Analog Devices Inc./Maxim Integrated MAX6391KA26

Overview

MAX6391KA26 from Maxim Integrated is a dual-voltage microprocessor supervisory circuit with sequenced active-low open-drain reset outputs, factory-set VCC reset threshold of 2.63V (min 2.55V, max 2.70V), guaranteed reset validity down to VCC = 1.0V, and fixed 140ms (min) RESET1 timeout. It monitors master supply VCC and adjustable secondary supply RESET IN2 (625mV internal reference), enabling controlled power-up sequencing in multivoltage systems such as industrial controllers and set-top boxes.

For engineers reviewing the MAX6391KA26 datasheet, MAX6391KA26 pinout, MAX6391KA26 application, or MAX6391KA26 equivalent, key selection criteria include its dual-supply monitoring capability, 140ms minimum RESET1 delay, internal 47kΩ pullup resistors on both RESET outputs, immunity to short negative VCC transients, and SOT23-8 package compatibility with space-constrained embedded designs.

Technical Context

The MAX6391KA26 implements two independent voltage-monitoring comparators: one for VCC (with factory-trimmed 2.63V threshold) and one for RESET IN2 (fixed 625mV reference). RESET1 asserts solely on VCC undervoltage and deasserts ≥140ms after VCC exceeds threshold; RESET2 asserts if either VCC or RESET IN2 falls below their thresholds and deasserts ≥140ms (or capacitor-adjustable) after both recover - always following RESET1 during power-up and preceding it during power-down.

Its BiCMOS process enables 15µA typical supply current and operation down to VCC = 1.0V while maintaining valid reset assertion. The CSRT pin selects between fixed 140ms RESET2 timeout (when tied to VCC) or user-adjustable timeout (via external capacitor), and internal 47kΩ pullup resistors on RESET1/R1 and RESET2/R2 support flexible VOH level translation without external components.

Key Specifications

Pinout & Package

MAX6391KA26 is housed in an 8-pin SOT23 package (JEDEC MO-178 compliant, 1.95mm × 2.80mm footprint, 1.30mm height), suitable for high-density PCB layouts and reflow-compatible assembly.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual-voltage supervisory applications.

Compared with MAX6391KA23 and TPS3808G125, the MAX6391KA26 uniquely delivers factory-trimmed 2.63V VCC supervision with built-in secondary supply monitoring, sequenced reset timing, and internal pullups - eliminating discrete components and layout complexity required by alternatives.

Availability

MAX6391KA26 is available at Aetrix Electronics and suitable for industrial controllers, set-top boxes, and servers/workstations requiring stable component supply, long-term lifecycle support, and guaranteed reset performance across -40°C to +85°C.

Supply support for MAX6391KA26 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 semiconductor company specializing in precision analog, mixed-signal, and power management ICs for industrial, communications, and computing applications.

The MAX6391/MAX6392 product line was designed specifically for multivoltage microprocessor systems requiring coordinated, sequenced reset generation - addressing the need for reliable power-up/down sequencing without external timing components or logic.

FAQ

What is the factory-set VCC reset threshold voltage for MAX6391KA26?

The MAX6391KA26 has a factory-trimmed VCC reset threshold of 2.63V (nominal), with a guaranteed range of 2.55V to 2.70V across temperature and process variation. This value is laser-trimmed during manufacturing and cannot be adjusted. The threshold applies exclusively to the VCC pin; RESET IN2 uses a separate fixed 625mV internal reference. This precise 2.63V setting makes MAX6391KA26 ideal for supervising standard 3.3V logic rails with appropriate noise margin.

Does MAX6391KA26 support adjustable RESET2 timeout, and how is it configured?

Yes, MAX6391KA26 supports both fixed and adjustable RESET2 timeout. To select fixed 140ms (min) timeout, connect the CSRT pin directly to VCC. To enable user-adjustable timeout, connect an external capacitor from CSRT to ground; the timeout period is calculated as tRP2 ≈ 2.08 × 10⁶ × CCSRT (seconds), yielding ~3.1ms with 1500pF. The internal 600nA current source ramps voltage across CCSRT until it reaches the 1.25V internal reference, triggering RESET2 deassertion. This flexibility allows fine-tuning of peripheral initialization delays without adding external timers.

How does MAX6391KA26 ensure proper reset sequencing between RESET1 and RESET2 during power-up and power-down?

MAX6391KA26 guarantees strict sequencing: during power-up, RESET1 deasserts ≥140ms after VCC exceeds 2.63V, and RESET2 deasserts only after RESET1 is deasserted plus its own timeout (140ms min or capacitor-adjusted). During power-down, RESET2 asserts before RESET1 whenever VCC falls below threshold - confirmed by tRD2 < tRD1 timing specifications. This behavior is hardwired in the BiCMOS logic and does not rely on external components, ensuring deterministic sequencing critical for avoiding bus contention or latch-up in multivoltage systems using MAX6391KA26.

Can MAX6391KA26 monitor a 1.8V supply as the secondary voltage, and what resistor values are needed?

Yes, MAX6391KA26 can monitor a 1.8V supply on RESET IN2 using an external resistor divider. With the internal 625mV reference, calculate R3 and R4 using VRST = VTH2 × (R3 + R4)/R4. For VRST = 1.8V and VTH2 = 0.625V, selecting R4 = 500kΩ yields R3 ≈ 940kΩ. This configuration scales the 1.8V input down to 625mV at RESET IN2, enabling accurate undervoltage detection. The MAX6391KA26's high-impedance input draws only 50nA, minimizing divider current and power loss - essential for battery-operated systems.

What is the minimum VCC voltage at which MAX6391KA26 guarantees valid reset operation?

MAX6391KA26 guarantees valid reset assertion and deassertion functionality down to VCC = 1.0V across the full -40°C to +85°C temperature range. This specification ensures reliable brownout detection and controlled shutdown even during severe supply droop, such as in battery-powered or unregulated adapter scenarios. Unlike many supervisors that cease operation near 1.8V, the MAX6391KA26 maintains comparator accuracy, timer function, and output drive capability at 1.0V - a key differentiator for mission-critical applications requiring fail-safe behavior under marginal power conditions.