LTR-526AB Phototransistor Datasheet - Dark Blue Package - Infrared Detector - English Technical Document

1. Product Overview

The LTR-526AB is a high-performance silicon NPN phototransistor designed for infrared (IR) detection applications. Its core function is to convert incident infrared light into an electrical current. A key feature of this component is its special dark blue plastic package, which acts as a visible light filter. This design significantly reduces the sensor's sensitivity to ambient visible light, making it specifically suitable for applications where the detection signal is purely in the infrared spectrum, thereby improving signal-to-noise ratio and reliability.

Core Advantages: The device offers high photo sensitivity paired with low junction capacitance, enabling fast response times essential for data communication and sensing. Its high cut-off frequency supports applications requiring rapid signal modulation. The combination of fast switching time (rise/fall time typically 50 ns) and robust construction makes it ideal for demanding environments.

Target Market: This phototransistor is targeted at designers and engineers working on infrared-based systems. Typical applications include infrared remote control receivers, proximity sensors, object detection, industrial automation (e.g., counting, sorting), interruptive optical switches (e.g., printers, encoders), and basic optical data links.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

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

• Power Dissipation (P_D): 150 mW maximum. This is the total power the device can safely dissipate as heat, primarily determined by the product of collector-emitter voltage and collector current.
• Reverse Voltage (V_R): 30 V maximum. This is the maximum voltage that can be applied in reverse bias across the emitter-collector junction without causing breakdown.
• Operating Temperature Range (T_A): -40°C to +85°C. The device is guaranteed to function within its specified parameters across this industrial temperature range.
• Storage Temperature Range (T_stg): -55°C to +100°C. The device can be stored without degradation within these limits.
• Lead Soldering Temperature: 260°C for 5 seconds, measured 1.6mm from the package body. This defines the conditions for wave or hand soldering.

2.2 Electro-Optical Characteristics

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

• Reverse Breakdown Voltage (V_(BR)R): Minimum 30 V (I_R = 100 µA). This confirms the device's robust voltage handling capability, aligning with the absolute maximum rating.
• Reverse Dark Current (I_D(R)): Maximum 30 nA (V_R = 10V, E_e = 0 mW/cm²). This is the leakage current when no light is incident. A low value is critical for applications requiring high sensitivity to weak signals, as it represents the noise floor of the detector.
• Open Circuit Voltage (V_OC): Typical 350 mV (λ = 940nm, E_e = 0.5 mW/cm²). This is the voltage generated across the open terminals when illuminated, a parameter more relevant to photovoltaic mode operation but specified here.
• Rise Time (T_r) & Fall Time (T_f): Typical 50 ns each (V_R = 10V, λ = 940nm, R_L = 1 kΩ). These parameters define the switching speed. The 50 ns specification indicates suitability for medium-speed data transmission and fast-sensing applications.
• Short Circuit Current (I_S): 1.7 µA (Min), 2 µA (Typ) (V_R = 5V, λ = 940nm, E_e = 0.1 mW/cm²). This is the photocurrent generated when the output is shorted (or virtually shorted by a transimpedance amplifier). It is a direct measure of responsivity at a given irradiance.
• Total Capacitance (C_T): Maximum 25 pF (V_R = 3V, f = 1 MHz). Low junction capacitance is crucial for achieving high bandwidth and fast response times, as it limits the RC time constant of the circuit.
• Peak Sensitivity Wavelength (λ_SMAX): Typical 900 nm. The device is most sensitive to infrared light at this wavelength. It is well-matched to common infrared emitters (like GaAs LEDs) which typically emit around 880-950 nm.

3. Performance Curve Analysis

The datasheet provides several key graphs illustrating device behavior under varying conditions.

3.1 Dark Current vs. Reverse Voltage

This curve shows that the reverse dark current (I_D) remains very low (in the pA to low nA range) up to the maximum rated voltage of 30V. This confirms excellent junction quality and low leakage, essential for stable operation in dark conditions.

3.2 Capacitance vs. Reverse Voltage

The graph demonstrates that the junction capacitance (C_T) decreases with increasing reverse bias voltage (V_R). This is a characteristic of semiconductor junctions. Operating at a higher reverse voltage (e.g., 10V as in the switching test) minimizes capacitance, thereby maximizing bandwidth and speed.

3.3 Photocurrent vs. Irradiance

This is a critical transfer characteristic. It shows that the photocurrent (I_P) has a highly linear relationship with the incident infrared irradiance (E_e) over a wide range. This linearity is vital for analog sensing applications where the light intensity needs to be measured accurately, not just detected.

3.4 Relative Spectral Sensitivity

This curve plots the device's normalized responsivity across different wavelengths. It peaks around 900 nm and has a significant bandwidth, typically spanning from roughly 800 nm to 1050 nm. The dark blue package effectively attenuates sensitivity below ~700 nm (visible light), as indicated by the sharp drop-off on the left side of the curve.

3.5 Temperature Dependence

Separate curves illustrate how dark current and photocurrent vary with ambient temperature. Dark current increases exponentially with temperature (a fundamental semiconductor property), which can raise the noise floor in high-temperature operation. Photocurrent also shows variation, typically decreasing slightly as temperature increases. These factors must be considered in designs intended for operation across the full -40°C to +85°C range.

4. Mechanical and Package Information

4.1 Package Dimensions

The LTR-526AB comes in a standard 3mm radial leaded package. Key dimensions include a body diameter of approximately 3.0 mm and a typical lead spacing of 2.54 mm (0.1 inches) where leads emerge from the package. The overall height includes the lens dome. The dark blue tint is integral to the plastic molding.

4.2 Polarity Identification

The device has two leads. The longer lead is typically the collector, and the shorter lead is the emitter. This is the standard convention for phototransistors in this package style. Always verify polarity with the specific datasheet diagram before installation.

4.3 Package Notes

• All dimensions are in millimeters, with tolerances typically ±0.25mm unless specified.
• A small protrusion of resin under the flange is allowed, with a maximum height of 1.5mm.
• Lead spacing is measured at the exit point from the package body, which is critical for PCB footprint design.

5. Soldering and Assembly Guidelines

For hand or wave soldering, the leads can be subjected to a temperature of 260°C for a maximum duration of 5 seconds. The measurement point for this temperature is 1.6mm (0.063") from the package body. It is recommended to use standard PCB soldering practices. Avoid excessive mechanical stress on the leads, especially near the package body. The device should be stored in its original moisture-barrier bag under the specified storage temperature conditions (-55°C to +100°C) to prevent degradation before use.

6. Application Suggestions and Design Considerations

6.1 Typical Application Circuits

The most common configuration is the switched (or digital) mode. Here, the phototransistor is connected in a common-emitter configuration: Collector to a positive supply voltage (V_CC) via a pull-up resistor (R_L), and emitter to ground. The output is taken from the collector. When no light is present, the transistor is off, and the output is high (V_CC). When sufficient IR light hits the base, the transistor turns on, pulling the output low. The value of R_L affects switching speed (lower R_L gives faster speed but lower output swing) and current consumption.

For analog or linear sensing, a transimpedance amplifier (TIA) circuit is recommended. This op-amp-based circuit converts the photocurrent directly into a voltage (V_out = I_photo * R_feedback) while keeping the phototransistor in a virtual short-circuit condition (zero bias voltage), which minimizes the effects of junction capacitance and extends linearity.

6.2 Design Considerations

• Biasing: Applying a reverse bias (V_CE) reduces junction capacitance, improving speed. The datasheet switching parameters are given at V_R=10V.
• Load Resistor (R_L): Choose R_L based on required speed and output voltage swing. A smaller R_L yields faster response but a smaller output voltage change.
• Ambient Light Immunity: The dark blue package provides good rejection of visible light. However, for operation in environments with strong incandescent light (which contains IR) or direct sunlight, additional optical filtering (an IR-pass filter) or modulation/demodulation techniques may be necessary.
• Optical Alignment: Ensure proper alignment between the IR emitter and the phototransistor. The lens has a directional sensitivity pattern; for maximum signal, aim the light source at the center of the dome.
• Electrical Noise: In electrically noisy environments, keep traces short, use decoupling capacitors near the device, and consider shielding the sensor assembly.

7. Technical Comparison and Differentiation

Compared to a standard clear-packaged phototransistor, the LTR-526AB's primary differentiator is its visible light rejection due to the dark blue package. This makes it superior in applications where ambient visible light is present, as it prevents false triggering or saturation from room lights, etc.

Compared to a photodiode, a phototransistor provides internal gain (h_FE of the transistor), resulting in much higher output current for the same light level, simplifying subsequent amplification circuitry. However, phototransistors are generally slower than photodiodes due to the base charge storage effect. The LTR-526AB's 50 ns speed represents a good balance between high sensitivity and reasonably fast response.

8. Frequently Asked Questions (FAQ)

Q: What is the purpose of the dark blue package?

A: It acts as a built-in filter that blocks most visible light while allowing infrared wavelengths (especially around 900 nm) to pass through. This significantly improves the signal-to-noise ratio in IR-only applications.

Q: Can I use this with a 850 nm IR LED?

A: Yes. While peak sensitivity is at 900 nm, the spectral sensitivity curve shows substantial responsivity at 850 nm. You will get a strong signal, though slightly less than with a 900 nm source.

Q: How do I choose the value of the load resistor (R_L)?

A: It involves a trade-off. For maximum output voltage swing, use a larger R_L (e.g., 10kΩ). For maximum speed (fastest rise/fall times), use a smaller R_L (e.g., 1kΩ or less), as it reduces the RC time constant formed with the device's junction capacitance. Refer to the rise/fall time test condition (R_L=1kΩ).

Q: Does the device require a reverse bias voltage to operate?

A: It can operate with zero bias (photovoltaic mode), generating a small voltage. However, for optimal speed and linearity in most circuit configurations (common-emitter switch or with a TIA), applying a reverse bias voltage (e.g., 5V to 10V as per datasheet conditions) is recommended.

9. Practical Application Examples

Example 1: Infrared Remote Control Receiver. The LTR-526AB is an ideal candidate for the detector in a TV or AC remote receiver. The dark blue package rejects interference from indoor lighting. It would be connected in a common-emitter configuration with an appropriate R_L. The output pulse train would then be fed into a decoder IC. The 50 ns response time is more than adequate for standard remote control carrier frequencies (typically 36-40 kHz).

Example 2: Object Proximity Sensor. In a vending machine or industrial counter, an IR LED and the LTR-526AB can be placed on opposite sides of a chute (through-beam mode) or next to each other facing the same direction (reflective mode). When an object interrupts or reflects the IR beam, the change in the phototransistor's output state is detected by a microcontroller, triggering a count or action. The linear photocurrent vs. irradiance characteristic can even be used in reflective mode to roughly gauge distance or reflectivity.

10. Operating Principle

A phototransistor is fundamentally a bipolar junction transistor (BJT) where light acts on the base region. In the LTR-526AB (NPN type), photons with energy greater than the bandgap of silicon (corresponding to wavelengths shorter than ~1100 nm) are absorbed in the base-collector junction region. This absorption creates electron-hole pairs. The electric field in the reverse-biased collector-base junction sweeps these carriers, generating a base current. This photogenerated base current is then amplified by the transistor's current gain (h_FE), resulting in a much larger collector current. Thus, a small optical input produces a significant electrical output current. The dark blue package material absorbs higher-energy photons (visible light), preventing them from generating carriers, while lower-energy infrared photons pass through to the silicon chip.

11. Technology Trends

The trend in discrete optoelectronic components like the LTR-526AB is towards further miniaturization (smaller surface-mount packages), higher integration (combining the photodetector with amplification and logic circuits in a single package), and enhanced functionality (e.g., integrated daylight filters, higher speed for data communication). There is also a drive for components that operate at lower voltages to be compatible with modern digital systems. While basic phototransistors remain highly relevant for cost-sensitive, high-volume applications, more complex solutions like integrated optical sensors and ambient light sensors are addressing needs for smarter, digitally interfaced sensing.