STMicroelectronics E-UC3845BN

Overview

E-UC3845BN from STMicroelectronics is a high-performance current-mode PWM controller IC designed for fixed-frequency DC-DC and off-line power supplies. It features an internally trimmed 5 V reference, UVLO with 8.4 V turn-on/7.6 V turn-off thresholds, <50% maximum duty cycle (due to internal toggle flip-flop), oscillator frequency guaranteed at 250 kHz (up to 500 kHz), and ±1 A totem-pole output capable of directly driving N-channel MOSFET gates. It is used in low-duty-cycle isolated flyback converters requiring precise current-mode control and cycle-by-cycle current limiting.

For engineers reviewing the E-UC3845BN datasheet, E-UC3845BN pinout, E-UC3845BN application, or E-UC3845BN equivalent, key selection criteria include its 8-pin DIP package, 7.6–8.4 V UVLO window, 47–50% max duty cycle, 5 V ±2% reference accuracy, and 150 ns typical rise/fall time - all critical for stable operation in auxiliary power supplies and AC-DC adapters.

Technical Context

The E-UC3845BN implements current-mode control using a voltage-controlled oscillator with RT/CT programming, a high-gain error amplifier (90 dB open-loop gain, 1 MHz bandwidth), and a latched PWM comparator that enables strict cycle-by-cycle current limiting. Its internal toggle flip-flop halves the effective duty cycle, restricting maximum conduction to under 50% - a defining architectural difference versus UC3842B/3B.

It integrates a precision 5 V reference (±2% over temperature), under-voltage lockout with hysteresis, automatic feed-forward compensation via the ISENSE input, and a high-current totem-pole output stage optimized for fast MOSFET gate drive. The COMP pin provides direct access to the error amplifier output for loop compensation network design.

Key Specifications

Pinout & Package

E-UC3845BN is housed in an 8-pin plastic dual in-line package (Minidip), compliant with JEDEC MS-001, with 2.54 mm lead pitch and through-hole mounting.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar current-mode PWM controller applications.

Compared with UC3842BN, E-UC3845BN trades higher duty cycle for tighter UVLO and inherent half-cycle limitation - ideal for low-input, low-power flyback designs. Versus NCP1014ST, it offers full external frequency control and higher drive strength but requires discrete MOSFET and more external components.

Availability

E-UC3845BN is available at Aetrix Electronics and suitable for AC-DC adapters, flyback auxiliary supplies, and LED driver control circuits requiring stable component supply, long-term lifecycle support, and consistent parametric performance across production batches.

Supply support for E-UC3845BN includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, specializing in power management, analog, microcontrollers, and automotive ICs.

E-UC3845BN belongs to the UC384xB family of current-mode PWM controllers, engineered specifically for cost-effective, high-reliability fixed-frequency switch-mode power supplies in consumer, industrial, and computing applications.

FAQ

What is the minimum recommended RT/CT combination to achieve 250 kHz operation?

Per Figure 2 in the datasheet, a 6.2 kΩ RT resistor with 1 nF CT capacitor achieves 250 kHz at 25°C. For production robustness, ST recommends using RT = 10 kΩ and CT = 3.3 nF - this yields ~225 kHz at low temperature and ~275 kHz at high temperature, staying within guaranteed 250 kHz ±10% tolerance across the full operating range.

Can E-UC3845BN drive a 1000 pF gate capacitance directly without a buffer?

Yes. With 150 ns typical rise/fall time into 1 nF (per datasheet test condition), E-UC3845BN delivers sufficient ±1 A peak current to charge/discharge 1000 pF gates within ~150 ns. Layout best practices - short traces, local 0.1 µF bypass at VCC/GND, and ground plane under pins 4–5 - are essential to maintain this performance.

How does the internal toggle flip-flop affect transformer design in flyback applications?

The toggle flip-flop forces alternating clock cycles to be blanked, limiting maximum duty cycle to <50%. This mandates transformer turns ratio and core selection for safe reset under worst-case low-line conditions, preventing saturation. Designers must size the primary inductance and air gap accordingly - unlike UC3842B, no margin exists beyond 50% conduction.

Is the 5 V reference output (pin 8) internally current-limited?

Yes. The VREF pin is specified to deliver up to 20 mA while maintaining ±2% accuracy. Beyond 20 mA, output voltage drops due to internal series resistance; Figure 8 shows reference voltage decreasing by ~25 mV at 60 mA load. For loads >10 mA, add a local decoupling capacitor (e.g., 1 µF X7R) to sustain regulation during transients.