6N138 6N139 Photocoupler Datasheet - 8-Pin DIP Package - High Gain Split Darlington - English Technical Document

1. Product Overview

The 6N138 and 6N139 are high-performance, low-input-current photocouplers featuring a split Darlington phototransistor output stage. These devices are designed to provide a very high current transfer ratio (CTR), enabling reliable signal transmission with minimal input drive current. They are housed in a standard 8-pin Dual In-line Package (DIP), with options for wide lead spacing and surface-mount configurations. The primary function is to provide electrical isolation between input and output circuits, protecting sensitive logic from voltage spikes and ground loops.

1.1 Core Advantages and Target Market

The key advantage of these photocouplers is their exceptionally high typical CTR of 2000%, which allows for direct interfacing with low-current logic signals without requiring additional amplification. They are certified by major international safety agencies (UL, cUL, VDE, SEMKO, NEMKO, DEMKO, FIMKO) and offer a high isolation voltage of 5000 Vrms. These features make them ideal for industrial, telecommunications, and computing applications where noise immunity, safety isolation, and signal integrity are critical. The target markets include industrial automation, power supply feedback loops, digital interface isolation, and communication line receivers.

2. Technical Parameter Deep Dive

This section provides an objective interpretation of the key electrical and optical parameters specified in the datasheet.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage may occur. The input infrared LED has a maximum continuous forward current (IF) of 20 mA and can withstand a 1 A peak transient current for very short pulses (<1 µs). The output transistor's maximum collector current (IO) is 60 mA, and its power dissipation (PO) is limited to 100 mW. The device can operate in an ambient temperature range from -40°C to +85°C. The 5000 Vrms isolation voltage is a key safety parameter, tested with all input pins shorted together and all output pins shorted together.

2.2 Electrical Characteristics

The electrical characteristics are guaranteed over the commercial temperature range of 0°C to 70°C. For the input LED, the typical forward voltage (VF) is 1.3V at IF = 1.6 mA. The output section parameters differ slightly between the 6N138 and 6N139. The 6N139 typically offers a lower logic high output current (IOH) leakage of 0.01 µA compared to the 6N138's 100 µA under the same condition (IF=0mA, VCC=18V). The supply current in the logic low state (ICCL) is typically 0.6 mA for both when the LED is driven with 1.6 mA.

2.3 Transfer Characteristics

The Current Transfer Ratio (CTR) is the most critical parameter, defined as (IC / IF) * 100%. The 6N139 has a minimum CTR of 400% at IF=0.5mA and 500% at IF=1.6mA. The 6N138 has a minimum CTR of 300% at IF=1.6mA. The typical value for both is 2000-2500%, indicating high sensitivity. The logic low output voltage (VOL) is specified under various load conditions, with a maximum of 0.4V, ensuring compatibility with standard TTL and CMOS logic levels.

2.4 Switching Characteristics

Switching speed is dependent on the input drive current and load resistance. Propagation delay times (tPLH, tPHL) are provided for specific test conditions. For example, the 6N139 with IF=0.5mA and RL=4.7kΩ has a typical tPHL of 5 µs and tPLH of 16 µs. Increasing IF to 12mA with RL=270Ω drastically improves speed to 0.2 µs and 1.7 µs, respectively. The 6N138 is generally slower under its specified test condition (IF=1.6mA, RL=2.2kΩ). Common Mode Transient Immunity (CMTI) is specified at a minimum of 1000 V/µs for both logic high and low states, indicating good noise rejection against fast voltage transients across the isolation barrier.

3. Mechanical and Package Information

The devices are provided in a standard 8-pin DIP package. The pin configuration is as follows: Pin 1: No Connection, Pin 2: Anode, Pin 3: Cathode, Pin 4: No Connection, Pin 5: Ground (Gnd), Pin 6: Output (Vout), Pin 7: Base (VB), Pin 8: Supply Voltage (VCC). The base pin (7) provides access to the phototransistor's base, which can be used to connect a speed-up resistor or capacitor to trade off bandwidth for stability. Package options include standard DIP, wide-lead bend (0.4-inch spacing), and surface-mount lead forms (S and low-profile S1).

4. Soldering and Assembly Guidelines

The absolute maximum rating for soldering temperature is 260°C for 10 seconds. This is typical for wave or reflow soldering processes. Standard precautions for handling electrostatic discharge (ESD)-sensitive devices should be observed. The devices should be stored in conditions within the specified storage temperature range of -55°C to +125°C.

5. Packaging and Ordering Information

The part number follows the format: 6N13XY(Z)-V. 'X' is the part number (8 for 6N138, 9 for 6N139). 'Y' indicates the lead form option: none for standard DIP (45 units/tube), 'M' for wide lead bend (45 units/tube), 'S' for surface mount, 'S1' for low-profile surface mount. 'Z' specifies tape and reel option for SMD parts: 'TA' or 'TB' (1000 units/reel). 'V' is an optional suffix for VDE approval. Users must select the correct combination based on assembly requirements.

6. Application Suggestions

6.1 Typical Application Scenarios

The datasheet lists several key applications: Digital logic ground isolation, RS-232C line receivers, low input current line receivers, microprocessor bus isolation, and current loop receivers. Their high CTR makes them excellent for interfacing microcontroller GPIO pins directly, isolating sensor signals in noisy environments, or providing galvanic isolation in serial communication lines like RS-232 or RS-485.

6.2 Design Considerations

1. Input Current Limiting: An external series resistor must be used to limit the LED forward current (IF) to a value within the absolute maximum and desired operating range. The required resistor value is (Vdrive - VF) / IF. 2. Output Loading: The output transistor acts as a current sink. The load resistor (connected between VCC and pin 6) should be chosen to set the desired output voltage swing and switching speed. A smaller resistor increases speed but also increases power consumption. 3. Speed vs. Stability: Connecting a resistor (typically 10kΩ to 1MΩ) from the base pin (7) to ground can improve stability and noise immunity but will reduce the CTR and slow down the switching speed. A capacitor can be added in parallel for further filtering. 4. Power Supply Decoupling: Good practice involves placing a 0.1 µF ceramic capacitor close to the VCC pin (8) to ground to suppress noise.

7. Technical Comparison and Differentiation

The primary differentiation of the 6N138/6N139 family lies in its split Darlington configuration and very high CTR. Compared to standard single-transistor photocouplers (e.g., 4N25 series), these devices offer significantly higher sensitivity, allowing them to be driven directly by low-current CMOS logic. Compared to newer digital isolators, they offer a simpler, analog solution that is highly cost-effective for applications requiring basic isolation without the need for ultra-high speed or complex protocols. The availability of a base pin provides designers with a unique degree of freedom to tailor the frequency response and noise immunity.

8. Frequently Asked Questions (Based on Technical Parameters)

Q1: What is the main difference between the 6N138 and 6N139?
A1: The key differences are in their electrical specifications. The 6N139 generally offers better performance: a higher minimum CTR (500% vs. 300% at IF=1.6mA), lower output leakage current in the off state, and slightly different switching characteristics under test. The 6N138 is the lower-specification variant.

Q2: How do I choose the value for the input current limiting resistor?
A2: Determine the required forward current (IF) from your application (e.g., 1.6 mA for a good balance of speed and CTR). Measure or use the typical VF from the datasheet (1.3V). If your drive voltage is 5V, the resistor R = (5V - 1.3V) / 0.0016A = 2312.5Ω. A standard 2.2kΩ resistor would be a suitable choice.

Q3: Why is my photocoupler switching slowly?
A3: Switching speed is heavily influenced by IF and the load resistor RL. To increase speed, you can: a) Increase the LED drive current (IF). b) Decrease the value of the load resistor (RL) on the output collector. c) Optionally, use the base pin (7) with a small resistor to ground to remove stored charge, but this will reduce CTR.

Q4: What does \"Common Mode Transient Immunity\" mean?
A4: It measures the device's ability to ignore fast voltage spikes that appear equally on both the input and output sides of the isolation barrier. A high CMTI (like 1000 V/µs) means the output will not falsely toggle due to such noise, which is crucial in noisy power environments.

9. Practical Design Case

Case: Isolating a Microcontroller UART Signal for RS-232 Communication.
A microcontroller's 3.3V UART TX line needs to be isolated before connecting to an RS-232 transceiver chip on a different ground plane. A 6N139 can be used. The microcontroller pin drives the LED through a 1kΩ resistor (IF ~ (3.3V-1.3V)/1k = 2mA). The output collector (pin 6) is connected to the RS-232 chip's input pin via a 4.7kΩ pull-up resistor to the RS-232 chip's VCC (5V). The base pin (7) is left open or connected to ground via a large resistor (e.g., 1MΩ) for stability. This simple circuit provides robust isolation, protects the microcontroller from ground shifts or surges on the RS-232 line, and maintains signal integrity.

10. Principle of Operation

The device operates on the principle of optoelectronic coupling. An electrical current applied to the input pins (Anode and Cathode) causes the infrared Light Emitting Diode (LED) to emit light. This light travels across a transparent isolation gap and strikes the photosensitive base region of a split Darlington pair of silicon phototransistors. The incident light generates a base current, which is amplified by the two transistor stages, resulting in a much larger collector current at the output. The \"split\" configuration typically means the base of the first transistor is accessible (pin 7), allowing external biasing. The complete electrical isolation between the input LED and the output transistors is provided by the molded plastic package, which has high dielectric strength.

11. Industry Trends and Context

Photocouplers like the 6N138/139 represent a mature and reliable isolation technology. Current trends in signal isolation include the growth of digital isolators based on CMOS and RF or capacitive coupling, which offer superior speed, power efficiency, and integration (multiple channels in one package). However, photocouplers maintain strong advantages in certain areas: they offer very high working voltage isolation (several kV), excellent common-mode transient immunity, simplicity, and robustness against high-voltage dv/dt stresses. They are often preferred in high-noise industrial environments, power supply feedback loops, and applications where proven reliability and safety certifications are paramount. The development of new LED and detector materials continues to improve the speed and CTR of optocouplers, ensuring their relevance alongside newer technologies.