Series:	--
Packaging:	Tube
Part Status:	Active
Output Type:	Transistor Driver
Function:	Step-Up, Step-Down, Step-Up/Step-Down
Output Configuration:	Positive, Isolation Capable
Topology:	Buck, Boost, Flyback, Forward Converter
Number of Outputs:	1
Output Phases:	1
Voltage - Supply (Vcc/Vdd):	7.6 V ~ 30 V
Frequency - Switching:	500kHz
Duty Cycle (Max):	48%
Synchronous Rectifier:	No
Clock Sync:	No
Serial Interfaces:	--
Control Features:	Frequency Control
Operating Temperature:	0°C ~ 70°C (TA)
Package / Case:	8-SOIC (0.154", 3.90mm Width)
Supplier Device Package:	8-SOIC
Base Part Number:	UC3845A

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UC3845AD8 Application Field and Working Principle

Introduction

The UC3845AD8 is a current-mode pulse-width modulation (PWM) switching controller designed for off-line applications. It is available in a PLCC-20 package.

Application Field

The UC3845AD8 is ideally suited for a wide range of industrial, medical, and consumer applications, such as air conditioners, welding machines, power supplies, dimming, and solar photovoltaic systems. The UC3845AD8 can be used in buck, boost, flyback, and forward converter topologies, utilizing either discrete or integrated power MOSFETs. In combination with its current-mode control capability, the UC3845AD8 provides Proportional Integral (PI) response and excellent line and load regulation. The UC3845AD8 also features lower component count, wide range of frequency operation, integrated precision adjustable oscillator, soft-start, over current protection and output short circuit protection.

UC3845AD8 Overview

The UC3845AD8 is a current-mode PWM switching controller that combines several control concepts in a single device. It provides a high frequency precision adjustable oscillator, a current-sense comparator, a temperature compensation system, a voltage reference, and an error amplifier in one integrated circuit.The current-mode control circuitry of the UC3845AD8 adjusts the on-time of the switch to the accurately programmed output current or voltege through pulse-width modulation of the constant frequency signal provided by the oscillator. This current-mode control provides fast transient response and stable loop gain.

Working Principle

The UC3845AD8 control circuitry is organized as a 6-terminal block as shown in Figure 1. The input terminal serves as a voltage feedback control port and an error amplifier is included to amplify the difference between the desired and the actual output voltage. The error amplifier drives the control level at the current sense of the oscillator, which modulates its ON-time. Because of the current drive capability of the UC3845AD8, a single Proportional Integral (PI) controller is not needed. This simplifies the design procedure, reduces the number of components, and improves the performance and reliability.Figure 1: UC3845AD8 Control Circuitry

Conclusion

The UC3845AD8 is an excellent choice for off-line applications such as air conditioners, welding machines, power supplies, dimming and solar photovoltaic systems. It is available in a PLCC-20 package. It provides fast transient response, wide frequency operation, adjustable soft-start, temperature compensation, integrated voltage reference, over current protection and short-circuit protection in one device. With its efficient use of the current-mode control, a single PI controller is not needed, reducing the number of components and improving the performance and reliability.

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