UC2845BD1G Enables Frequency Programming via External RC Components

^{September 6, 2025 — Amid the ongoing trend toward higher efficiency and reliability in switching power supply technology, the current-mode PWM controller UC2845BD1G is increasingly becoming a mainstream solution in industrial power, communications equipment, and consumer electronics due to its excellent stability and precise control performance. The chip adopts advanced BCD process technology, supports a wide input voltage range of 8V to 30V, and provides efficient control support for various power topologies such as flyback and forward converters. According to relevant technical specifications, the device incorporates comprehensive protection functions and environmentally friendly features, ensuring safe and reliable operation of power systems under various working conditions.}

^{The UC2845BD1G is packaged in SOIC-8 and integrates a high-gain error amplifier, a precise duty cycle control circuit, and a temperature-compensated precision reference. The chip supports a maximum operating frequency of 500kHz and allows duty cycle adjustment from 0% to nearly 100%. Its built-in 36V clamp circuit provides overvoltage protection for the error amplifier output, while also featuring an undervoltage lockout (UVLO) function with a typical startup threshold of 16V and shutdown threshold of 10V.}

^{The chip utilizes a current-mode control architecture, delivering excellent line and load regulation. Its integrated high-current totem pole output stage can directly drive MOSFETs, with a peak output current of ±1A. The built-in programmable oscillator allows the operating frequency to be set via external resistors and capacitors, while also featuring soft-start functionality and programmable current limiting. The chip operates over a junction temperature range of -40°C to 125°C, meeting industrial-grade application requirements.}

Core Philosophy: Current Mode Control

This diagram illustrates the principle of Current Mode Control. Unlike traditional Voltage Mode Control, it features two control loops:

• Outer Loop: A slower voltage loop responsible for setting the correct output level.
• Inner Loop: A faster current loop responsible for real-time monitoring and limiting of the power switch current.

​This structure enables faster dynamic response and inherent cycle-by-cycle current limiting, significantly improving power supply reliability and performance.

^{In-Depth Analysis of Key Modules}

^{1. Voltage Loop — "The Commander"
Core Components: Error Amplifier (Error Amp) + 5.0V Reference}

^{Working Process:}

^{The chip generates an extremely stable 5.0V reference voltage, which is divided down to 2.5V and supplied to the non-inverting input (+) of the error amplifier.}

^{The power supply’s output voltage is divided by external resistors and fed into the inverting input (-) of the error amplifier — the FB (Pin 2).}

^{The error amplifier continuously compares the FB voltage with the internal 2.5V reference.}

^{The comparison result is output from the COMP (Pin 1) as an error voltage. The level of this voltage directly indicates how much power needs to be supplied:}

^{Output voltage too low → COMP voltage rises}

^{Output voltage too high → COMP voltage falls}

^{Key Detail:
The COMP pin requires an external RC compensation network. The design of this network is critical — it determines the stability of the entire power supply loop (i.e., whether the system will oscillate).}

2. Clock and Timing — "The Metronome"
Core Component: Oscillator

^{Working Process:}

^{A resistor (RT) and capacitor (CT) are connected between the RT/CT (Pin 4) and ground.}

^{An internal constant current source charges the CT capacitor (slope determined by RT), forming the rising edge of the sawtooth wave.}

^{When the voltage reaches a specific threshold, internal circuitry rapidly discharges the capacitor, creating the falling edge.}

^{This generates a fixed-frequency sawtooth wave, which determines the PWM switching frequency. The start of each sawtooth cycle provides a clock signal that sets the PWM latch and initiates a new output pulse.}

^{3. Power Supply and Protection — "Logistics and Security"}

^{Undervoltage Lockout (UVLO):}

^{Monitors the voltage at Vcc (Pin 7).}

^{The chip begins operation only when Vcc exceeds the startup threshold (≈16V), preventing unstable PWM operation under insufficient voltage.}

^{Once activated, the chip continues to operate as long as Vcc remains above the shutdown threshold (≈10V).}

^{This mechanism ensures stable and reliable startup behavior.}

^{5V Reference (Vref):}

^{Serves not only as the reference for the error amplifier, but is also output through the VREF (Pin 8).}

^{It provides a clean and stable 5V supply to external circuits (such as voltage-divider resistors or RT), enhancing the system’s noise immunity and overall stability.}

^{Signal Flow Summary (The Big Picture)}

^{The clock signal initiates the cycle and sets the output to turn on the MOSFET. The rising current is converted into a sampled voltage, which is compared in real time with the COMP voltage that represents power demand. When the two voltages are equal, the output turns off immediately, thereby determining the pulse width. This process repeats continuously, forming an efficient and stable closed-loop control.}

^{The UC2845BD1G utilizes a standard SOIC-8 package, delivering complete current-mode PWM control functionality through a streamlined pin layout. Its core pins include power supply input (VCC), totem-pole output (OUTPUT), error compensation (COMP), feedback input (FB), current sensing (ISENSE), and oscillator frequency setting (RT/CT). The device also provides a precise 5V reference output (VREF), supporting external circuit implementations for over-current protection, soft-start, and frequency adjustment. With high integration and system reliability, it is suitable for a wide range of isolated and non-isolated power topologies.}

^{In the field of industrial power supplies, it is used in AC/DC converters, inverter power systems, and motor drive controllers.
In communication equipment, it is applied in base station power supplies and network device power modules.
For consumer electronics, it is suitable for LCD display power supplies, adapters, and chargers.
In the automotive electronics sector, it is utilized in on-board chargers and auxiliary power systems.}

The UC2845BD1G offers the following key performance parameters:

These specifications highlight the device’s suitability for a wide range of power conversion applications requiring precise regulation and robust performance.

^{The product meets the following environmental regulations and standards:}

^{RoHS Compliance: Conforms to EU Directive 2015/863 requirements}

^{Halogen-Free: Chlorine content < 900 ppm, Bromine content < 900 ppm}

^{REACH Compliance: Contains no Substances of Very High Concern (SVHC)}

^{Lead-Free: Complies with JEDEC J-STD-020 standard}

^{Packaging: Uses environmentally friendly lead-free packaging materials}

^{All compliance information is based on manufacturer's specifications and industry standards.}

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