Eliminating Output Capacitors with XC6501 Series (Marking AB4): A High-Speed LDO Design Guide

Executive Summary: What is the XC6501 Series (Marking AB4)?

The XC6501 Series (Marking AB4) is a high-speed CMOS Low Dropout (LDO) voltage regulator designed specifically for stable operation without the need for an external output capacitor (CL-less). Specifically, the "AB4" laser marking identifies the XC6501B351 variant, which provides a fixed 3.5V output in a compact SOT-25 or USP-4 package.

• Market Position: High-efficiency, space-saving regulator for ultra-compact mobile and wearable electronics.

• Top Features: Capacitor-less stability, low quiescent current (13µA), and high ripple rejection (50dB @ 1kHz).

• Primary Audience: Ideal for IoT hardware engineers, smartphone PCB designers, and procurement managers looking to reduce Bill of Materials (BOM) count.

• Supply Status: Active (Note: Often misidentified in the field as an NXP component due to package similarities).

1. Technical Specifications & Performance Analysis

1.1 Core Architecture (CMOS LDO)

The XC6501 utilizes a high-speed CMOS process that allows for a very low dropout voltage and fast transient response. The standout architectural feature is its internal phase compensation circuit, which ensures the regulator remains stable even when the output capacitor is omitted. This "CL-less" capability is a game-changer for designs where PCB real estate is at a premium.

1.2 Key Electrical Characteristics

Engineers must adhere to the following parameters to ensure long-term reliability:

• Input Voltage Range: 1.4V to 6.0V (Absolute maximum; exceeding this will damage the CMOS gate).

• Fixed Output Voltage: 3.5V (Specific to the AB4 marking).

• Max Output Current: 200mA.

• Dropout Voltage: Remarkably low at 150mV (typical) at a 100mA load.

• Quiescent Current: Only 13µA, making it suitable for always-on battery-powered sensors.

1.3 Interfaces and Connectivity

As a linear regulator, the interface is straightforward but includes a critical CE (Chip Enable) pin. *   CE Function: Allows the system MCU to shut down the regulator to save power. *   Logic Level: High = Active; Low = Standby.

2. Pinout, Package, and Configuration

2.1 Pin Configuration Guide

The XC6501 Series (Marking AB4) is most commonly found in the SOT-25 (SOT-23-5) package.

1. VIN: Input Voltage (1.4V - 6.0V).

2. VSS: Ground.

3. CE: Chip Enable (Active High).

4. NC: No Connection (Internal).

5. VOUT: Regulated 3.5V Output.

2.2 Naming Convention & Ordering Codes

Understanding the Torex naming convention is vital for procurement: 

XC6501: Series Name. 

B: High Speed / 3-terminal + CE. 

35: 3.5V Output. 

1: 0.1V increments. 

MR-G: SOT-25 Package / Halogen Free. 

Marking "AB4": The specific production code for the 3.5V fixed variant.

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

Pro Tip: While the XC6501 is stable without an output capacitor, adding a 0.1µF ceramic capacitor at the input is still recommended if the regulator is located far from the main power source.

3.1 Hardware Implementation

• Bypass Capacitors: Though "CL-less," a small output capacitor (0.1µF to 1.0µF) can still be used to further improve load transient response if space allows.

• PCB Layout: Keep the VIN and VOUT traces wide to minimize parasitic inductance.

• Thermal Management: The SOT-25 package has a power dissipation limit (approx. 250mW-600mW depending on the board). At 200mA with a 2V drop (5V in, 3V out), the chip will dissipate 400mW—ensure adequate copper pour for heat sinking.

3.2 Common Design Challenges

• Manufacturer Confusion: Users frequently mistake "AB4" for an NXP part. Fix: Verify the Torex logo (a stylized 'T' or 'X') on the datasheet. NXP does not use this specific marking sequence for their LDOs.

• Input Voltage Spikes: Since the max input is 6.0V, using this with a 4-cell AA battery pack (which can reach 6.4V when fresh) or a noisy USB line can be risky. Fix: Use a Zener or TVS diode if the input rail is unregulated.

4. Typical Applications & Use Cases

4.1 Real-World Example: Wireless Sensor Node

In a smart home environment, a sensor node using an ESP32 or nRF52 often requires a stable 3.5V rail for specific analog sensors. The XC6501 (AB4) allows the designer to remove the bulky output capacitor, reducing the total footprint by roughly 15%, which is critical for coin-cell operated devices.

5. Alternatives and Cross-Reference Guide

If the XC6501 is unavailable, consider these high-performance alternatives:

• Direct Replacements: NXP LD6806 (Note: requires a capacitor) or Texas Instruments TLV70035 (3.5V variant).

• Better Performance: Onsemi NCP160 (Higher PSRR for RF applications).

• Cost-Effective Options: Microchip MIC5365 (Very similar footprint and specs).

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between XC6501 Series (Marking AB4) and NXP regulators?

• A: While the packages look identical, the XC6501 is a Torex Semiconductor part. The "AB4" marking is specific to Torex's internal coding for the 3.5V output version.

• Q: Can XC6501 Series (Marking AB4) be used in Automotive designs?

• A: Unless specified as AEC-Q100 qualified, this series is intended for consumer, industrial, and communication equipment.

• Q: Is the output capacitor really optional?

• A: Yes. The XC6501 is designed for "CL-less" operation, meaning it remains stable without an output capacitor, though adding one doesn't hurt performance.

• Q: How do I identify the manufacturer if the logo is small?

• A: Look for the marking "AB4". Cross-reference this with the Torex XC6501 datasheet. NXP markings usually follow different alphanumeric patterns (e.g., three characters starting with 'p').

7. Resources

• Datasheet: Visit the Torex Semiconductor official website and search for "XC6501".

• CAD Models: Available on SnapEDA or Ultra Librarian for SOT-25 footprints.

• Evaluation Boards: Contact Torex distributors for the CL-less LDO demo kit.