Texas Instruments UC3903N

Overview

UC3903N from Texas Instruments is a quad supply and line monitor IC designed for power system supervision in industrial and embedded systems. It monitors up to four independent voltage rails-including one negative rail via internal inverter-with programmable under- and over-voltage thresholds centered on a 2.5-V precision reference, 30 mA open-collector fault outputs, and 8–40 V supply operation. It is used in telecom power shelves, server PSUs, and industrial PLCs for early fault detection and power-good signaling.

For engineers reviewing the UC3903N datasheet, UC3903N pinout, UC3903N application, or UC3903N equivalent, this page delivers verified technical context, exact pin functions for the 18-pin PDIP package, real-world timing behavior of UV/OV delay circuits, and validated alternative parts with documented functional trade-offs for multi-rail monitoring designs.

Technical Context

The UC3903N implements a fault window architecture where OV and UV thresholds are generated as ±offsets from its 2.5-V internal reference, with offset magnitude and hysteresis both proportional to the voltage applied at Pin 4 (Window Adjust). Each sense input (Pins 6–9, 15) connects to dedicated comparator inputs with clamping diodes and 2-kΩ series resistors for overvoltage protection.

It integrates three high-current open-collector outputs: UV FAULT (Pin 17), OV FAULT (Pin 16), and POWER OK (Pin 10), each sinking ≥30 mA; a dedicated inverter (Pins 5/14) enables sensing of negative supplies at SENSE 4; and a general-purpose op-amp (Pins 11–13) provides 20-mA source capability for auxiliary signal conditioning.

Key Specifications

Pinout & Package

UC3903N is housed in an 18-pin plastic dual in-line package (PDIP, N package), with 0.3-inch body width and through-hole mounting. Pin numbering follows standard DIP convention (Pin 1 = bottom-left corner, counterclockwise).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar quad supply monitoring applications.

Compared with UC2903N, UC3903N offers tighter thermal specification compliance at lower cost for commercial environments, while TPS3431 reduces integration but sacrifices fault coordination capability-making UC3903N irreplaceable in systems requiring synchronized multi-rail validation.

Availability

UC3903N is available at Aetrix Electronics and suitable for telecom power shelves, industrial PLCs, and server PSUs requiring stable component supply with long-term lifecycle support and traceable sourcing.

Supply support for UC3903N 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and power management technologies, with decades of heritage in high-reliability power supervision ICs.

The UC3903N belongs to TI's legacy UCx903 family of quad supply monitors, engineered specifically for robust, multi-rail power system supervision in commercial-grade industrial and communications equipment.

FAQ

What is the operating temperature range of the UC3903N?

The UC3903N is specified for 0°C to +70°C ambient operation, making it suitable for commercial-grade applications such as network switches, industrial HMIs, and office equipment power supplies. This differs from the UC2903N (–40°C to +85°C) and UC1903 (–55°C to +125°C), which target industrial and military environments. The UC3903N datasheet confirms this range in the Electrical Characteristics table under "UC3903" column headers.

How does the UC3903N sense a negative supply voltage?

The UC3903N senses negative supplies using its dedicated SENSE 4 inverter circuit (Pins 5 and 14). When Pin 5 (INVERT INPUT) is tied high, the internal emitter-follower inverts the negative voltage applied to Pin 14 (SENSE 4 INVERT INPUT), producing a positive-referenced signal at Pin 9 (SENSE 4) that the comparator can evaluate. This eliminates need for external level-shifting components, as confirmed in Figure 6 and "Sensing a Negative Voltage Level" section of the UC3903N datasheet.

Can the UC3903N monitor more than four voltage rails?

No-the UC3903N has exactly four dedicated sense inputs (SENSE 1–4, Pins 6–9 and 15), with no provision for expansion. Unused inputs must be tied to the 2.5-V reference or another active sense node to prevent floating conditions. While the general-purpose op-amp (Pins 11–13) can condition additional signals, it does not feed into the fault logic; thus, true multi-rail monitoring beyond four rails requires cascading or external comparators.

What is the maximum sink current of the UC3903N's open-collector outputs?

All three open-collector outputs (POWER OK, UV FAULT, OV FAULT) on the UC3903N sink ≥30 mA when saturated at 2 V, per the Electrical Characteristics table. Test conditions specify V_OUT = 2 V, and typical performance reaches 70 mA. This allows direct driving of optocoupler LEDs (e.g., PC817), small relays, or microcontroller interrupt pins without external transistors.

How is the fault window programmed on the UC3903N?

The fault window is programmed via the WINDOW ADJUST pin (Pin 4): applying 0.5–2.5 V sets the magnitude of the ±offset from the 2.5-V reference, with hysteresis scaling proportionally (e.g., 10–30 mV/V). A simple resistive divider from REF (Pin 2) to GND (Pin 3) generates this voltage-no active components required. Equation 1 in the datasheet explicitly defines fault window = ±10 × V_ADJ, confirming linear, predictable scaling.