LTR-301 Phototransistor Datasheet - Side-Looking Package - Clear Transparent - 30V Collector-Emitter - English Technical Document

1. Product Overview

The LTR-301 is a silicon NPN phototransistor designed for infrared detection applications. It is housed in a side-looking plastic package with a clear transparent lens, optimized for sensing infrared radiation, typically at a wavelength of 940nm. This component is engineered to convert incident infrared light into a corresponding electrical current at its collector terminal.

The primary function of this device is as a light-to-current transducer. When infrared light strikes the photosensitive base region of the transistor, it generates electron-hole pairs. This photogenerated current acts as the base current, which is then amplified by the transistor's current gain (beta), resulting in a significantly larger collector current. This amplified signal is easier to interface with subsequent electronic circuitry like microcontrollers or amplifiers.

Its core advantages include a wide operating range for the collector current, which provides design flexibility across different sensitivity requirements. The integrated lens enhances its sensitivity by focusing incoming light onto the active area. The side-looking package orientation is particularly useful for applications where the light source is parallel to the PCB surface, such as in slot-type interrupters or reflective sensors. The clear package allows for broad spectral response, though it is optimized for infrared.

The target market for this component includes consumer electronics, industrial automation, security systems, and various sensing applications. Typical uses are in object detection, position sensing, rotary encoders, paper detection in printers, and touchless switches.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. Operation under these conditions is not guaranteed.

• Power Dissipation (P_D): 100 mW. This is the maximum total power the device can dissipate as heat. Exceeding this limit risks thermal runaway and failure.
• Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter pins when the base is open (no light).
• Emitter-Collector Voltage (V_ECO): 5 V. The maximum reverse voltage allowed between emitter and collector.
• Operating Temperature (T_A): -40°C to +85°C. The ambient temperature range for reliable operation.
• Storage Temperature (T_stg): -55°C to +100°C.
• Lead Soldering Temperature: 260°C for 5 seconds at a distance of 1.6mm from the package body. This is critical for wave or hand soldering processes.

2.2 Electrical & Optical Characteristics

These parameters are specified at an ambient temperature (T_A) of 25°C and define the device's performance under specific test conditions.

• Collector-Emitter Breakdown Voltage, V_(BR)CEO: 30 V (min). Tested with I_C = 1mA and no illumination (E_e = 0 mW/cm²). This confirms the Absolute Maximum Rating.
• Emitter-Collector Breakdown Voltage, V_(BR)ECO: 5 V (min). Tested with I_E = 100µA and no illumination.
• Collector-Emitter Saturation Voltage, V_CE(SAT): 0.4 V (max). This is the voltage drop across the transistor when it is fully "on" (saturated) with I_C = 0.1mA under an irradiance of 1 mW/cm². A low V_CE(SAT) is desirable for switching applications to minimize power loss.
• Rise Time (T_r) & Fall Time (T_f): 10 µs (typ) and 15 µs (typ) respectively. These parameters define the switching speed. Measured with V_CC=5V, I_C=1mA, and R_L=1kΩ. The asymmetry is common in phototransistors due to charge storage effects.
• Collector Dark Current (I_CEO): 100 nA (max). This is the leakage current that flows from collector to emitter when the device is in complete darkness (E_e = 0 mW/cm²) and V_CE = 10V. A low dark current is crucial for good signal-to-noise ratio, especially in low-light sensing.

3. Binning System Explanation

The LTR-301 employs a binning system for its key parameter, the On-State Collector Current (I_C(ON)). Binning is a quality control process where components are sorted based on measured performance into specific ranges or "bins." This ensures consistency for the end-user.

The parameter binned is I_C(ON), measured under standardized conditions: V_CE = 5V, E_e = 1 mW/cm², and λ = 940nm. The device is sorted into one of eight bins (A through H) based on its measured current output.

• Bin A: 0.20 - 0.60 mA
• Bin B: 0.40 - 1.08 mA
• Bin C: 0.72 - 1.56 mA
• Bin D: 1.04 - 1.80 mA
• Bin E: 1.20 - 2.40 mA
• Bin F: 1.60 - 3.00 mA
• Bin G: 2.00 - 3.84 mA
• Bin H: 2.56 mA (Min)

Design Implication: When designing a circuit, you must account for the bin you are using. For example, choosing a device from Bin H guarantees a higher minimum sensitivity than one from Bin A. This is critical for setting comparator thresholds or analog gain stages. If your design requires a minimum signal level, you must specify a bin code that meets that requirement.

4. Performance Curve Analysis

The datasheet provides several characteristic curves that illustrate how parameters vary with operating conditions.

4.1 Collector Dark Current vs. Ambient Temperature (Fig. 1)

This graph shows I_CEO increasing exponentially with temperature. At 85°C, the dark current can be orders of magnitude higher than at 25°C. This is a fundamental semiconductor behavior (leakage currents double approximately every 10°C). Design Consideration: In high-temperature environments, the increased dark current can be mistaken for a genuine light signal. Circuits may need temperature compensation or a higher detection threshold.

4.2 Collector Power Derating vs. Ambient Temperature (Fig. 2)

This curve shows the maximum allowable power dissipation (P_C) decreasing linearly as ambient temperature (T_A) increases above 25°C. At 85°C, the maximum power dissipation is significantly reduced. Design Consideration: Ensure the operating power (V_CE * I_C) remains below the derated line for the maximum expected T_A to prevent thermal overload.

4.3 Rise/Fall Time vs. Load Resistance (Fig. 3)

This graph demonstrates the trade-off between switching speed and signal amplitude. As the load resistor (R_L) increases, the rise and fall times also increase. A larger R_L gives a larger output voltage swing (ΔV = I_C * R_L) but slows down the response. Design Consideration: For high-speed applications (e.g., data communication), use a smaller R_L. For maximizing voltage output in slower applications (e.g., ambient light sensing), a larger R_L can be used.

4.4 Relative Collector Current vs. Irradiance (Fig. 4)

This is a transfer characteristic, showing that the collector current (I_C) is approximately linear with incident light power (irradiance, E_e) over a certain range when V_CE is fixed (5V). This linearity is key for analog light measurement applications.

4.5 Sensitivity Diagram (Fig. 5)

This polar diagram illustrates the angular sensitivity of the device. The phototransistor is most sensitive to light arriving perpendicular to the lens (0°). Sensitivity decreases as the angle of incidence increases, typically falling to 50% (half-angle) at a specific angle (e.g., ±10° to ±20° as suggested by the graph). Design Consideration: This defines the field of view. Proper mechanical alignment between the emitter and detector is crucial. It can also be used to reject stray light from unwanted directions.

5. Mechanical & Package Information

The device uses a side-looking, clear transparent plastic package. The term "side-looking" indicates that the photosensitive area is on the side of the package, parallel to the leads, rather than on the top. This is ideal for sensing in the plane of the PCB.

Key Dimensional Notes:

• All dimensions are in millimeters, with a general tolerance of ±0.25mm unless specified otherwise.
• Lead spacing is measured at the point where the leads exit the package body, which is critical for PCB footprint design.
• The package includes a lens molded into the plastic to enhance optical collection efficiency.

Polarity Identification: The longer lead is typically the Collector. However, always refer to the package drawing in the full datasheet for definitive identification, often indicated by a flat side on the package or a marker on the lens.

6. Soldering & Assembly Guidelines

The critical parameter provided is the lead soldering temperature: 260°C maximum for 5 seconds, measured at a point 1.6mm (0.063") from the package body. This is a standard rating for through-hole components.

Process Recommendations:

• Wave Soldering: Ensure the temperature profile does not exceed the specified limit at the lead/package junction. Preheat is essential to minimize thermal shock.
• Hand Soldering: Use a temperature-controlled iron. Apply heat to the lead/pad junction quickly and efficiently, avoiding prolonged contact with the component body.
• Cleaning: Use cleaning agents compatible with the plastic package material. Avoid ultrasonic cleaning unless verified to be safe for the device.
• Storage: Store in a dry, anti-static environment within the specified temperature range (-55°C to +100°C) to prevent moisture absorption (which can cause "popcorning" during reflow) and electrostatic discharge damage.

7. Application Notes & Design Considerations

7.1 Typical Application Circuits

1. Digital Switch (Object Detection): The phototransistor is used in series with a pull-up resistor (R_L) connected to V_CC. The collector node is connected to a digital input (e.g., microcontroller GPIO or Schmitt trigger). In darkness, I_C is very low (I_CEO), so the output is pulled high to V_CC. When illuminated, I_C increases, pulling the output voltage low towards V_CE(SAT). The value of R_L is chosen based on the desired switching speed (see Fig. 3) and the required logic low voltage level: R_L ≈ (V_CC - V_CE(SAT)) / I_C(ON).

2. Analog Light Meter: The phototransistor is connected in a similar configuration, but the collector voltage is fed to an Analog-to-Digital Converter (ADC) input. Due to the approximate linearity shown in Fig. 4, the ADC reading can be correlated to light intensity. A higher R_L provides greater voltage swing for better ADC resolution but reduces bandwidth.

7.2 Critical Design Factors

• Source Matching: For optimal performance, pair the phototransistor with an infrared LED emitter at the same peak wavelength (940nm).
• Electrical Loading: The phototransistor is a current source. The load resistor converts this current to a voltage. Choose R_L to balance signal level, speed, and power consumption.
• Ambient Light Rejection: The device responds to all light, not just IR. Use optical filters (black IR-transmissive plastic) or modulated (pulsed) light sources with synchronous detection to reject ambient 50/60Hz light noise and DC ambient light.
• Biasing: Ensure the operating V_CE is within the recommended range (well below 30V) and that the power dissipation (V_CE * I_C) is within limits, especially at high temperature.

8. Technical Comparison & Differentiation

Compared to a photodiode, a phototransistor provides internal gain, yielding a much larger output signal for the same light input, simplifying subsequent amplifier design. However, this comes at the cost of slower response times (µs vs. ns for photodiodes) and higher temperature sensitivity of the dark current.

The LTR-301's specific differentiators are its side-looking package, which is not as common as top-looking types, and its clear lens (vs. tinted or black). The clear lens offers a broader spectral response, which can be an advantage or disadvantage depending on the need for visible light rejection. The detailed binning system allows for precise selection of sensitivity, which is a key advantage for volume production requiring consistent performance.

9. Frequently Asked Questions (FAQs)

Q: What is the difference between Bins? Which one should I choose?

A: Bins categorize devices by their sensitivity (I_C(ON)). Choose a bin based on your circuit's required minimum signal current. For higher sensitivity/longer range, choose a higher bin (e.g., H). For cost-sensitive applications where lower sensitivity is acceptable, a lower bin (e.g., A) may suffice.

Q: Why is my output signal noisy or unstable?

A: This is often caused by ambient light (sunlight, fluorescent lamps) or electrical noise. Solutions include: 1) Using a modulated IR source and filtering the received signal. 2) Adding a capacitor (10nF - 100nF) in parallel with the load resistor R_L to filter high-frequency noise (this will slow the response). 3) Ensuring proper shielding and grounding.

Q: Can I use this with a visible light source?

A: Yes, the clear package means it will respond to visible light as well as IR. However, its sensitivity is typically characterized and optimized for 940nm IR. The response to visible light will be different and not guaranteed by the datasheet.

Q: How do I calculate the responsivity or sensitivity?

A: Responsivity is not directly given. You can estimate it from the I_C(ON) specification. For example, for Bin E (min 1.20mA at 1 mW/cm²), the minimum responsivity is approximately 1.20 mA / (1 mW/cm²) = 1.20 mA/(mW/cm²). Note that this is a crude estimate as the active area is not specified.

10. Practical Use Case Example

Scenario: Paper Detection in a Printer. A reflective sensor is built using the LTR-301 and an IR LED. They are placed side-by-side facing the paper path. The IR LED constantly emits light. When no paper is present, the light reflects off a distant surface weakly, and the phototransistor output is low. When paper passes directly beneath the sensor, it reflects a strong signal back to the phototransistor, causing a sharp increase in I_C and a corresponding voltage drop at the collector node.

Design Steps:

1. Select a bin (e.g., Bin D or E) that provides enough signal current from the expected paper reflection.

2. Choose R_L. For a 5V supply and a target logic-low voltage of 0.8V, and using I_C(ON,min) for Bin D (1.04mA): R_L ≤ (5V - 0.8V) / 1.04mA ≈ 4.0kΩ. A standard 3.3kΩ resistor would be suitable, providing good signal margin.

3. Connect the collector node to a comparator or microcontroller interrupt pin. Set a threshold voltage at the comparator's inverting input (e.g., 2.5V) to reliably detect the presence/absence of paper.

4. Mechanically align the sensor so the IR LED's beam and the phototransistor's field of view intersect at the paper's surface.

11. Operational Principle

A phototransistor is fundamentally a bipolar junction transistor (BJT) where the base current is generated by light instead of an electrical connection. In an NPN phototransistor like the LTR-301:

1. Infrared photons with sufficient energy (wavelength ≤ 1100nm for silicon) penetrate the clear package and are absorbed in the semiconductor material, primarily in the base-collector depletion region.
2. This absorption creates electron-hole pairs.
3. The electric field in the reverse-biased base-collector junction sweeps these carriers apart: electrons to the collector, holes to the base.
4. The accumulation of holes in the base region lowers the base-emitter potential barrier, effectively acting as a positive base current (I_B).
5. This photogenerated base current is then amplified by the transistor's current gain (β or h_FE), resulting in a collector current: I_C = β * I_B(photo). This is the source of the device's gain.

The side-looking package positions this photosensitive junction on the side, with a lens to focus incoming light for improved efficiency.

12. Technology Trends

Phototransistors like the LTR-301 represent a mature, cost-effective technology. Current trends in optosensing include:

• Integration: Moving towards integrated solutions that combine the photodetector, amplifier, digitizer, and logic (e.g., I²C output light sensors) on a single chip, reducing external component count and simplifying design.
• Miniaturization: Development of phototransistors in smaller surface-mount device (SMD) packages for space-constrained applications.
• Specialization: Devices with built-in spectral filters (e.g., for RGB sensing or specific IR bands) or daylight blocking filters are becoming more common for robust operation in varied environments.
• Speed: While phototransistors are generally slower than photodiodes, there is ongoing development to improve their bandwidth for data communication applications (e.g., IR remote control, simple optical data links).

Despite these trends, discrete phototransistors remain highly relevant due to their simplicity, low cost, high sensitivity, and the design flexibility they offer in configuring gain and bandwidth through external components.