Monolithic Power Systems Inc. NB621EV-LF-Z

Overview

NB621EV-LF-Z from Monolithic Power Systems is a synchronous step-down DC/DC converter with integrated 5V/3.3V selectable LDO, delivering up to 8A continuous output current from a 5.5V–25V input. It features internal high-side and low-side MOSFETs, fixed 600kHz switching frequency (syncable 300kHz–2MHz), 0.8V–15V adjustable output, and power-good signaling - deployed in notebook I/O power rails where compactness and dual-regulator integration are critical.

For engineers reviewing the NB621EV-LF-Z datasheet, NB621EV-LF-Z pinout, NB621EV-LF-Z application, or NB621EV-LF-Z equivalent, key selection criteria include its integrated LDO voltage select (SELH), bootstrap capacitor requirements (BST pin), thermal shutdown threshold (150°C), and EN/SYNC dual-function pin behavior under external clock synchronization.

Technical Context

The NB621EV-LF-Z operates in peak-current-mode control with internal compensation, enabling fast transient response and stable loop behavior without external Type-II/III networks. Its error amplifier compares FB voltage against a precise 0.808V reference and drives a COMP node that directly modulates high-side switch conduction time.

It integrates two independent regulation paths: a primary synchronous buck stage (8A, 600kHz) and a secondary LDO (100mA, 5V/3.3V selectable via SELH) powered from VIN or REGO. The LDO activates only when ENLDO is high and switches to REGO output when VOUT exceeds LDO's switchover threshold (4.7V/3.15V).

Key Specifications

Pinout & Package

Package: 4mm × 5mm QFN-28 with exposed thermal pad (JEDEC MO-220 VHGD-3 compliant). Thermal resistance θJA = 40°C/W on 4-layer PCB.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar synchronous buck converter with integrated LDO applications.

Compared with NB621EV-LF-Z, NB700 offers superior thermal performance and automotive qualification, while NB693A retains mechanical compatibility and adds EMI mitigation - both eliminate obsolescence risk without redesigning PCB layout or firmware.

Availability

NB621EV-LF-Z is available at Aetrix Electronics and suitable for notebook systems, networking infrastructure, digital set-top boxes, and flat-panel display power management requiring stable component supply during legacy product maintenance and end-of-life transition phases.

Supply support for NB621EV-LF-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 mixed-signal ICs for power management, motion control, and automotive applications.

The NB621 belongs to MPS's high-current synchronous buck converter product line, designed specifically for space-constrained computing and consumer electronics where integration of buck regulation and auxiliary LDO functionality reduces system-level complexity and cost.

FAQ

What does "END OF LIFE, REFER TO NB700 OR NB693A" mean for NB621EV-LF-Z?

This designation indicates NB621EV-LF-Z has reached end-of-life status per Monolithic Power Systems' product lifecycle policy. Production has ceased, and no new wafers are being fabricated. Aetrix Electronics maintains limited legacy stock for last-time buys and supports transition to NB700 (enhanced successor) or NB693A (pin-compatible replacement) with full documentation and cross-reference guidance.

Can NB621EV-LF-Z operate without an external bootstrap capacitor?

No. An external bootstrap capacitor (0.1µF–1µF) between BST and SW pins is mandatory to generate the floating gate drive voltage for the internal high-side MOSFET. Omitting it prevents high-side switch activation, resulting in zero output voltage and potential thermal stress on the low-side FET due to uncontrolled conduction.

How is the LDO output voltage selected between 3.3V and 5V?

The SELH pin determines LDO output: grounding SELH sets regulated output to 5V (±2.5%); pulling SELH to ≥2.5V selects 3.3V (±3.3%). This selection is active only when ENLDO is high. The switchover threshold (4.7V/3.15V) ensures seamless handoff from VIN to REGO when buck output exceeds LDO target.

What is the function of the REGO pin, and when should it be connected?

REGO connects the internal LDO's input to the buck regulator's output instead of VIN. When VOUT exceeds the switchover threshold (4.7V for 5V mode, 3.15V for 3.3V mode), REGO improves LDO efficiency by reducing dropout voltage and power dissipation. It must be connected directly to the buck output node for this feature to activate.

Does NB621EV-LF-Z require external compensation components?

No. The NB621EV-LF-Z uses internal compensation optimized for typical 0.8V–15V output configurations with standard inductor and capacitor values. Only the soft-start capacitor (on SS pin) and feedback resistors (on FB pin) are required external components - no compensation network (e.g., Type-II/III RC) is needed for stable operation.

What protection features are implemented, and how do they behave under fault conditions?

Three layered protections are active: (1) Cycle-by-cycle overcurrent limit trips at 9.5A, forcing hiccup mode if FB drops below 30% of target; (2) Thermal shutdown disables all circuitry at 150°C, re-enabling at 140°C; (3) Input UVLO disables operation below 5.06V (rising) with 450mV hysteresis. All features operate independently without external components.

Is the EN/SYNC pin compatible with CMOS or TTL logic levels?

Yes. EN/SYNC accepts standard CMOS/TTL logic: low ≤0.4V disables the device; high ≥2V enables it. For synchronization, apply a clean 300kHz–2MHz square wave with rise/fall times <50ns and logic-high level ≥2V. Internal hysteresis prevents false triggering near threshold voltages.