Analog Devices Inc./Maxim Integrated MAX687CPA

Overview

The MAX687CPA from Maxim Integrated is a precision 3.3V fixed-output, low-dropout linear regulator requiring an external PNP pass transistor (e.g., FZT749), delivering up to 1A output current with 10mA base drive, 2.96V latched shutdown threshold, and integrated power-fail monitoring (PFO). It operates from 2.7V–11.0V input and targets battery-powered portable telephones where automatic low-voltage cutoff prevents cell reversal.

For engineers reviewing the MAX687CPA datasheet, MAX687CPA pinout, MAX687CPA application, or MAX687CPA equivalent, key selection criteria include its latched shutdown behavior, PFO sourcing/sinking capability, dropout voltage dependency on external transistor V_CE(SAT), and compatibility with 8-pin plastic DIP packaging for through-hole prototyping and industrial assembly.

Technical Context

The MAX687CPA implements a precision bandgap reference and dual comparators: one monitors V_OUT for PFO activation at 170mV below nominal (3.13V), and another triggers latched shutdown at 2.96V. Its ON pin requires a high pulse to initiate startup and must remain asserted until V_OUT exceeds the shutdown threshold to achieve stable latching.

Base drive is delivered via the BASE pin with internal current limiting controlled by a resistor between BASE and BLIM (IBASE ≤ 0.1V / (RB + 5Ω)), enabling accurate output current limiting without collector-sense resistors. The CC pin accepts a 10nF non-polarized compensation capacitor to stabilize loop response and suppress start-up overshoot.

Key Specifications

Pinout & Package

MAX687CPA is housed in an 8-pin plastic DIP package (0°C to +70°C operating range), with lead spacing of 0.300" and standard through-hole footprint.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar low-dropout linear regulator applications.

Compared with MAX688CPA, the MAX687CPA provides autonomous latched shutdown ideal for unattended battery systems; versus LT1528CN#PBF, it trades integration for flexibility in thermal management and current scaling via external PNP selection.

Availability

MAX687CPA is available at Aetrix Electronics and suitable for portable telephones, battery-powered memory backup systems, and industrial handheld terminals requiring stable component supply with long-term manufacturability and consistent parametric performance.

Supply support for MAX687CPA 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) designs precision analog and mixed-signal ICs for power management, sensing, and interface applications in industrial, automotive, and communications markets.

The MAX687CPA belongs to Maxim's high-accuracy LDO regulator family, engineered specifically for battery-powered systems demanding autonomous low-voltage protection, power-fail signaling, and external pass-transistor flexibility.

FAQ

What is the function of the ON pin on the MAX687CPA?

The ON pin on the MAX687CPA is an active-high startup control that must be pulsed high to initiate regulator operation. It must remain asserted until the output voltage exceeds 2.96V to achieve latched-on state. Once latched, the MAX687CPA stays enabled until output voltage falls below 2.96V, at which point it shuts down and remains off until the ON pin is pulsed again. This behavior is unique to the MAX687CPA and differs from the SHDN-controlled MAX688CPA/MAX689CPA.

Does the MAX687CPA require an external PNP transistor, and why?

Yes, the MAX687CPA requires an external PNP transistor (e.g., FZT749 or 2N2907A) to serve as the pass element. The MAX687CPA itself only provides precision reference, control logic, base drive, and monitoring functions. Using an external transistor allows scalable current handling (up to >1A), optimized thermal dissipation via heatsinking, and dropout voltage determined solely by the transistor's V_CE(SAT) - a key advantage over monolithic LDOs.

How does the PFO output behave on the MAX687CPA compared to MAX688CPA/MAX689CPA?

On the MAX687CPA, the PFO output is push-pull (sources and sinks current), whereas on MAX688CPA and MAX689CPA it is open-drain (sinks only). Additionally, PFO on the MAX687CPA remains active (low) during shutdown, providing continuous fault indication. In contrast, MAX688CPA/MAX689CPA PFO is disabled when SHDN is pulled below 0.2V. This makes MAX687CPA's PFO more suitable for systems requiring persistent power-fail signaling even after shutdown.

What is the minimum output capacitance required for stable operation of the MAX687CPA?

The MAX687CPA requires a minimum output capacitance of 10µF for loads up to 100mA. For higher loads, the recommended value is 1µF per 10mA (e.g., 68µF for 500mA, 470µF for 4A). Capacitors must be low-ESR (ESR ≤ 1% of load resistance), and Sanyo OS-CON or equivalent polymer types are preferred for low-temperature operation. Ceramic capacitors alone are insufficient due to inadequate ESR for loop stability.

Can the MAX687CPA be used with input voltages below 3.0V?

Yes, the MAX687CPA operates with input voltages as low as 2.7V, but regulation is only maintained while VIN − VOUT exceeds the external PNP's V_CE(SAT). At very low VIN (e.g., 2.9V), dropout may occur depending on load and transistor selection. The internal reference and comparators remain functional down to 2.7V, enabling PFO and shutdown functions even when V_OUT is out of regulation - a critical feature for accurate battery end-of-life detection in the MAX687CPA.