Monolithic Power Systems Inc. NB671AGQ-Z

Overview

NB671AGQ-Z from Monolithic Power Systems is a fully integrated 5V–22V input, synchronous step-down DC/DC converter delivering 6A continuous and 9A peak output current with 0.604V–5.5V adjustable output voltage. It employs Constant-On-Time (COT) control for fast transient response, integrates low-R_DS(ON) MOSFETs (30 mΩ HS / 15 mΩ LS), and features open-drain power-good signaling, internal 7ms soft-start, and full protection (OCP, OVP, UVP, thermal shutdown). It targets compact, high-efficiency point-of-load regulation in space-constrained consumer and industrial power rails.

For engineers reviewing the NB671AGQ-Z datasheet, NB671AGQ-Z pinout, NB671AGQ-Z application, or NB671AGQ-Z equivalent, key selection considerations include its QFN16 (3mm×3mm) package thermal performance (θ_JA = 70°C/W), COT-based stability with ceramic output capacitors, valley-current OCP threshold of 8.5A (typ), and VCC UVLO at 4.6V - all critical for reliable 6A buck design in laptop, tablet, and distributed power systems.

Technical Context

The NB671AGQ-Z implements a proprietary COT control architecture with internal ramp compensation, enabling stable operation with pure ceramic output capacitors without external ESR dependency. Its valley-current sensing OCP uses the low-side MOSFET's R_DS(ON) as the current-sense element, triggering cycle-by-cycle shutdown when inductor valley current exceeds 8.5A (typ).

Power sequencing is managed via an enable pin with 1.25V threshold and 100mV hysteresis, while the open-drain PG signal asserts after FB reaches 95% V_REF with 0.5ms delay and de-asserts at 85% V_REF. Internal 5V LDO (VCC) powers control circuitry and requires ≥1µF local decoupling; bootstrap capacitor between BST and SW enables high-side gate drive.

Key Specifications

Pinout & Package

Package: QFN16 (3mm × 3mm, 0.5mm pitch), thermally enhanced with exposed pad (pins 8, 9) connected to PGND for optimal heat dissipation and low-inductance grounding.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar synchronous step-down converter applications.

Compared with MP2451DT-LF-Z and RT6253BGQW, the NB671AGQ-Z uniquely combines 6A/9A current capability, COT-based transient performance, and integrated output discharge in a single QFN16 package - making it optimal for new 6A PoL designs where simplicity, thermal robustness, and sequencing integrity are prioritized.

Availability

NB671AGQ-Z is available at Aetrix Electronics and suitable for laptop computer power delivery, tablet SoC regulation, and networking PHY supply requiring stable component supply across production lifecycles.

Supply support for NB671AGQ-Z 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

Monolithic Power Systems (MPS) is a fabless semiconductor company specializing in high-performance analog and power ICs, with over two decades of leadership in DC/DC conversion, motor drivers, and LED lighting controllers.

The NB671AGQ-Z belongs to MPS's high-current synchronous buck converter product line, designed specifically for compact, high-efficiency point-of-load regulation in consumer portable electronics and infrastructure equipment where thermal density and transient response are critical.

FAQ

What is the recommended input capacitor configuration for NB671AGQ-Z?

Use ≥22µF total input capacitance with low-ESR ceramic capacitors (e.g., three 10µF X7R 0805 parts) placed as close as possible to VIN and PGND pins. Wide copper pours and multiple vias to the PGND plane are mandatory to minimize high-frequency loop inductance and prevent switching noise coupling into the FB node.

Can NB671AGQ-Z operate with 100% duty cycle?

No - the NB671AGQ-Z cannot achieve true 100% duty cycle due to minimum off-time limitation (300 ns typ) and bootstrap capacitor recharge requirement. The maximum practical duty cycle is ~95% at VIN = 5.5V and VOUT = 5V; for near-unity ratios, consider using an LDO post-regulator or selecting a different topology.

How does the internal output discharge function behave during thermal shutdown?

During thermal shutdown, the NB671AGQ-Z disables both high-side and low-side switches, then activates the internal 6Ω discharge MOSFET between VOUT and PGND. Discharge continues until VOUT falls below ~0.4V, independent of EN state - ensuring safe de-energization even during fault conditions.

Is external ramp compensation required for stability with 22µF ceramic output capacitors?

No - the NB671AGQ-Z's internal ramp compensation is sufficient for stable operation with typical 22µF X7R ceramic output capacitors (e.g., three 10µF 0805 parts). External ramp is only needed in edge cases involving very low-ESR/ESL layouts or ultra-low output voltages (<0.8V) with high C_OUT (>100µF).

What is the maximum allowable voltage on the BST pin relative to SW?

The BST pin voltage must not exceed V_SW + 5.5V per absolute maximum ratings. Since V_SW swings up to VIN during high-side conduction, the BST capacitor must be rated for ≥VIN + 5.5V (e.g., 25V for 19V input). Use 0.1µF X7R 0603 ceramics with low ESL for reliable bootstrap operation.

Does NB671AGQ-Z support forced PWM mode to eliminate acoustic noise?

No - the NB671AGQ-Z operates exclusively in COT-controlled PWM mode under heavy load and automatically transitions to skip mode at light load. There is no pin or register to force continuous PWM; acoustic noise mitigation requires careful inductor selection (shielded type) and layout optimization to suppress mechanical vibration.

How is the power-good (PG) signal affected when pulled up to a voltage other than VCC?

If PG is pulled up to a voltage >VCC (e.g., 3.3V or 5V rail), it will not de-assert (go low) when EN is low or VIN drops below 1.75V - violating safe power sequencing. MPS explicitly recommends pulling PG to VCC to ensure coordinated shutdown and avoid false "good" assertions during brown-out or disable events.