Analog Devices Inc./Maxim Integrated MAX1856EUB+T

Overview

The MAX1856EUB+T from Maxim Integrated is a wide-input-range, synchronizable current-mode PWM controller for SLIC (Subscriber Line Interface Circuit) power supplies, generating dual negative outputs (e.g., −24V and −72V) or single outputs (e.g., −48V, −185V) using off-the-shelf flyback transformers. It operates from 3V to 28V input, delivers low audio-band noise on talk battery, handles ring-trip transients robustly, and supports Idle Mode™ for high light-load efficiency in voice-enabled CPE.

For engineers reviewing the MAX1856EUB+T datasheet, MAX1856EUB+T pinout, MAX1856EUB+T application, or MAX1856EUB+T equivalent, key selection criteria include its adjustable 100–500kHz switching frequency, internal soft-start, LDO-regulated 5V bias rail, current-sense threshold (85–115mV), and 10-pin µMAX package compatibility with space-constrained broadband infrastructure designs.

Technical Context

The MAX1856EUB+T implements an inverting flyback topology with direct-summing current-mode control-eliminating a traditional error amplifier-and integrates a BiCMOS comparator that simultaneously processes feedback error, current-sense voltage, and slope compensation. Its PWM operation maintains fixed-frequency regulation across load, while Idle Mode™ dynamically reduces switching pulses at light loads to minimize quiescent current.

It features a 5V LDO output (VLDO = 4.5–5.5V, 12mA max) powering all internal circuitry and the EXT gate driver, a programmable oscillator (fOSC = 100–500kHz via ROSC), and dual-mode SYNC/SHDN pin supporting shutdown or external clock synchronization (100–500kHz range) with automatic fallback to ROSC-based operation if sync is lost.

Key Specifications

Pinout & Package

Package: 10-pin µMAX (3mm × 3mm, 0.5mm pitch, exposed pad), rated for −40°C to +85°C operating temperature.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar SLIC power supply applications.

Compared with MAX1857EUB+T, the MAX1856EUB+T trades extended temperature rating for lower unit cost in commercial CPE; compared with LT3751EMS#TRPBF, it delivers plug-and-play dual-output regulation without external compensation, reducing design time and layout risk for −24V/−72V and −48V applications.

Availability

MAX1856EUB+T is available at Aetrix Electronics and suitable for VoIP gateways, cable modems, FTTH ONUs, and DSL line driver supplies requiring stable component supply and long-lifecycle support in broadband infrastructure programs.

Supply support for MAX1856EUB+T 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

Maxim Integrated (now part of Analog Devices) is a semiconductor company specializing in high-performance analog, mixed-signal, and power-management ICs for communications, computing, and industrial markets.

The MAX1856 belongs to Maxim's SLIC power supply controller product line, designed specifically for cost-sensitive, space-constrained voice-over-broadband equipment requiring reliable negative voltage generation with minimal external components.

FAQ

What is the primary function of the MAX1856EUB+T in a SLIC power supply?

The MAX1856EUB+T serves as a current-mode PWM controller for flyback converters generating negative voltages required by Subscriber Line Interface Circuits. It regulates outputs such as −24V (off-hook), −72V (ringer), or −48V (IP phone) using standard off-the-shelf transformers, with integrated LDO, soft-start, and Idle Mode™ for high efficiency across load ranges. The MAX1856EUB+T eliminates need for external loop compensation and supports both free-running and synchronized operation.

Can the MAX1856EUB+T generate both −24V and −72V simultaneously?

Yes, the MAX1856EUB+T supports dual-output configurations using split feedback-connecting separate resistor dividers from each output to REF and FB. In the standard application circuit (Figure 1), it delivers −24V at up to 400mA and −72V at up to 100mA with <±1.5% cross-regulation. The MAX1856EUB+T achieves this without additional controllers by leveraging its inverting flyback topology and precise 0V FB threshold referenced to REF.

How does the SYNC/SHDN pin operate in synchronization mode?

In synchronization mode, a rising edge on the SYNC/SHDN pin initiates each switching cycle, locking the MAX1856EUB+T to an external clock between 100kHz and 500kHz. The MAX1856EUB+T still requires the ROSC resistor to be populated, and if the external clock is lost while SYNC/SHDN is high, the device seamlessly reverts to ROSC-based operation within one cycle. Idle Mode™ is disabled during sync to ensure deterministic timing, preserving regulation during intermittent clock signals.

What is the role of the LDO pin on the MAX1856EUB+T?

The LDO pin provides a regulated 5V output (4.5–5.5V) that powers all internal circuitry-including the EXT gate driver-and can supply up to 12mA for small external loads. It derives from VCC with ≤300mV dropout, enabling operation down to VCC = 3V. The MAX1856EUB+T requires a ≥1µF ceramic capacitor from LDO to PGND for stability, and its output drives the EXT pin directly-eliminating need for external bias supplies in flyback designs.

Does the MAX1856EUB+T require external compensation components?

No, the MAX1856EUB+T uses direct-summing current-mode control with no traditional error amplifier, so it requires no external compensation components. Its feedback loop relies on the FB input referenced to the 1.25V REF output, and regulation stability is inherent to the architecture. This simplifies design, reduces bill-of-materials, and avoids phase-margin tuning-making the MAX1856EUB+T ideal for rapid deployment in cost-sensitive CPE applications.