onsemi UC3842BD1

Overview

UC3842BD1 from STMicroelectronics is a high-performance current-mode PWM controller IC designed for fixed-frequency off-line and DC-DC power supplies. It features a trimmed oscillator (250 kHz guaranteed), latching PWM for cycle-by-cycle current limiting, 5 V ±1% internal reference, under-voltage lockout (16 V on / 10 V off), and a totem-pole output capable of ±1 A peak drive - enabling direct N-channel MOSFET gate control in flyback, forward, and boost converters.

For engineers reviewing the UC3842BD1 datasheet, UC3842BD1 pinout, UC3842BD1 application, or UC3842BD1 equivalent, key selection criteria include UVLO threshold compatibility with 115/230 VAC rectified bus, duty cycle range (up to 96%), oscillator programming flexibility (RT/CT), current-sense gain (3 V/V), and SO8 thermal performance (R_{th j-amb} = 150 °C/W).

Technical Context

The UC3842BD1 implements current-mode control via a high-speed current-sense comparator that terminates switch conduction when sensed inductor current reaches the error amplifier's output voltage level. Its oscillator uses an internal constant-current source charging CT through RT, delivering precise frequency control with <±1% variation over temperature and supply voltage.

Internal circuitry includes a trimmed 5 V reference with ±25 mV load regulation, a rail-to-rail error amplifier (90 dB open-loop gain, 1 MHz GBW), and UVLO with 6 V hysteresis - ensuring reliable start-up and brown-out recovery in unregulated input applications.

Key Specifications

Pinout & Package

UC3842BD1 is housed in an SO8 (Small Outline 8-pin) package with standard 1.27 mm pitch, JEDEC MS-012AC compliant, and rated for 800 mW power dissipation at T_amb ≤ 25°C.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC3842N, UC3842BD1 offers superior thermal performance in surface-mount systems and eliminates lead-related parasitics; compared with TL494CN, it delivers simpler current-mode implementation with built-in sensing and latch protection - reducing BOM count and layout complexity in single-switch topologies.

Availability

UC3842BD1 is available at Aetrix Electronics and suitable for AC-DC adapters, flyback SMPS, DC-DC boost converters, and offline forward converters requiring stable component supply, long-term lifecycle support, and consistent SO8 packaging.

Supply support for UC3842BD1 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-grade ICs.

The UC384xB series was developed specifically for cost-sensitive, high-reliability current-mode switch-mode power supplies - targeting industrial, consumer, and telecom power applications where precision timing, robust fault handling, and minimal external components are critical.

FAQ

What is the maximum operating frequency of UC3842BD1?

The UC3842BD1 oscillator is guaranteed at 250 kHz but supports operation up to 500 kHz using appropriate RT/CT values. At 500 kHz, design considerations include reduced minimum on-time, increased gate drive losses, and tighter PCB layout for current-sense signal integrity. The internal current-sense comparator propagation delay (≤300 ns) remains sufficient for stable operation within this range.

Can UC3842BD1 drive a P-channel MOSFET directly?

No - UC3842BD1's OUTPUT pin is a totem-pole stage referenced to ground and designed to sink/source current into an N-channel MOSFET gate. Driving a P-channel device would require level-shifting circuitry or an external inverting driver, as the IC lacks high-side floating drive capability or bootstrap support. Its architecture assumes low-side N-MOSFET configuration.

How does the UVLO hysteresis affect system reliability?

The 6 V hysteresis (16 V on / 10 V off) prevents oscillatory start-up during brown-out conditions by ensuring the IC remains latched off until input voltage recovers sufficiently to sustain regulation. This avoids repeated attempts to switch under marginal VCC, protecting the power switch and reducing stress on input bulk capacitors and transformer insulation.

Is slope compensation required when using UC3842BD1 in DCM flyback?

No - slope compensation is only necessary in continuous-conduction-mode (CCM) flyback or boost converters to prevent subharmonic oscillation at duty cycles >50%. In discontinuous-conduction-mode (DCM) operation, the inductor current fully resets each cycle, eliminating the need for external ramp injection - simplifying design and reducing component count.