Overview
The UC284xB/UC384xB series are high-performance current mode PWM controllers. They are tailored for precise frequency control with a trimmed oscillator guaranteed at 250kHz, accommodating operation up to 500kHz. Their main functions include automatic feed-forward compensation, cycle-by-cycle current limiting through latching PWM, a high current totem pole output, as well as under-voltage lockout (UVLO) with hysteresis. Design features result in low start-up and operating currents, contributing to their efficiency. Available packaging options include Minidip and SO8.
Product Variants and Features
Variants within the series differ by UVLO thresholds and maximum duty cycle percentages. The UC3842B and UC3844B are designed with UVLO thresholds of 16V (on) and 10V (off), aligning well with off-line applications. Meanwhile, the UC3843B and UC3845B offer lower UVLO thresholds of 8.5V (on) and 7.9V (off). The UC3842B and UC3843B are capable of nearly 100% duty cycles, while the UC3844B and UC3845B are limited to a 0 to 50% range, achieved by an internal toggle flip flop that turns the output off every other clock cycle.
Description
The UC384xB control ICs are intended for implementing off-line or DC to DC fixed-frequency current mode control schemes. They are engineered with an internally trimmed oscillator for exact duty cycle control and a precision reference enhanced for error amplifier input accuracy. Additionally, the control ICs are equipped with under-voltage lockout featuring a start-up current below 0.5mA, logic electronics supporting latched operation, and a PWM comparator for current sense comparison.
Block Diagram and Pin Functions
A block diagram outlines the internals of the IC, including a 5V reference (VREF) with 50mA current, error amplifier, PWM latch, oscillator (OSC), current sense comparator, and more. Specific to the UC3844B and UC3845B is a toggle flip flop addition.
Pin functions for elements like the error amplifier output (COMP), inverting input (VFB), current sense (IsENSE), oscillator frequency programming (Rt/Ct), ground, output to MOSFET gate, supply voltage (Vcc), and reference output (Vref) are described, giving a comprehensive view of connections and configurations necessary for IC functionality.
Absolute Maximum Ratings and Ordering Numbers
The datasheet specifies absolute maximum ratings for supply voltage, output current and energy, analog inputs, error amplifier sink current, power dissipation, storage and junction operating temperatures, and soldering lead temperature. For easy component selection, ordering numbers are provided for both SO8 and Minidip package types across the series variants: UC2842B/3B/4B/5B and UC3842B/3B/4B/5B.
References:
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