CT, preferably a ceramic CT, from the CT pin to ground, and place the CT as close to the UCX846/7 as possible.
Place resistor RT from RT pin to GND and place RT as close to UCX846/7 as possible.

layout example