Analog Devices Inc./Maxim Integrated MAX690ACSA+

Overview

The MAX690ACSA+ from Maxim Integrated is a microprocessor supervisory circuit providing precision reset generation, battery-backup switchover, watchdog timer, and power-fail detection for +5V systems. It asserts a 200ms active-low RESET pulse when VCC drops below 4.65V, switches CMOS RAM to VBATT during brownout, monitors PFI against a 1.25V reference, and triggers reset if WDI is un-toggled for 1.6s - used in battery-powered controllers and critical μP monitoring.

For engineers reviewing the MAX690ACSA+ datasheet, MAX690ACSA+ pinout, MAX690ACSA+ application, or MAX690ACSA+ equivalent, key selection criteria include guaranteed RESET assertion down to VCC = 1V, 50nA quiescent current in battery-backup mode, ±2% power-fail accuracy (shared with MAX802L), and compatibility with lithium backup batteries up to 4.80V.

Technical Context

The MAX690ACSA+ integrates four independent functional blocks: a precision voltage monitor with 4.65V threshold and 40mV hysteresis, a monostable reset generator producing 200ms pulses, a 1.6s watchdog timer cleared by WDI edges or RESET assertion, and a battery switchover circuit using dual PMOS switches (5Ω for VCC→VOUT, 80Ω for VBATT→VOUT) with 40mV hysteresis to prevent chatter.

Its power-fail comparator operates independently with a fixed 1.25V internal reference and 400ns response time; PFO is forced low during battery-backup mode. The device guarantees RESET output validity down to VCC = 1V and supports bidirectional μP reset I/O via external 4.7kΩ series resistor per application note.

Key Specifications

Pinout & Package

MAX690ACSA+ is housed in an 8-pin SOIC package (SO) with gull-wing leads, 1.27mm pitch, and JEDEC MS-012AC outline. Pin 1 is marked by a beveled corner or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar microprocessor supervisory applications.

Compared with MAX802LCSA+, MAX690ACSA+ offers identical core supervision but slightly looser power-fail accuracy tolerance; versus MAX706PESA+, it provides essential +5V-specific features - battery switchover, PFI/PFO, and 4.65V reset - making it irreplaceable in legacy 5V industrial controllers.

Availability

MAX690ACSA+ is available at Aetrix Electronics and suitable for battery-powered controllers, intelligent instruments, and critical μP power monitoring requiring stable component supply across industrial temperature ranges (0°C to +70°C).

Supply support for MAX690ACSA+ 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 systems.

The MAX690A family was engineered specifically for robust +5V microprocessor supervision - integrating reset, watchdog, battery switchover, and power-fail detection into a single 8-pin SOIC to reduce BOM count and improve system-level reliability.

FAQ

What is the guaranteed minimum VCC level at which MAX690ACSA+ asserts RESET?

The MAX690ACSA+ guarantees RESET assertion down to VCC = 1.0V, ensuring reliable microprocessor hold condition even during deep brownout events. This specification is verified across the full 0°C to +70°C operating range and enables deterministic reset behavior in systems where supply voltage may sag significantly before complete collapse. The MAX690ACSA+ maintains this guarantee while drawing only 200µA total supply current.

Does MAX690ACSA+ support lithium backup batteries, and what is the maximum allowable voltage?

Yes, MAX690ACSA+ supports lithium backup batteries with open-circuit voltages from 2.8V to 4.80V, as specified in Table 2 of the datasheet. Voltages exceeding 4.80V risk forward-biasing the internal substrate diode (D1) when VCC is near the 4.65V reset threshold, causing unintended current flow. The device's 80Ω VBATT-to-VOUT switch and 50nA backup-mode quiescent current optimize compatibility with standard CR2032 and BR2032 cells.

How does the watchdog timer in MAX690ACSA+ interact with the RESET output?

The MAX690ACSA+ watchdog timer triggers a 200ms active-low RESET pulse if WDI remains static (high or low) for 1.6s. RESET assertion clears the watchdog timer, and the timer only resumes counting once RESET goes high. If WDI is tied high or low continuously, the device generates repeating RESET pulses every ~1.8s (1.6s timeout + 200ms pulse width). This behavior is inherent to the MAX690ACSA+ architecture and requires no external components.

Can MAX690ACSA+ monitor negative supply rails, and how is this implemented?

Yes, MAX690ACSA+ can monitor negative rails using an external resistor network that references PFI to ground, as shown in Figure 7 of the datasheet. When the negative rail degrades (e.g., rises toward zero), the voltage at PFI falls below 1.25V, causing PFO to go low. Accuracy depends on resistor tolerance and PFI input bias (±25nA), but the MAX690ACSA+ itself requires no modification - the function leverages its existing 1.25V comparator without additional ICs.

Is MAX690ACSA+ compatible with microprocessors having bidirectional reset pins, such as the Motorola 68HC11?

Yes, MAX690ACSA+ can interface safely with bidirectional reset pins using a 4.7kΩ series resistor between RESET and the μP I/O, as documented in Figure 8 of the datasheet. This resistor prevents contention when the μP attempts to drive reset low while MAX690ACSA+ drives it high. The MAX690ACSA+ RESET output remains fully functional - delivering 3.2mA sink capability and guaranteed low voltage - while eliminating indeterminate logic states in shared-reset configurations.