LTR-306 Phototransistor Data Sheet - Side-Looking Package - Collector Current up to 2.4mA - Voltage 30V - English Technical Document

1. Product Overview

The LTR-306 is a silicon NPN phototransistor housed in a side-looking plastic package. This component is designed to detect infrared radiation, converting incident light into an electrical current at its collector terminal. Its primary function is as a light sensor in various electronic circuits, where it acts as a light-controlled switch or an analog light intensity sensor. The side-looking package orientation is a key feature, meaning the sensitive area faces perpendicular to the direction of the leads, which is optimal for applications where the light source is positioned to the side of the PCB.

The core advantages of this device include its wide operating range of collector current, which provides design flexibility across different sensitivity requirements. The integrated lens is engineered to enhance sensitivity by focusing incoming infrared light onto the active semiconductor region. Furthermore, the use of a low-cost plastic package makes it an economical choice for high-volume consumer and industrial applications where cost-effectiveness is crucial without sacrificing essential performance parameters.

The target market for the LTR-306 encompasses a broad spectrum of applications requiring reliable infrared detection. This includes, but is not limited to, object detection and counting systems, slot sensors (e.g., in printers and vending machines), tape-end sensors, proximity sensing, and industrial automation equipment. Its robust design and specified performance make it suitable for integration into both simple and complex electronic systems.

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. The maximum power dissipation is 100 mW at an ambient temperature (T_A) of 25°C. The collector-emitter voltage (V_CE) must not exceed 30 V, while the reverse emitter-collector voltage (V_EC) is limited to 5 V. The device is rated for operation within an ambient temperature range of -40°C to +85°C and can be stored in temperatures from -55°C to +100°C. For soldering, the leads can withstand 260°C for 5 seconds when measured 1.6mm from the package body, which is a standard requirement for wave or reflow soldering processes.

2.2 Electrical & Optical Characteristics

All electrical and optical parameters are specified at T_A=25°C, providing a baseline for performance comparison.

• Collector-Emitter Breakdown Voltage (V_(BR)CEO): Minimum 30V (I_C = 1mA, E_e=0). This is the voltage at which the junction breaks down in the absence of light.
• Emitter-Collector Breakdown Voltage (V_(BR)ECO): Minimum 5V (I_E = 100μA, E_e=0). This parameter is important for reverse bias conditions.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): Typically 0.1V, with a maximum of 0.4V (I_C = 100μA, E_e=1 mW/cm²). This low voltage indicates good switching performance when the transistor is fully on.
• Rise Time (T_r) & Fall Time (T_f): Maximum 20 μs each (V_CC=5V, I_C=1mA, R_L=1kΩ). These parameters define the switching speed of the phototransistor in response to a light pulse.
• Collector Dark Current (I_CEO): Maximum 100 nA (V_CE = 10V, E_e=0). This is the leakage current when no light is present, a critical parameter for low-light sensitivity and signal-to-noise ratio.

3. Binning System Explanation

The LTR-306 employs a binning system for its key parameter, the On-State Collector Current (I_C(ON)). Binning is a quality control and sorting process that groups components based on measured performance within specified ranges. This ensures consistency for the end-user. The device is tested under standard conditions (V_CE = 5V, E_e = 1 mW/cm², λ=940nm).

The bins are labeled A through F, each representing a specific range of I_C(ON):

• Bin A: 0.20 mA to 0.60 mA
• Bin B: 0.40 mA to 1.08 mA
• Bin C: 0.72 mA to 1.56 mA
• Bin D: 1.04 mA to 1.80 mA
• Bin E: 1.20 mA to 2.40 mA
• Bin F: Minimum 1.60 mA (no upper limit specified in the provided data)

This system allows designers to select a bin that matches their circuit's required sensitivity. For example, a circuit needing high output current for direct drive of a relay or LED might specify Bin E or F, while a low-power sensing circuit might use Bin A or B to minimize power consumption.

4. Performance Curve Analysis

The data sheet includes several typical characteristic curves that illustrate how key parameters vary with operating conditions. These are essential for understanding device behavior beyond the single-point specifications.

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

This curve shows that the collector dark current (I_CEO) increases exponentially with rising ambient temperature. At -40°C, it is in the picoampere range, but it can rise to around 100 μA at 120°C. This characteristic is crucial for high-temperature applications, as the increasing dark current acts as an offset or noise source, potentially reducing the effective sensitivity and dynamic range of the sensor.

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

This graph demonstrates the derating of the maximum allowable power dissipation as ambient temperature increases. While the device can dissipate 100 mW at 25°C, this rating must be reduced linearly at higher temperatures to prevent thermal runaway and ensure reliability. The curve provides the necessary data for thermal management in the application design.

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

This plot reveals the trade-off between switching speed and load resistance. Rise and fall times (T_r, T_f) increase significantly as the load resistor (R_L) value increases. For a 1kΩ load, the time is around 20μs, but it can exceed 150μs for a 10kΩ load. Designers must choose R_L to balance the need for fast response time against the desired output voltage swing or current level.

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

This is a fundamental transfer characteristic. It shows that the collector current is relatively linear with incident light irradiance (E_e) in the lower range (0-2 mW/cm²) when V_CE is held at 5V. This linear region is where the device can be used for analog light measurement. At higher irradiance levels, the response may begin to saturate.

4.5 Sensitivity Diagram (Fig. 5)

This polar diagram illustrates the angular sensitivity of the phototransistor. The relative sensitivity is plotted against the angle of incident light. It shows that the device has a specific viewing angle where sensitivity is maximum (typically on-axis, 0°). The sensitivity decreases as the light source moves off-axis. This diagram is vital for mechanical alignment in the final application to ensure optimal coupling between the light source and the sensor.

5. Mechanical & Packaging Information

The LTR-306 uses a plastic side-looking package. The dimensions are provided in the data sheet with all measurements in millimeters (inches in parentheses). Key dimensional tolerances are typically ±0.25mm unless otherwise specified. The lead spacing is measured at the point where the leads emerge from the package body, which is critical for PCB footprint design. The package includes a lens molded into the plastic to enhance optical collection efficiency. The side-looking orientation means the active sensing area is on the side of the component, not the top. Clear polarity identification (emitter and collector pins) is provided in the package drawing, which is essential for correct circuit board assembly.

6. Soldering & Assembly Guidelines

The device is suitable for standard PCB assembly processes. The absolute maximum rating specifies that the leads can withstand a soldering temperature of 260°C for 5 seconds when measured 1.6mm (0.063") from the package body. This rating is compatible with typical wave soldering and reflow soldering profiles. It is recommended to follow standard JEDEC or IPC guidelines for moisture sensitivity handling if applicable, though the plastic package is generally robust. During soldering, care should be taken to avoid excessive thermal stress on the package. After assembly, cleaning should be performed with solvents compatible with the plastic material. For storage, the specified range of -55°C to +100°C should be observed, and components are typically supplied in moisture-barrier bags with desiccant.

7. Application Recommendations

7.1 Typical Application Scenarios

• Object Detection/Interruption: Used in pairs with an infrared LED to detect the presence or absence of an object breaking the beam. Common in printers, copiers, vending machines, and industrial counters.
• Proximity Sensing: Detecting the reflection of infrared light from a nearby object.
• Light Barrier/Slot Sensors: Detecting the edge of a tape, paper, or other material.
• Encoders: Used in optical rotary or linear encoders to read patterns on a code wheel or strip.
• Simple Remote Control Receivers: For basic infrared command detection (though dedicated receiver modules are more common for complex protocols).

7.2 Design Considerations

• Biasing: The phototransistor can be used in two common configurations: switch mode (with a pull-up resistor) or analog mode (in a common-emitter amplifier configuration). The value of the load resistor (R_L) critically affects both output voltage/current and response speed (see Fig. 3).
• Ambient Light Immunity: For reliable operation in environments with varying ambient light (e.g., sunlight, room lights), modulation of the infrared source and corresponding filtering or demodulation of the phototransistor signal is often necessary.
• Lens and Alignment: Proper mechanical alignment between the infrared emitter and the phototransistor, considering its side-looking orientation and angular sensitivity pattern (Fig. 5), is essential for maximizing signal strength and reliability.
• Temperature Effects: Design must account for the variation of dark current (Fig. 1) and sensitivity with temperature, especially in outdoor or harsh environments.
• Electrical Noise: In sensitive analog circuits, shielding and proper grounding may be required to prevent noise pickup on the high-impedance phototransistor node.

8. Technical Comparison & Differentiation

Compared to a standard photodiode, a phototransistor like the LTR-306 provides internal gain, resulting in a much higher output current for the same light input. This eliminates the need for an external transimpedance amplifier in many simple detection circuits, reducing component count and cost. Compared to other phototransistors, the LTR-306's specific advantages lie in its side-looking package, which is a distinct mechanical form factor suited for specific optical paths, its wide collector current binning offering flexibility, and its integrated lens for enhanced sensitivity. Its specified rise/fall times and voltage ratings make it a robust general-purpose component for medium-speed applications.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: What does the bin code (A, B, C, etc.) mean for my design?
A: The bin code indicates the guaranteed range of collector current the device will produce under standard test conditions. Choose a bin that provides sufficient signal current for your downstream circuitry (e.g., comparator, microcontroller ADC) while considering power consumption. Higher bins (E, F) give more current but may have slightly higher dark current.

Q: Can I use this sensor in sunlight?
A: Direct sunlight contains a significant amount of infrared radiation that will saturate the sensor and render it unusable for detecting a separate IR source. For outdoor use, optical filtering (an IR-pass filter that blocks visible light) and/or modulated light sources with synchronous detection are mandatory.

Q: Why is the rise/fall time dependent on the load resistor?
A> The phototransistor's speed is limited by the RC time constant formed by its junction capacitance and the load resistance (R_L). A larger R_L creates a larger time constant, slowing down the voltage swing at the collector, hence increasing rise and fall times. For faster response, use a smaller R_L, but this will also reduce the output voltage swing.

Q: How do I interpret the sensitivity diagram?
A: The diagram shows the relative response of the sensor to light coming from different angles. A value of 1.0 (or 100%) is typically at 0° (straight on to the lens). The curve shows how much the signal decreases if the light source is misaligned. Use this to design the mechanical housing and alignment features in your product.

10. Practical Design Example

Scenario: Designing a Paper-Present Sensor for a Printer. An infrared LED is placed on one side of the paper path, and the LTR-306 is placed directly opposite, creating a beam. When no paper is present, the IR light hits the phototransistor, turning it on and pulling its collector voltage low. When paper passes through, it blocks the beam, the phototransistor turns off, and its collector voltage goes high (via a pull-up resistor). This voltage transition is detected by a microcontroller.

Design Steps:
1. Select an appropriate bin (e.g., Bin C) to ensure a strong enough current change to reliably drive the chosen pull-up resistor voltage across the expected operating temperature range.
2. Choose a load/pull-up resistor (R_L). A 4.7kΩ resistor with a 5V supply would give a good voltage swing. Refer to Fig. 3 to ensure the resulting ~100μs response time is fast enough for the paper speed.
3. Mechanically design the holder so the LED and LTR-306 are aligned according to the 0° axis in the sensitivity diagram (Fig. 5). The side-looking package simplifies this as both components can be mounted flat on the PCB facing each other.
4. Implement the IR LED driver with modulation (e.g., a 1kHz square wave) to make the sensor immune to constant ambient IR light. The microcontroller would then read the sensor signal synchronously with this modulation.

11. Operating Principle

A phototransistor is a bipolar junction transistor where the base region is exposed to light. In the LTR-306 (NPN type), incident photons with sufficient energy (infrared light at ~940nm) are absorbed in the base-collector junction, generating electron-hole pairs. These photogenerated carriers are separated by the electric field in the reverse-biased base-collector junction. The resulting photocurrent acts as a base current for the transistor. Due to the transistor's current gain (beta/h_FE), this small photocurrent is amplified, producing a much larger collector current. This internal amplification is the key difference from a photodiode. The collector current is primarily proportional to the intensity of the incident light and the device's gain.

12. Technology Trends

Phototransistors like the LTR-306 represent a mature and cost-effective technology for simple light sensing. Current trends in optoelectronics include the integration of phototransistors with on-chip amplification and signal conditioning circuits to create digital output sensors or analog sensors with improved linearity and temperature compensation. There is also a move towards miniaturization and surface-mount packages with even smaller footprints. For higher-speed and more precise applications, photodiodes with external transimpedance amplifiers or dedicated optical ICs are often preferred. However, for basic, low-cost, medium-speed detection tasks, discrete phototransistors remain highly relevant due to their simplicity, robustness, and low component count.