STMicroelectronics UC3845BD1

Overview

UC3845BD1 from STMicroelectronics is a high-performance current-mode PWM controller IC designed for fixed-frequency DC-DC and off-line power supplies. It features 8.4 V turn-on / 7.6 V turn-off UVLO thresholds, ≤50% maximum duty cycle (due to internal toggle flip-flop), 250 kHz guaranteed oscillator frequency, and 5 V ±100 mV reference output. It drives N-channel MOSFETs directly via a totem-pole output capable of ±1 A peak current, and is used in low-duty-cycle isolated flyback converters requiring precise cycle-by-cycle current limiting.

For engineers reviewing the UC3845BD1 datasheet, UC3845BD1 pinout, UC3845BD1 application, or UC3845BD1 equivalent, key selection criteria include its 7.6–8.4 V UVLO window, 47–50% max duty cycle limitation, 3 V/V current sense gain, 2.5 V error amp input reference, and SO8/Minidip package compatibility with legacy UC384xB designs.

Technical Context

The UC3845BD1 implements current-mode control using an internal 2.5 V precision error amplifier, a 3 V/V gain current sense comparator with 150–300 ns propagation delay, and a trimmed RC oscillator whose frequency is set by external RT/CT components. Its latch-based PWM logic ensures cycle-by-cycle current limiting, while automatic feed-forward compensation improves transient response in voltage-mode-derived topologies.

Unlike UC3842B/3B, it incorporates an internal toggle flip-flop that blanks the output every other clock cycle-enforcing a hard ≤50% duty cycle limit ideal for forward and half-bridge topologies where transformer reset must be guaranteed. The 5 V reference provides stable biasing for RT/CT timing and supports external ramp injection for slope compensation.

Key Specifications

Pinout & Package

UC3845BD1 is available in SO8 and Minidip (PDIP-8) packages. Both share identical pinout and functional mapping per JEDEC MS-012 and STMicroelectronics D95IN332.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC3845N, UC3845BD1 offers SO8 surface-mount compatibility for higher-density layouts; compared with TL494CN, it delivers tighter UVLO hysteresis, built-in 5 V reference, and hardware-enforced 50% duty cap-making it superior for compact, low-component-count flyback designs.

Availability

UC3845BD1 is available at Aetrix Electronics and suitable for AC-DC adapters, industrial DC-DC converters, and telecom power supplies requiring stable component supply, long-term lifecycle support, and RoHS-compliant sourcing.

Supply support for UC3845BD1 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, designing and manufacturing analog, microcontroller, power, and sensor solutions for industrial, automotive, and consumer markets.

The UC384xB family was developed specifically for cost-sensitive, high-reliability current-mode switch-mode power supplies-emphasizing minimal external components, robust fault handling, and compatibility with legacy 384x designs.

FAQ

What is the maximum recommended operating temperature for UC3845BD1 in SO8 package?

The UC3845BD1 SO8 package has a maximum junction temperature of 150°C and a thermal resistance (R_{th j-amb}) of 150°C/W. At 25°C ambient, continuous power dissipation must remain below 800 mW. Derating is required above 25°C ambient-e.g., at 70°C ambient, max power drops to ~530 mW to maintain T_j ≤ 150°C.

Can UC3845BD1 replace UC3842BD1 in an existing design?

UC3845BD1 is not a drop-in replacement for UC3842BD1 due to critical differences: UVLO thresholds (7.6/8.4 V vs. 10/16 V) and max duty cycle (≤50% vs. ≤100%). Swapping requires redesigning the startup circuit and verifying transformer reset margin; it is only viable if the original design already operates below 50% duty and uses low-voltage biasing.

Does UC3845BD1 support external synchronization?

Yes-pin 4 (RT/CT) accepts external clock signals via AC-coupled injection. A series diode clamp is required if sync amplitude exceeds 300 mV below ground. External sync enables multi-phase interleaving or noise spreading, as documented in Figure 22 of the datasheet with 47 Ω series resistor and 0.01 µF coupling capacitor.

What is the typical propagation delay from ISENSE input to OUTPUT turn-off?

The current sense comparator propagation delay is specified as 150–300 ns (typ. 225 ns) from ISENSE voltage crossing 1.0 V to OUTPUT transition. This delay directly impacts minimum detectable overcurrent pulse width and must be included in worst-case short-circuit response time calculations-especially critical in high-frequency flyback designs.