Monolithic Power Systems Inc. MP28257DD-LF-Z

Overview

MP28257DD-LF-Z from Monolithic Power Systems is a fully integrated synchronous step-down DC/DC converter delivering up to 4A continuous output current, operating from 4.2V to 20V input, with programmable switching frequency and constant-on-time (COT) control for fast transient response. It integrates 120mΩ high-side and 50mΩ low-side MOSFETs, supports output adjustment from 0.815V to 13V, and features power-good indication and comprehensive protection in a 2mm×3mm QFN12 package-used in compact networking power rails.

For engineers reviewing the MP28257DD-LF-Z datasheet, MP28257DD-LF-Z pinout, MP28257DD-LF-Z application, or MP28257DD-LF-Z equivalent, key selection criteria include its COT-based transient performance, internal MOSFET R_DS(on) values, thermal shutdown threshold (150°C), PG timing delay (500μs), and latch-off OCP behavior under sustained overcurrent.

Technical Context

The MP28257DD-LF-Z employs constant-on-time (COT) control architecture, where ON-time is determined by input voltage and external resistor R7, enabling input-voltage feedforward and stable operation across wide VIN ranges without loop compensation. Its internal high-side and low-side MOSFETs eliminate external switch requirements and reduce conduction loss.

It operates in continuous conduction mode (CCM) at heavy loads and automatically transitions to skip mode at light loads to maintain efficiency, with dead-time control preventing shoot-through. Protection logic includes cycle-by-cycle OCP with 50μs hold-off, latch-off SCP, UVLO (3.1V rising), and non-latching thermal shutdown with 25°C hysteresis.

Key Specifications

Pinout & Package

The MP28257DD-LF-Z is housed in a thermally enhanced 2mm × 3mm QFN12 package with exposed thermal pad, RoHS-compliant and lead-free.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MP28257DD-LF-Z, MP2384DJ-LF-Z offers higher current headroom but sacrifices latch-off protection, while TPS544B20RTWR adds digital configurability at the cost of analog simplicity and higher BOM complexity.

Availability

MP28257DD-LF-Z is available at Aetrix Electronics and suitable for networking equipment power, distributed telecom power, industrial PLC I/O modules, and embedded vision system power requiring stable component supply and long-term design-in support.

Supply support for MP28257DD-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 power ICs, with design centers in the US, China, and Taiwan.

The MP28257 belongs to MPS's high-density synchronous buck converter product line, engineered for space-constrained, fast-transient applications in networking, telecom, and industrial automation where minimal external components and robust protection are essential.

FAQ

What is the recommended input capacitor configuration for MP28257DD-LF-Z?

A minimum 22μF X5R/X7R ceramic capacitor rated ≥25V should be placed directly at the IN pin with short, wide traces and multiple vias to GND. For systems with high input ripple current, parallel a second 10μF capacitor. Total RMS current rating must exceed half the maximum output current (≥2A RMS) to ensure reliability under worst-case 2×VOUT input conditions.

Does MP28257DD-LF-Z require an external ramp for ceramic output capacitors?

Yes - when using low-ESR ceramic output capacitors, an external ramp (R4/C4 network) must be added to the FB pin to stabilize COT control in PWM mode. The ramp slope must satisfy equation 9 per the datasheet; omitting it risks subharmonic oscillation and output voltage instability under medium loads.

How does the latch-off OCP behave during a sustained overload condition?

When inductor current exceeds the 5.5–7A limit for five consecutive cycles within a 50μs window, the device enters latch-off mode: HS-FET and LS-FET turn off permanently, PG goes low, and VOUT collapses. Recovery requires full power cycle - no automatic retry - making it suitable for fault-isolation in safety-critical subsystems.

Can MP28257DD-LF-Z operate with an output voltage below 1V?

Yes - the 0.815V feedback reference enables stable outputs as low as 0.815V. For 0.9V output, use R1 = 100kΩ and R2 = 91kΩ (standard 1% values). Layout must minimize FB trace length and avoid noisy nodes, as the 10–50nA FB bias current makes the node sensitive to leakage and coupling.

What is the thermal performance of the QFN12 package under 4A load at 12V input?

At TA = 85°C, VIN = 12V, VOUT = 3.3V, and 4A load on a 4-layer JEDEC-standard PCB, junction temperature rises ~65°C above ambient (θJA = 70°C/W), reaching ~150°C - triggering thermal shutdown. To sustain full load, add 2–4 thermal vias under the exposed pad and use ≥1in² copper pour for heatsinking.

Is MP28257DD-LF-Z pin-compatible with other MPS buck converters?

No - MP28257DD-LF-Z has a unique QFN12 pinout (e.g., BST at Pin 3, PG at Pin 6) incompatible with MP2384 or MPQ4571. Substitution requires PCB redesign. The "NOT RECOMMENDED FOR NEW DESIGNS" note in the datasheet refers to functional obsolescence, not physical compatibility.