Analog Devices Inc./Maxim Integrated MAX688CSA+T

Overview

MAX688CSA+T from Maxim Integrated is a precision 3.3V fixed-output, low-dropout linear regulator requiring an external PNP pass transistor (e.g., FZT749) to deliver >1A output current. It features active-low shutdown control (SHDN), power-fail monitoring (PFO), 150µA operating supply current, and ±2% output voltage accuracy over 0°C to +70°C. Used in portable telephones and battery-powered memory backup supplies.

For engineers reviewing the MAX688CSA+T datasheet, MAX688CSA+T pinout, MAX688CSA+T application, or MAX688CSA+T equivalent, key selection criteria include its 3.3V fixed output, SHDN-controlled standby/shutdown sequencing, PFO open-drain warning signal, dropout performance down to 40mV at 200mA, and SO-8 package compatibility with board-level thermal management for high-current designs.

Technical Context

The MAX688CSA+T implements a precision bandgap reference and high-gain error amplifier to regulate output via base drive to an external PNP transistor. Its SHDN input provides three-level operation: ON (>1.25V), standby (<1.2V, <25µA I_IN), and full shutdown (<0.2V, <1µA I_IN), with 70mV hysteresis between modes.

PFO is an open-drain comparator output that asserts low when V_OUT drops 170mV below nominal (3.3V), enabling early power-fail detection without latching. Base current limiting is achieved via a resistor (RB) between BASE and BLIM pins, enforcing I_BASE ≤ 0.1V/(R_B + 5Ω) to constrain output current and quiescent rise in dropout.

Key Specifications

Pinout & Package

MAX688CSA+T is housed in an 8-pin SO (Small Outline) package with 1.27mm pitch, JEDEC MS-012AC compliant, body size 4.9mm × 6.0mm × 1.75mm, suitable for automated SMT assembly and moderate-power thermal dissipation.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MAX687CSA+T, MAX688CSA+T trades automatic battery-protection shutdown for precise, externally controllable power sequencing; versus LT1528CS8#TRPBF, it delivers lower dropout and 150× lower quiescent current at the cost of requiring an external PNP and PCB layout effort.

Availability

MAX688CSA+T is available at Aetrix Electronics and suitable for portable telephones, battery-powered memory backup systems, and portable test instruments requiring stable component supply with commercial temperature range and SO-8 footprint compatibility.

Supply support for MAX688CSA+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 U.S.-based semiconductor company specializing in high-performance analog and mixed-signal ICs for power, sensing, and communication applications.

The MAX687/MAX688/MAX689 family was designed specifically for high-accuracy, low-dropout regulation in battery-critical portable systems, leveraging external PNP transistors to achieve scalable current delivery with minimal dropout and intelligent power-fail signaling.

FAQ

What is the function of the BLIM pin on the MAX688CSA+T?

The BLIM pin on the MAX688CSA+T serves as the reference node for base-current limiting. When a resistor (RB) is connected between BASE and BLIM, the device enforces I_BASE ≤ 0.1V/(R_B + 5Ω), preventing excessive drive to the external PNP transistor during dropout or overload. This design eliminates the need for a collector-sense resistor, preserving low dropout voltage while enabling predictable output current limiting. The MAX688CSA+T uses this mechanism to maintain regulation integrity across varying load and input conditions.

Does the MAX688CSA+T have built-in short-circuit protection?

The MAX688CSA+T does not provide direct short-circuit protection on its OUT pin. Instead, it limits output current indirectly by capping base drive to the external PNP transistor via the BLIM–BASE resistor network. If the external PNP saturates or the load draws excessive current, the regulated base current restricts collector current per I_LOAD ≈ β × I_BASE. System-level short-circuit robustness therefore depends on proper selection of the external PNP's SOA and thermal design. The MAX688CSA+T itself remains functional under sustained overload as long as power dissipation limits are observed.

Can the MAX688CSA+T be used with a P-channel MOSFET instead of a PNP bipolar transistor?

No, the MAX688CSA+T is specifically designed to drive PNP bipolar transistors and is not compatible with P-channel MOSFETs. Its BASE output delivers current-sourced drive optimized for bipolar base-emitter junctions, and lacks the voltage swing or gate-drive strength required for MOSFET enhancement-mode operation. Attempting to use a P-MOSFET would result in improper regulation or failure to turn on. For MOSFET-based LDOs, consider dedicated high-side switch controllers or integrated PMOS LDOs such as the MAX1725 or TPS7A16.

What is the minimum output capacitance required for stable operation of the MAX688CSA+T?

The MAX688CSA+T requires a minimum output capacitance of 10µF for loads up to 100mA. For higher loads, the datasheet specifies 1µF per 10mA of load current (e.g., 50µF for 500mA). Capacitors must be low-ESR types-such as ceramic, OS-CON, or polymer-and their ESR must remain below 1% of the load resistance to ensure loop stability and transient response. Using undersized or high-ESR capacitors risks oscillation or excessive output voltage deviation during load steps. This requirement is explicitly defined for the MAX688CSA+T in its Electrical Characteristics table and Applications Information section.

How does the PFO output behave during shutdown of the MAX688CSA+T?

During shutdown of the MAX688CSA+T-when the SHDN pin is pulled below 0.2V-the PFO output is forced low and actively sinks current, regardless of the actual V_OUT level. This behavior is guaranteed per the datasheet's ELECTRICAL CHARACTERISTICS table (V_OL = 0.06V max at I_SINK = 1.2mA). It ensures a deterministic logic-low signal to downstream monitoring circuitry, enabling reliable system-level power-fail assertion even when regulation has ceased. This forced-low state is intrinsic to the MAX688CSA+T's internal architecture and does not require external pull-down components.