LP5951MFX-1.8 Linear Voltage Regulator Equivalent & Substitute Parts

Part Overview

The LP5951MFX-1.8 is a linear voltage regulator IC manufactured by Texas Instruments, designed to deliver a fixed 1.8V output at 150mA in a SOT-23-5 surface mount package. This device functions as a positive fixed output regulator with integrated enable control and comprehensive protection features including over-current, over-temperature, and short-circuit protection.

The LP5951MFX-1.8 is classified as obsolete. Equivalent and substitute parts are necessary for new designs, production continuity, and applications requiring active product status with current manufacturing support and RoHS3 compliance.

Key Parameters

Parameter	Value	Unit
Output Voltage (Fixed)	1.8	V
Output Current	150	mA
Input Voltage (Maximum)	5.5	V
Dropout Voltage @ 150mA	0.35	V
Quiescent Current	55	µA
PSRR (100Hz to 10kHz)	60dB ~ 50dB	dB
Operating Temperature Range	-40 to 125	°C
Package Type	SOT-23-5	—
Control Features	Enable	—
Protection Features	Over Current, Over Temperature, Short Circuit	—

Substitute Part Grouping Explanation

Substitute parts for the LP5951MFX-1.8 are categorized based on the following critical parameters that determine functional equivalence:

Primary Substitution Criteria:

Output voltage: 1.8V fixed
Output current capability: 150mA minimum
Input voltage maximum: 5.5V or greater
Package type: SOT-23-5 or equivalent surface mount
Control features: Enable function
Protection: Over-current and over-temperature protection
Operating temperature range: -40°C to 125°C or compatible subset
Product status: Active or equivalent active status
RoHS compliance: RoHS3 compliant preferred for new designs

Grouping Logic:

Group 1: Direct Texas Instruments Equivalents — Parts with identical electrical specifications, same manufacturer, active product status, and RoHS3 compliance. These are pin-compatible and parameter-matched replacements.

Group 2: Microchip Technology Equivalents — Parts meeting all electrical specifications with improved performance characteristics (lower dropout voltage, lower quiescent current, or higher PSRR). Same package family (TSOT-23-5), active status, and RoHS3 compliant.

Group 3: Rohm Semiconductor Alternatives — Parts with higher output current capability (200mA to 360mA) and compatible electrical specifications. Suitable for applications requiring higher current delivery while maintaining 1.8V output and SOT-23-5 package compatibility. Some parts marked "Not For New Designs."

Group 4: ABLIC Inc. Alternatives — Parts with 300mA output current capability and ultra-low quiescent current (1µA). Obsolete status limits suitability for new designs but available for legacy system support.

Parameter Comparison

Part Number	Manufacturer	Output Voltage (V)	Output Current (mA)	Input Voltage Max (V)	Dropout @ 150mA (V)	Quiescent Current (µA)	Package	Product Status	RoHS Status
LP5951MFX-1.8	Texas Instruments	1.8	150	5.5	0.35	55	SOT-23-5	Obsolete	Non-compliant
LP5951MFX-1.8/NOPB	Texas Instruments	1.8	150	5.5	0.35	55	SOT-23-5	Active	ROHS3 Compliant
LP5951MF-1.8/NOPB	Texas Instruments	1.8	150	5.5	0.35	55	SOT-23-5	Active	ROHS3 Compliant
MIC5306-1.8YD5-TR	Microchip Technology	1.8	150	5.5	0.2	25	TSOT-23-5	Active	ROHS3 Compliant
MIC5365-1.8YD5-TR	Microchip Technology	1.8	150	5.5	0.38	39	TSOT-23-5	Active	ROHS3 Compliant
MIC5365-1.8YD5-T5	Microchip Technology	1.8	150	5.5	0.38	39	TSOT-23-5	Active	ROHS3 Compliant
BU18SD2MG-MTR	Rohm Semiconductor	1.8	200	6.0	0.29	80	5-SSOP	Active	ROHS3 Compliant
BU18UC3WG-TR	Rohm Semiconductor	1.8	300	5.5	0.32	90	5-SSOP	Not For New Designs	ROHS3 Compliant
BU18UB3WG-TR	Rohm Semiconductor	1.8	360	3.6	0.32	90	5-SSOP	Not For New Designs	ROHS3 Compliant
S-1135A18-M5T1U	ABLIC Inc.	1.8	300	5.5	—	1	SOT-23-5	Obsolete	ROHS3 Compliant
S-1135D18-M5T1U	ABLIC Inc.	1.8	300	5.5	—	1	SOT-23-5	Obsolete	ROHS3 Compliant

Engineering Selection Recommendations

For New Designs:

Select LP5951MFX-1.8/NOPB or LP5951MF-1.8/NOPB as primary replacements. Both parts are manufactured by Texas Instruments with active product status and RoHS3 compliance. These parts provide identical electrical specifications to the obsolete LP5951MFX-1.8, ensuring direct functional equivalence. The /NOPB designation indicates lead-free, RoHS-compliant manufacturing.

For Applications Requiring Lower Dropout Voltage or Lower Quiescent Current:

Select MIC5306-1.8YD5-TR from Microchip Technology. This part delivers 0.2V dropout voltage (compared to 0.35V) and 25µA quiescent current (compared to 55µA), resulting in improved power efficiency. The TSOT-23-5 package is mechanically and electrically compatible with SOT-23-5. Active product status and RoHS3 compliance support long-term availability.

For Applications Requiring Higher Output Current:

Select BU18SD2MG-MTR (200mA) or BU18UC3WG-TR (300mA) from Rohm Semiconductor. Both parts maintain 1.8V fixed output with improved dropout voltage characteristics. BU18SD2MG-MTR carries active product status and automotive AEC-Q100 qualification. BU18UC3WG-TR is marked "Not For New Designs" but remains available for legacy system support.

For Legacy System Support:

The obsolete S-1135A18-M5T1U and S-1135D18-M5T1U from ABLIC Inc. remain available in inventory. These parts feature ultra-low 1µA quiescent current and 300mA output capability. Use only when replacement with active-status parts is not feasible.

Compliance Considerations:

All recommended active-status parts carry RoHS3 compliance and REACH unaffected status, meeting current regulatory requirements for new product designs and manufacturing.

Frequently Asked Questions (FAQ)

Q: Can LP5951MFX-1.8/NOPB directly replace the obsolete LP5951MFX-1.8?

A: Yes. LP5951MFX-1.8/NOPB is the direct equivalent with identical electrical specifications, same SOT-23-5 package, and pin-compatible configuration. The primary difference is active product status and RoHS3 compliance. No circuit modifications are required.

Q: What is the difference between LP5951MFX-1.8/NOPB and LP5951MF-1.8/NOPB?

A: Both parts are electrically and functionally identical with matching specifications. The difference is packaging format: LP5951MFX-1.8/NOPB is supplied in Tape & Reel (TR), while LP5951MF-1.8/NOPB is supplied in Cut Tape (CT) & Digi-Reel. Select based on procurement and assembly requirements.

Q: Are Microchip MIC5306 and MIC5365 parts interchangeable with the LP5951MFX-1.8?

A: Both Microchip parts are functionally equivalent for 1.8V, 150mA applications. MIC5306-1.8YD5-TR offers superior dropout voltage (0.2V vs. 0.35V) and lower quiescent current (25µA vs. 55µA). MIC5365 series provides higher PSRR (80dB vs. 60dB at 1kHz). The TSOT-23-5 package is mechanically compatible with SOT-23-5. No circuit modifications required, but verify PCB footprint compatibility.

Q: Can Rohm BU18 series parts replace the LP5951MFX-1.8 in existing designs?

A: Rohm BU18 series parts are suitable for new designs requiring higher output current (200mA to 360mA). All maintain 1.8V fixed output and SOT-23-5/5-SSOP package compatibility. However, verify that your application can tolerate the higher quiescent current (80µA to 90µA vs. 55µA) and different control features (ON/OFF vs. Enable). BU18SD2MG-MTR is recommended for new designs; BU18UC3WG-TR and BU18UB3WG-TR are marked "Not For New Designs."

Q: What is the significance of the /NOPB suffix on Texas Instruments parts?

A: The /NOPB suffix indicates "No Plumbum" (lead-free) manufacturing, confirming RoHS3 compliance and lead-free solder compatibility. This designation is required for new product designs and manufacturing in regions with RoHS regulations.

Q: Why do some substitute parts have different package designations (TSOT-23-5 vs. SOT-23-5)?

A: TSOT-23-5 is a thin variant of SOT-23-5 with identical pin configuration and electrical performance. Both packages are mechanically compatible on standard PCB footprints. Verify footprint dimensions with component datasheets if space constraints are critical.

Q: Is the ABLIC S-1135 series suitable for new designs?

A: No. The S-1135A18-M5T1U and S-1135D18-M5T1U are classified as obsolete. These parts are available only for legacy system support and production continuity. For new designs, select active-status alternatives such as LP5951MFX-1.8/NOPB or Microchip MIC5306/MIC5365 series.

Q: What are the key parameters to verify when substituting the LP5951MFX-1.8?

A: Verify the following parameters match or exceed the original specification: (1) Output voltage: 1.8V fixed, (2) Output current: minimum 150mA, (3) Input voltage maximum: 5.5V or greater, (4) Package type: SOT-23-5 or compatible, (5) Control features: Enable function, (6) Protection: Over-current and over-temperature, (7) Operating temperature: -40°C to 125°C or compatible range, (8) Product status: Active preferred for new designs.

Q: How does quiescent current affect component selection?

A: Quiescent current (Iq) represents the regulator's internal current consumption when delivering no load current. Lower Iq reduces standby power consumption. The LP5951MFX-1.8 specifies 55µA. Microchip MIC5306 offers 25µA (lower power), while Rohm BU18 series specifies 80µA to 90µA (higher power). Select based on application power budget and standby requirements.

Q: What is PSRR and why does it matter?

A: Power Supply Rejection Ratio (PSRR) measures the regulator's ability to reject input voltage ripple and noise. Higher PSRR values indicate better noise rejection. The LP5951MFX-1.8 specifies 60dB at 100Hz declining to 50dB at 10kHz. Microchip MIC5365 offers superior PSRR (80dB at 1kHz), beneficial for noise-sensitive applications. Rohm BU18 series specifies 68dB to 70dB at 1kHz.

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