Analog Devices Inc./Maxim Integrated MAX1681ESA+

Product Summary MAX1681ESA Charge Pump IC from Analog Devices Inc./Maxim Integrated

The MAX1681ESA from Analog Devices Inc./Maxim Integrated is a compact, frequency-selectable, switched-capacitor charge pump designed for engineers seeking efficient voltage conversion without the need for inductors. Encapsulated in a standard 8-SOIC (3.90mm width) package, the MAX1681ESA provides versatile solutions for generating negative or doubled voltages in circuits requiring up to 125mA output current. This device is engineered for low-board-space designs, replacing more complex inductor-based regulators, with minimal external components and simple configuration.

Highlighted Attributes of the MAX1681ESA Charge Pump

The MAX1681ESA distinguishes itself through several critical features:

Digital frequency selection (500kHz or 1MHz), enabling the use of smaller capacitors for compact PCB layouts.

Output configurations as either a voltage inverter or doubler, with a selectable, ratiometric positive or negative fixed output.

High efficiency (up to 90%) and low output resistance (typically 3.5Ω), allowing for minimal voltage drop even at full 125mA load.

Low quiescent current (<1μA in shutdown), ensuring negligible standby power consumption.

Wide input voltage range (3.0V to 5.5V in inverter mode; 4.0V to 5.5V in doubler mode), supporting a range of battery-operated and logic-level systems.

Logic-controlled shutdown feature for system-level power management integration.

Industrial temperature operation (-40°C to +85°C), suitable for demanding environments.

Functional Diagram and Operational Modes of the MAX1681ESA

The MAX1681ESA operates as a switched-capacitor charge pump, utilizing two principal modes: inverter and doubler. In inverter mode, a positive input voltage is inverted to produce a negative output, making the device suitable for generating local negative supplies for analog and mixed-signal subsystems. In doubler mode, the input voltage is multiplied, providing an output close to twice the input potential, used in boosting supplies for specific logic or interface circuits.

Frequency selection is handled via the FSEL pin, allowing engineers to choose between 500kHz and 1MHz operation. This flexibility is vital for designers balancing capacitor size, switching losses, and layout constraints. Additionally, the SHDN pin provides shutdown control, putting the device into a low-power state by disabling the output. This logic input is especially beneficial in portable or “sleep mode” applications where energy savings and system control are priorities.

Electrical Specifications and Performance Evaluation of the MAX1681ESA

From a performance standpoint, the MAX1681ESA is optimized for stable operation across a variety of input voltages and capacitive load configurations. The device maintains a regulated output with minimal resistance-added drop: at 125mA load, the output drop is typically less than 0.44V due to the low 3.5Ω effective output resistance (using recommended capacitor values with low ESR).

Supply current scales with switching frequency, supporting careful trade-offs between dynamic power dissipation and physical capacitance selection. For instance, operating at 500kHz consumes 10-18mA typical supply current, while at 1MHz, supply current increases to 20-36mA. Output ripple voltage and source resistance are tightly linked to both the capacitor selection (C1 and C2) and switching frequency.

The device exhibits robust efficiency (80–90%) under typical operating conditions, with minimal shutdown leakage current. The MAX1681ESA is also designed for tolerances in real-world circuits, rated for reliable function from -40°C to +85°C, and is insensitive to moderate temperature and supply variations.

Design Factors and Application Tips for Selecting the MAX1681ESA

When specifying the MAX1681ESA for a new design, engineers should consider the following:

Choose inverter or doubler configuration based on required output polarity and magnitude.

Ensure the input voltage falls within device recommendations: 3.0V–5.5V for inverter mode and 4.0V–5.5V for doubler mode.

Evaluate the output load profile: for applications drawing less than 125mA, output voltage drop and ripple will be minimal with recommended capacitors.

Integrate SHDN and FSEL control into system logic or microcontroller GPIO as needed for dynamic system control and efficiency management.

Consider parallel or cascaded operation for higher current or multi-stage voltage needs, keeping in mind cumulative output resistance and efficiency effects.

The device excels in applications such as op-amp power supplies, local analog negative rails, MOSFET bias voltage generators, and interface power systems – especially where board space, cost and simplicity are pressing constraints.

Guidelines for Choosing Capacitors and PCB Layout for the MAX1681ESA

Capacitor selection is paramount for optimal MAX1681ESA performance. The part is validated with 2.2μF (or greater) low-ESR capacitors for C1 (the charge-pump capacitor) and C2 (the output capacitor). Low-ESR types such as multilayer ceramic chip capacitors from reputable brands (e.g., AVX, Nichicon, TDK, Vishay) are recommended to minimize ripple and loss. Engineers are encouraged to scale capacitance up for maximum load/current, lower ESR, or where supply noise immunity is mission-critical.

The input supply should be well bypassed with a capacitor (typically equal in value to C1), placed close to the IN pin, to ensure supply stability and to filter switching transients. Thoughtful PCB layout minimizing trace inductance and maximizing ground plane area will enhance performance, reduce EMI, and improve transient response.

For higher output voltage or current, cascading or paralleling devices is possible, with proportional changes in output resistance and efficiency to be accounted for in system design.

Comparison and Recommended Use Cases for the MAX1681ESA

Compared with inductor-based converters, the MAX1681ESA streamlines implementation where minimal external components and small footprint are required. The device is particularly well-suited for designs constrained by space or electromagnetic compatibility (EMC), where inductors may introduce cost, complexity, or noise.

Use cases include:

Providing negative rails in mixed-signal or sensor systems.

Boosting supply for legacy interface ICs or specialized analog blocks.

Supplying MOSFET gate drive voltages in compact battery-powered platforms.

Serving as a low-ripple, low-EMI alternative to pulse inductor regulators in portable and consumer electronics.

Alternative and Replacement Options for the MAX1681ESA

Given the MAX1681ESA’s status as an obsolete product, selection engineers and procurement teams should consider proven alternatives with similar functional characteristics. Potential replacement models to evaluate include:

MAX1680ESA (Maxim Integrated): Similar topology, with selectable lower switching frequencies (125kHz/250kHz), best suited where lower supply current is prioritized and slightly larger capacitors are acceptable.

MAX860/MAX861 (Maxim Integrated): Lower current devices in smaller packages, recommended for applications with output currents below 50mA.

Other inductorless switched-capacitor voltage converters from Maxim Integrated or Analog Devices, cross-referenced for required voltage, current, form factor, and compliance.

When qualifying alternatives, engineers must verify pin compatibility, input/output ranges, and frequency options to ensure a seamless transition in existing systems.

Conclusion

The MAX1681ESA charge pump from Analog Devices Inc./Maxim Integrated offers an effective, compact solution for generating inverting or doubled voltages within space- and cost-sensitive designs. Its high-frequency operation, logic-controlled shutdown, and flexible configuration and control make it a strong fit for analog power rails, interface supplies, and bias generator roles. While now obsolete, the MAX1681ESA’s family and functional equivalents remain valuable options for new and legacy designs alike. Proper capacitor selection, attention to layout, and careful comparison to potential replacements will ensure optimal performance and long-term supply assurance in engineering applications.