Analog Devices Inc./Maxim Integrated MAX1681ESA

MAX1681ESA Switched Cap Voltage Converter – General Description

The MAX1681ESA, manufactured by Analog Devices Inc./Maxim Integrated, is a high-efficiency, inductorless switched-capacitor voltage converter designed to offer both voltage inversion and doubling capabilities. Delivered in an industry-standard 8-SOIC (3.9mm width) package, the MAX1681ESA is engineered to provide up to 125mA output current and offers frequency selection up to 1MHz. Its robust design makes it a compelling solution for engineers seeking compact, high-performance alternatives to traditional inductor-based DC-DC converter solutions, particularly when board space and external component count are at a premium.

Essential Functions and Main Characteristics of the MAX1681ESA

The MAX1681ESA stands out due to its frequency-selectable switched-capacitor topology, which supports both inverter and doubler configurations. Functionally, it operates on a 2.0V to 5.5V supply and can output either the negative of the input voltage or twice the input, depending on the mode. This flexibility is attributable to the integrated charge-pump architecture and the logic-selectable FSEL pin, which allows designers to switch between 500kHz and 1MHz operation. The device’s output resistance is typically just 3.5Ω, which means minimal voltage drop under load and improved efficiency compared to higher-impedance solutions. Key features include a logic-level shutdown function (quiescent current <1μA), simple operation with only a few ceramic capacitors, and compatibility with both positive and negative output applications. The package supports ‘-40°C to +85°C’ operation, covering a wide range of use cases within industrial, communication, and consumer electronics systems.

Primary Operating Specifications of the MAX1681ESA

System designers will appreciate the MAX1681ESA’s ability to deliver 125mA with up to 90% typical efficiency, depending on configuration and component selection. The input voltage range of 3.0V to 5.5V (inverter mode) or 4.0V to 5.5V (doubler mode) fits well with most logic and analog supply rails found in modern platforms. Notably, the output voltage drop at full current is limited to approximately 440mV, attributed to the device’s low output impedance. Power efficiency for typical loads is maintained above 80% for most supply and load conditions, with even higher values at lighter loads or higher input voltages. The on-chip oscillator frequency can be toggled by the FSEL pin, trading off between lower supply current at 500kHz and minimized output ripple plus smaller capacitor requirements at 1MHz. The device is not RoHS compliant, which should be factored into regulatory planning.

Input Frequency, Output Options, and Usage Scenarios

The MAX1681ESA’s frequency-selectable operation offers engineers flexibility during system optimization. Using the FSEL logic input, the device can operate at 500kHz or 1MHz, allowing engineers to balance output ripple, efficiency, and external capacitor size. At higher frequencies, smaller capacitance values are possible—down to 1μF for both flying and output capacitors—resulting in size and cost savings for compact board designs. The device supports two main configurations:

Voltage Inverter: Achieves -VIN output from positive supply; commonly used for biasing analog circuits and op-amps, or generating negative logic rails without a transformer or inductor.

Voltage Doubler: Outputs approximately 2×VIN; provides boosted rails for interface or sensor circuits.

In both modes, the design topology ensures inductorless operation, simple board layout, and fast time-to-market for power supply solutions. Additionally, the logic-level shutdown input provides a way to reduce system quiescent current for battery-powered or power-sensitive applications.

Design Tips Choosing Capacitors, Peripheral Circuitry, and Board Layout

Capacitor selection is paramount for the performance of the MAX1681ESA. The recommended value is 2.2μF or higher per capacitor, but as switching frequency increases, the minimum viable capacitance drops (for example, 1μF at 1MHz is acceptable for the maximum rated load). Low-ESR (Equivalent Series Resistance) ceramic capacitors—such as those from Murata, TDK, or AVX—are preferred, as ESR directly affects efficiency and output ripple. The total output resistance is a function of both device internal resistance and the ESR of C1 (charge-pump flying capacitor) and C2 (output capacitor).

For layouts, keep traces between the capacitors and the MAX1681ESA short and wide to reduce parasitic resistance and inductance. A large input bypass capacitor is recommended close to the IN pin to suppress supply ripple and switching noise, especially in high-frequency mode. As with all switched-capacitor circuits, EMI mitigation strategies may be necessary if operated near sensitive analog or RF circuitry—frequency selection can assist in minimizing interference.

Expanded Usage Linking and Combining MAX1681ESA Units

Where higher output voltages or increased output currents are required, MAX1681ESA provides support for both cascading and paralleling. Cascading two devices in series (e.g., two inverters) allows generation of -2×VIN, with some increase in overall output resistance and modest impact on efficiency due to the summed resistive drop of the stages. This is suitable for applications where multiple negative rails are needed without employing magnetics. Paralleling MAX1681ESA devices, on the other hand, reduces net output impedance and enables larger load currents for a given voltage drop. In practical terms, two devices in parallel halve the output resistance, improving transient capability and thermal performance under heavier loads.

Alternative and Substitute Devices Comparable to the MAX1681ESA

While the MAX1681ESA is now classified by the manufacturer as obsolete, engineers can consider alternative models in the Maxim Integrated family when designing new applications or sourcing replacement parts. Notable alternatives include:

MAX1680: Nearly identical in topology and features, but focused on lower switching frequencies (125kHz/250kHz). It remains suited for designs tolerating larger capacitors and lower EMI, with similar output characteristics.

MAX860/MAX861: Designed for lower output current applications (up to 50mA) in smaller uMAX packages—ideal for ultra-compact, low-power designs.

MAX629/MAx774: For requirements beyond the charge pump’s voltage or current, Maxim’s inductive switching regulators offer enhanced voltage range and efficiency for larger or more demanding loads.

Procurement teams should consider both device specifications and package compatibility when evaluating replacements, as well as compliance with environmental regulations (such as RoHS) which the MAX1681ESA does not meet.

Conclusion

The MAX1681ESA serves as a valuable inductorless DC-DC converter solution for engineers seeking efficient, compact voltage inversion or doubling capabilities within a 2V–5.5V input range and 125mA output current. Its frequency-selectable architecture, logic-level shutdown, and straightforward capacitor-based design simplify integration into a range of analog, interface, and biasing applications. While this device is now obsolete, understanding its capabilities and design requirements aids both product selection and retrofit projects, and guides the transition toward current-generation alternatives within Maxim Integrated’s power management portfolio. Careful consideration of capacitor selection, frequency operation, and advanced topologies such as cascaded or parallel configurations ensures optimal performance for demanding electronics design challenges.