Texas Instruments UCC3946D

Overview

UCC3946D from Texas Instruments is a microprocessor supervisor IC with integrated watchdog timer, programmable reset threshold (1.235 V ±2%), programmable reset period (160–260 ms with 64 nF), and programmable watchdog period (1.12–2.08 s with 64 nF), operating from 2 V to 5.5 V supply and supporting reset validity down to 1 V - used in embedded systems requiring reliable power-on and fault recovery.

For engineers reviewing the UCC3946D datasheet, UCC3946D pinout, UCC3946D application, or UCC3946D equivalent, key selection criteria include its 2% accurate reset threshold, 18 µA max supply current, glitch-immune reset detection, independent RES/WDO outputs, and support for external capacitor-based timing programming.

Technical Context

The UCC3946D implements dual independent timing blocks: a precision 400 nA current source charges CRP to set the reset period (TRP = 3.125 × CRP ms), and a separate 25 nA current source charges CWP to set the watchdog period (TWP = 25 × CWP ms). Both thresholds reference an internal 1.235 V bandgap.

Reset assertion is triggered when RTH voltage falls below 1.235 V for duration/voltage exceeding filter limits per Figure 1; WDO asserts low if WDI fails to toggle within TWP. Watchdog is disabled during active reset, and RES remains valid even at VDD = 1 V.

Key Specifications

Pinout & Package

UCC3946D is packaged in an 8-pin SOIC (D package), with gull-wing leads, 1.27 mm pitch, and JEDEC MS-012AC footprint.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MAX692AESA+ and TPS3808G33DBVR, the UCC3946D uniquely supports fully programmable reset voltage (via R1/R2), independently programmable reset and watchdog periods (via CRP/CWP), and guaranteed reset validity down to 1 V - making it optimal for multi-rail, noise-prone, or wide-VDD industrial designs.

Availability

UCC3946D is available at Aetrix Electronics and suitable for industrial PLC controllers, medical infusion pumps, automotive body control modules, and smart energy meters requiring stable component supply and long-term manufacturability.

Supply support for UCC3946D 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

Texas Instruments is a global semiconductor company headquartered in Dallas, Texas, delivering analog and embedded processing solutions for industrial, automotive, and personal electronics markets.

The UCC3946D belongs to TI's legacy microprocessor supervisor family, designed specifically for robust, capacitor-programmable reset and watchdog supervision in harsh environments with wide supply ranges and noise immunity requirements.

FAQ

What is the minimum supply voltage at which UCC3946D guarantees valid reset output?

The UCC3946D guarantees RES assertion remains valid down to VDD = 1 V, as specified in the Electrical Characteristics table under "Minimum VDD (Note 1)". This ensures reliable reset behavior during deep brownout conditions - a key differentiator versus many supervisors that require ≥2 V to function. The UCC3946D achieves this via optimized internal biasing and low-threshold comparators, enabling use in battery-failover or supercapacitor-backed systems where voltage decay extends below 2 V.

How do I program the reset threshold for UCC3946D?

To program the reset threshold for UCC3946D, connect a resistor divider (R1 + R2) between VDD and GND, with the midpoint tied to the RTH pin. The threshold voltage is calculated as V_RESET = 1.235 V × (R1 + R2)/R2. For example, to set a 3.0 V threshold, use R1 = 1.43 MΩ and R2 = 1 MΩ. Resistor values in the megaohm range minimize current draw while maintaining accuracy - and a momentary switch across R2 enables manual reset. This configuration is explicitly validated in the UCC3946D application diagram (Figure 4).

Can UCC3946D's RES and WDO outputs be directly wired together?

Yes, UCC3946D's RES and WDO outputs can be directly wired together, but with caveats: both are open-drain, so the dominant low state will prevail if one asserts low while the other is high. However, during mismatched states, additional current flows from VDD through the pull-up into the low-output pin - potentially exceeding 1 mA depending on pull-up value. TI recommends using an OR gate (e.g., 74LVC1G32) if current budget or noise immunity is critical. This behavior is documented in the "Connecting WDO to RES" section of the UCC3946D datasheet (SLUS247B).

What capacitor types are recommended for RP and WP pins on UCC3946D?

For RP and WP pins on UCC3946D, NPO (C0G) ceramic capacitors are strongly recommended due to their low leakage (<1 nA), minimal voltage coefficient, and stable capacitance over temperature and time. X7R ceramics may be used if leakage is verified <10 nA, but Y5V or electrolytic types must be avoided - high leakage invalidates the precision 400 nA (RP) and 25 nA (WP) current sources, causing significant timing errors in TRP and TWP. Layout best practices also require short traces and shielding from noisy signals near these nodes.

Does UCC3946D support watchdog disable during reset?

Yes, UCC3946D automatically disables the watchdog function whenever RES is asserted - i.e., during power-on reset or any subsequent undervoltage reset event. This prevents spurious WDO assertion while the microprocessor is held in reset and not yet executing code. Once RES deasserts and the system begins booting, the watchdog timer resumes counting from zero upon first WDI toggle. This behavior is explicitly stated in the UCC3946D description: "The watchdog function will be disabled during reset conditions."