Microchip Technology U2403B-M

Overview

U2403B-M from Atmel is a monolithic bipolar charge timer IC designed for time-controlled, constant-current NiCd/NiMH battery charging. It delivers programmable 3–24 h charge duration, 1.5 V internal reference voltage, open-collector LED indicator (pin 8), overtemperature shutdown at 140°C, and operates from 3.5–12 V supply. Used in cordless telephones and low-cost entertainment equipment battery chargers.

For engineers reviewing the U2403B-M datasheet, U2403B-M pinout, U2403B-M application, or U2403B-M equivalent, key selection criteria include its shunt-emitter current sensing (pin 2/3), oscillator timing via R4/C4 network (pin 4), test-mode switch (pin 5), and thermal protection behavior tied to collector voltage (pin 1) and junction temperature.

Technical Context

The U2403B-M implements a closed-loop constant-current source using an internal op-amp with 1.5 V reference applied to pin 3 (SENSE), regulating current through external R3. Its RC oscillator (pin 4) drives a programmable frequency divider (n = 8/17/26 depending on STM state) to set total charge time t_ch = 1/(f_osc × 2ⁿ).

Overtemperature protection activates at T_j ≥ 140°C, forcing pin 3 voltage to 0 V and halting oscillation until thermal recovery and V1 > VS condition is met. Trickle charge initiates post-timing with reduced 0.1 V reference, lowering current by factor ~15 versus fast-charge mode.

Key Specifications

Pinout & Package

U2403B-M is available in DIP8 and SO8 packages. Pin mapping is identical across both variants per Figure 2.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar battery charge timer applications.

Compared with MAX713 and UC3906, the U2403B-M offers fully self-contained timing and current regulation in a single 8-pin package-ideal for fixed-duration, cost-sensitive NiCd/NiMH chargers where simplicity and BOM reduction outweigh multi-cell flexibility or dual-loop precision.

Availability

U2403B-M is available at Aetrix Electronics and suitable for cordless telephones, entertainment equipment battery packs, low-cost cordless tool chargers, and emergency lighting backup systems requiring stable component supply and long-term lifecycle support.

Supply support for U2403B-M 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

Atmel Corporation, now part of Microchip Technology, is a semiconductor manufacturer specializing in microcontrollers, nonvolatile memory, and analog products for embedded applications.

The U2403B-M belongs to Atmel's legacy analog timing IC product line, engineered specifically for low-cost, discrete-component-based battery charging systems in consumer electronics.

FAQ

What is the minimum supply voltage required for reliable startup of the U2403B-M?

The U2403B-M features a power-on reset that releases at VS ≥ 3.1 V and asserts under-voltage reset below VS ≤ 2.9 V. Stable operation begins at 3.5 V per the Absolute Maximum Ratings table, making 3.5 V the practical minimum for guaranteed functional startup across temperature and process variation. The U2403B-M will not initiate charge sequencing until this threshold is met.

How does the U2403B-M implement overtemperature protection, and what happens to the charge cycle when it triggers?

The U2403B-M monitors junction temperature internally and disables the oscillator when T_j reaches 140°C, forcing pin 3 (V3) voltage to 0 V and halting the timing counter. Charge current drops to zero, and the IC remains in suspended state until temperature falls below the threshold and V1 exceeds VS. Only then does the U2403B-M resume counting remaining time - preserving total programmed duration despite thermal interruption.

Can the U2403B-M be used to charge batteries with higher voltage requirements, such as 24 V systems?

The U2403B-M itself has a 13.5 V absolute maximum supply rating and cannot directly operate from 24 V. However, Figure 9 in the datasheet shows an expander circuit using external transistors and NTC thermistor that allows U2403B-M-based charge control up to 30 V DC supply. In this configuration, the U2403B-M remains isolated at safe voltage levels while managing timing and logic for the high-side power stage.

What is the function of pin 5 (STM) on the U2403B-M, and how does it affect charge timing?

Pin 5 (STM) selects the frequency divider ratio applied to the oscillator output: open = n = 26, GND = n = 17, VS = n = 8. Since total charge time t_ch = 1/(f_osc × 2ⁿ), STM directly scales timing resolution - e.g., connecting STM to GND reduces nominal 6 h charge to 42 s for rapid functional verification. This makes the U2403B-M uniquely suited for production test without modifying R4/C4 components.

Does the U2403B-M support trickle charge, and how is its current level determined?

Yes, the U2403B-M automatically enters trickle charge mode after main timing completes, reducing the internal reference voltage from 1.5 V to ~0.1 V. This lowers regulated current by a factor of ~15 relative to fast-charge Ich = V3/R3. Total trickle current equals Ich/15 plus supply current IS and LED sink current I8. External R6 (Figure 8) can further reduce trickle current by diverting portion of I6 away from the battery.