Analog Devices Inc./Maxim Integrated MAX685EEE+

Overview

MAX685EEE+ from Maxim Integrated is a dual-output DC-DC converter IC designed for powering CCD imaging devices and LCD bias rails. It delivers independently regulated +24V (up to 10mA) and −9V (up to 10mA) from a single +2.7V to +5.5V input using one inductor, features 30mV_p-p output ripple, fixed-frequency PWM operation at 220kHz/400kHz, and integrated P-channel/N-channel power switches in a 16-pin QSOP package.

For engineers reviewing the MAX685EEE+ datasheet, MAX685EEE+ pinout, MAX685EEE+ application, or MAX685EEE+ equivalent, this page provides verified technical context, confirmed pin functions, real-world sequencing and synchronization behavior, exact output voltage setting methodology, and validated alternative options for CCD/LCD bias supply designs.

Technical Context

The MAX685EEE+ implements a time-multiplexed dual-regulator architecture: on alternating cycles, it operates as a step-up converter (LXP active, LXN grounded) to regulate the positive output, then as an inverting buck-boost (LXN active, LXP grounded) to regulate the negative output. Each regulator uses current-mode PWM control with independent feedback via FBP (1.25V threshold) and FBN (−0.1V threshold).

Switching frequency is selectable via SYNC pin (GND = 220kHz, VDD = 400kHz) or externally synchronized from 200kHz to 480kHz; actual switching at each LX node occurs at half that rate. Power-on sequencing is controlled by SEQ pin logic level, and POK asserts open-drain high only when both outputs are within ±10% of regulation.

Key Specifications

Pinout & Package

MAX685EEE+ is packaged in a 16-pin QSOP (0.150" wide, 0.025" lead pitch) with exposed pad not connected internally. Pin functions are electrically validated per Maxim's official datasheet revision 1 (June 2003).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual-output DC-DC converter applications.

Compared with MAX686EEE+, MAX685EEE+ provides tighter FBN threshold accuracy (±10mV vs. ±20mV) and lower POK threshold hysteresis, improving regulation margin in precision CCD biasing. Against TPS65131RGTR, MAX685EEE+ offers superior design flexibility via programmable sequencing and external clock sync - critical for noise-sensitive imaging systems.

Availability

MAX685EEE+ is available at Aetrix Electronics and suitable for CCD imaging bias, LCD panel power, and portable medical sensor applications requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for MAX685EEE+ 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, mixed-signal, and power management ICs for industrial, medical, and consumer applications.

The MAX685EEE+ belongs to Maxim's CCD/LCD DC-DC converter product line, engineered specifically to replace discrete charge pumps and transformer-based supplies in space-constrained, low-noise imaging and display subsystems.

FAQ

What is the maximum achievable negative output voltage for MAX685EEE+ without external components?

The MAX685EEE+ supports a native negative output range of −9V to −1.27V when using the internal reference and standard feedback configuration. This is defined by the FBN input threshold of −0.1V and REF output of 1.25V. The datasheet confirms −9V as the guaranteed minimum under all operating conditions across −40°C to +85°C, with no external components required beyond the standard resistor divider and output capacitors. Exceeding −9V requires the charge-pump-enhanced circuit shown in Figure 4 of the MAX685EEE+ datasheet.

How does the SEQ pin affect startup behavior in MAX685EEE+?

The SEQ pin on the MAX685EEE+ determines power-on sequencing order: when SEQ is logic-low (≤0.3×VDD), the negative output powers up first and must reach ≥90% of its target voltage before the positive output begins regulation; when SEQ is logic-high (≥0.7×VDD), the positive output powers first and stabilizes before enabling the negative rail. This prevents reverse biasing of CCD photodiodes or LCD driver inputs and is implemented entirely within the MAX685EEE+ control logic - no external timing components are needed.

Can MAX685EEE+ synchronize to an external clock, and what is the valid frequency range?

Yes, the MAX685EEE+ supports external clock synchronization via the SYNC pin. When driven with a clean CMOS-level square wave, the device locks its internal oscillator to frequencies between 200kHz and 480kHz. The actual switching frequency at each LX node is exactly half the applied SYNC frequency - so a 400kHz SYNC signal yields 200kHz operation per rail. This capability allows EMI reduction in noise-sensitive imaging systems and alignment with system master clocks.

What is the role of the REF pin in MAX685EEE+, and how is it used in negative output configuration?

The REF pin on the MAX685EEE+ provides a precision 1.25V output used as the reference point for the negative output feedback network. In standard operation, the FBN pin senses the voltage divider between VOUT− and REF (not GND), enabling accurate regulation of negative voltages relative to a stable positive reference. This architecture avoids errors from ground bounce and allows the negative rail to be set independently of the positive rail - a key feature confirmed in the MAX685EEE+ Electrical Characteristics table and Typical Operating Circuit (Figure 3).

Does MAX685EEE+ require external MOSFETs or diodes for basic operation?

No, the MAX685EEE+ integrates both P-channel and N-channel power switches, eliminating the need for external MOSFETs. Schottky diodes (e.g., MBR0520) are required externally per the standard application circuit (Figure 3), but these are low-cost, widely available components - not active switching elements. The IC's internal switches handle all high-current switching, and the diodes serve only as passive rectifiers in the charge-pump topology. No external gate drivers, controllers, or inductors beyond the single 22µH unit are needed for baseline operation.