Features Low startup current (<0.5mA) optimized for offline and DC-DC converters
Fine Tuning Oscillator Discharge Current Automatic Feed-Forward Compensation Pulse-by-Pulse Current Limiting Enhanced Load Response Characteristics Undervoltage Lockout with Hysteresis
DC-DC Converter Power Module Industrial PSU
battery powered PSU
illustrate
The ucx84x a series controller chip is an improved version of the ucx84x series for pin compatibility. Providing the necessary functions to control current-mode or switch-mode power supplies, this family of devices has many improved features: start-up current is less than 0.5 mA, oscillator discharge is adjusted to 8.3 mA, and at UV low voltage, the output stage can be Sink at least 10mA at voltages less than 1.2V, with VCC over 5V.

Overview
The ucx84xa family of fixed frequency pulse width modulator (pwm) controllers are designed to operate at a switching frequency of 500khz. These controllers are designed for peak current mode (pcm) and can be used in isolated and non-isolated power supply designs. These controllers can drive the FET directly from the output, which can source and sink up to 1A of gate drive current. These devices also have a built-in low impedance amplifier that can be used in non-isolated designs to control the power supply output voltage and feedback loop.
Functional block diagram

Characterization Pulse-by-Pulse Current Limiting Pulse-by-pulse limiting is an inherent characteristic of current-mode control schemes. The upper limit of peak current can be determined by simply clamping the error voltage. Precise current limiting allows optimization of magnetic and power semiconductor components while ensuring reliable power supply operation

undervoltage lockout
The UCX84XA devices feature undervoltage lockout protection circuitry for controlled operation during power-up and power-down sequences. The undervoltage lockout thresholds of the ucx842a, ucx843a, ucx844a, and ucx845a devices are optimized for two groups of applications: offline power supplies and dc-dc converters. Due to the wider range of vccon to vccoff, the ucx842a and ucx844a devices are ideal for offline AC input applications. The ucx843a and ucx845a controllers have a narrower vccon to vccoff hysteresis that can be used in dc to dc applications that are considered input regulation.
During uvlo, ic typically consumes 0.3ma of supply current. This VCC current is much smaller than the UCX84X series, so less power is drawn from the line. The reduced start-up current is of particular concern in off-line power supplies, where the IC is powered from a high-voltage DC rail and then directed to an auxiliary winding on the main transformer. The power is then dissipated in the start-up resistor, the size of which depends on the IC's start-up current. A 50% reduction in the UCX84X version series reduces resistive power loss by the same percentage compared to the UCX84X series. Once the turn-on threshold is crossed, the IC supply current typically increases to around 11 mA, and during undervoltage lockout, the UCX84XA family of devices prevents the power mosfet from parasitic turn-on due to the Miller effect at power-up. This improved design, with lower totem pole transistors operating under voltage lockout allows the IC to sink higher currents, up to 10mA, at saturation voltages as low as 0.7V, compared to ucx84x devices that will only sinks up to 0.2mA under the same conditions.

Slope Compensation Because the oscillator discharge current of the UCX84XA family has been trimmed, it is now possible to use UCX84XA family devices to operate accurately at high frequencies with precise maximum duty cycles (see Figure 7). This eliminates the effects of production variations in initial discharge current or dead time. Previous versions of ucx84x devices had a greater than 2:1 oscillator discharge current range, resulting in less reliable maximum duty cycle programming.
A small portion of the oscillator ramp can be resistively summed with the current sense signal to provide slope compensation for converters requiring duty cycles greater than 50%. Capacitor c and r2 form a filter to suppress leading edge switching spikes.
Device Functional Mode Normal Operation The IC can be used for peak current mode (pcm) control or voltage mode (vm) control. When the converter is operating in pcm, the voltage amplifier output will regulate the converter's peak current and duty cycle. When the integrated circuit is used for virtual machine control, the voltage amplifier output will regulate the duty cycle of the power converter. Adjustment of system peak current and duty cycle can be achieved using an integrated error amplifier and external feedback circuitry.
Under-Voltage Lockout (UVLO) Startup During system startup, the VCC voltage rises from 0. The IC operates in UVLO mode until the VCC voltage reaches its corresponding start-up threshold. After reaching the uvlo turns start threshold, the device will activate and the reference voltage will reach 5v.
UVLO Off Mode If the bias voltage of VCC drops below the minimum operating voltage, the PWM switching stops and the reference voltage becomes inactive, returning to 0 V. The device can be restarted by applying a voltage greater than the UVLO startup threshold to the VCC pin.

Application Information
The UCX84XA controller is a peak current mode pulse width modulator. These controllers have an on-board amplifier and can be used in isolated or non-isolated power supply designs. There is a totem pole gate driver on board capable of delivering 1 A peak current. This is a high-speed pulse width modulation capable of operating at switching frequencies up to 500 kHz.
typical application
A typical application of the UC3842A in an offline flyback converter is shown in Figure 10. The UC3842A uses an internal current control loop that contains a small current sense resistor that senses the primary inductor current ramp. This current sense resistor converts the inductor current waveform into a voltage signal that is directly input to the primary side pwm comparator. This inner loop determines the response to changes in the input voltage. The external voltage control loop includes comparing a portion of the output voltage with a reference voltage at the input of the error amplifier. When used in off-line isolated applications, the voltage feedback of the isolated output is accomplished using a secondary-side error amplifier and an adjustable voltage reference such as a tl431. The error signal passes through the primary-to-secondary isolation boundary of an opto-isolator whose collector is connected to the vref pin and the emitter is connected to fb. The external voltage control loop determines the response to load changes.
Typical Applications (continued)

Power Recommendations
TI recommends using the UCX84XA in isolated or non-isolated peak current mode control power supplies. The device can be used in buck, boost, flyback and forward converter based power topologies.
Layout Layout guidelines must use star grounding techniques.
The current loop must be as short and narrow as possible.
IC ground and power ground must meet at the return of the input bulk capacitor. Make sure that high frequencies and high currents from the power stage do not go through the signal ground path.
The high frequency bypass capacitor (CVCC1) must be placed between the VCC and GND pins as close to the pins as possible.
Resistor RS2 and capacitor CF form a low-pass filter for the current sense signal. CF must be as close as possible to the CS and GND pins.
The capacitor CVREF must be placed as close as possible to the VREF and GND pins.
SMD assembly with wave solder on single layer board. If multiple layers are used, some components can be rearranged to facilitate interconnection and reduce current loop area. If the soldering process allows, placing the SMD components vertically can improve the interconnect and loop area.