LTR-536AD Phototransistor Datasheet - Dark Green Package - 30V Reverse Voltage - 150mW Power Dissipation - English Technical Document

1. Product Overview

The LTR-536AD is a high-performance silicon NPN phototransistor specifically engineered for infrared (IR) detection applications. Its core function is to convert incident infrared radiation into an electrical current. A defining feature of this component is its special dark green plastic epoxy package. This material is formulated to attenuate or "cut" visible light wavelengths, significantly enhancing its sensitivity and signal-to-noise ratio specifically within the infrared spectrum, typically around 940nm. This makes it an ideal choice for applications where discrimination against ambient visible light is crucial.

Core Advantages:

• High Photo Sensitivity: Delivers a strong electrical output signal for a given level of infrared irradiance.
• Infrared Optimized: The dark green package acts as a visible light filter, making the device particularly suitable for pure IR sensing.
• Low Junction Capacitance: This parameter is critical for high-frequency operation, enabling faster response times.
• Fast Switching Characteristics: Features quick rise and fall times, suitable for pulsed IR systems and data communication.
• High Cut-off Frequency: Supports operation in higher frequency circuits.

Target Market: This phototransistor is targeted at designers and engineers working on infrared-based systems. Common applications include proximity sensors, object detection, touchless switches, IR data transmission links (like remote controls), industrial automation, and any system requiring reliable detection of infrared signals while rejecting interference from visible light sources.

2. In-Depth Technical Parameter Analysis

All parameters are specified at an ambient temperature (T_A) of 25°C unless otherwise noted. Understanding these parameters is essential for proper circuit design and ensuring reliable operation within the device's limits.

2.1 Absolute Maximum Ratings

These are the stress limits beyond which permanent damage to the device may occur. Operation should always be maintained within these limits.

• Power Dissipation (P_D): 150 mW. This is the maximum allowable power the device can dissipate as heat.
• Reverse Voltage (V_R): 30 V. The maximum voltage that can be applied in reverse bias across the collector-emitter junction.
• Operating Temperature Range (T_oper): -40°C to +85°C. The ambient temperature range for normal device operation.
• Storage Temperature Range (T_stg): -55°C to +100°C. The temperature range for non-operational storage.
• Lead Soldering Temperature: 260°C for 5 seconds, measured 1.6mm from the package body. This defines the reflow soldering profile constraints.

2.2 Electrical & Optical Characteristics

These parameters define the device's performance under specified test conditions.

• Reverse Breakdown Voltage (V_(BR)R): 30 V (Min). The voltage at which the reverse current (I_R) sharply increases (tested at 100µA). This correlates with the Absolute Max Rating.
• Reverse Dark Current (I_D(R)): 30 nA (Max). The leakage current that flows when the device is in reverse bias (V_R=10V) and in complete darkness (E_e=0). A lower value indicates better performance in low-light conditions.
• Open Circuit Voltage (V_OC): 350 mV (Typ). The voltage generated across the device under illumination (λ=940nm, E_e=0.5mW/cm²) with no external load (open circuit).
• Short Circuit Current (I_S): 1.7 µA (Min), 2 µA (Typ). The current that flows when the device is illuminated (λ=940nm, E_e=0.1mW/cm²) and the output is shorted (V_R=5V). This is a key measure of sensitivity.
• Rise Time (T_r) & Fall Time (T_f): 50 nsec (Typ). The time required for the output current to rise from 10% to 90% (rise) or fall from 90% to 10% (fall) of its final value in response to a step change in illumination. Critical for high-speed applications.
• Total Capacitance (C_T): 25 pF (Typ). The junction capacitance measured at V_R=3V and f=1MHz in darkness. Lower capacitance enables faster switching speeds.
• Wavelength of Max Sensitivity (λ_SMAX): 900 nm (Typ). The peak wavelength of infrared light to which the phototransistor is most responsive. It is optimized for emitters around 940nm.

3. Performance Curve Analysis

The datasheet provides several graphs illustrating the device's behavior under varying conditions. These are invaluable for detailed design work beyond the typical/min/max numbers.

3.1 Dark Current vs. Reverse Voltage (Fig.1)

This curve shows how the reverse dark current (I_D) increases with applied reverse voltage (V_R). It typically shows a very low, relatively constant current at lower voltages, with a gradual increase as voltage rises, culminating in the sharp rise at the breakdown voltage. Designers must ensure the operating V_R is sufficiently below the knee of this curve to minimize noise from leakage current.

3.2 Capacitance vs. Reverse Voltage (Fig.2)

This graph depicts the relationship between junction capacitance (C_T) and reverse bias voltage. Capacitance decreases with increasing reverse voltage. For high-speed circuit design, operating at a higher reverse voltage (within limits) can reduce C_T and improve bandwidth, but this must be balanced against increased dark current (from Fig.1).

3.3 Photocurrent & Dark Current vs. Ambient Temperature (Fig.3 & Fig.4)

Figure 3 illustrates how the photocurrent (I_P) changes with ambient temperature. Phototransistor sensitivity generally decreases as temperature increases. Figure 4 shows the exponential increase of dark current (I_D) with rising temperature. These two curves are critical for designing systems that must operate reliably over a wide temperature range (e.g., -40°C to +85°C). At high temperatures, the increasing dark current can swamp a weak optical signal, reducing the signal-to-noise ratio.

3.4 Relative Spectral Sensitivity (Fig.5)

This is perhaps the most important curve for application matching. It plots the normalized responsivity of the phototransistor across a range of wavelengths (typically ~800nm to 1100nm). The LTR-536AD shows peak sensitivity around 900nm and significant attenuation in the visible light spectrum (<800nm), a direct result of its dark green package. This curve must be cross-referenced with the emission spectrum of the intended IR LED or light source to ensure optimal coupling.

3.5 Photocurrent vs. Irradiance (Fig.6)

This graph shows the linear relationship between the incident infrared light power (irradiance E_e) and the resulting photocurrent (I_P). The slope of this line represents the device's responsivity. It confirms the device operates in a linear region for the tested irradiance range, which is desirable for analog sensing applications.

3.6 Total Power Dissipation vs. Ambient Temperature (Fig.8)

This derating curve shows the maximum allowable power dissipation (P_D) as a function of ambient temperature. The absolute maximum rating of 150mW applies only up to a certain temperature (likely 25°C). As ambient temperature increases, the device's ability to dissipate heat decreases, so the maximum allowed power must be reduced linearly to prevent overheating. This is crucial for reliability calculations.

4. Mechanical & Packaging Information

4.1 Package Dimensions

The LTR-536AD comes in a standard 3mm (T-1) through-hole package. Key dimensional notes from the datasheet include:

• All dimensions are in millimeters (inches provided in parenthesis).
• A standard tolerance of ±0.25mm (.010") applies unless specified otherwise.
• The maximum protrusion of resin under the flange is 1.5mm (.059").
• Lead spacing is measured at the point where the leads emerge from the package body.

Polarity Identification: The device has a flat side on the lens, which typically indicates the collector lead. The longer lead is usually the emitter. However, designers should always verify polarity with a multimeter in diode test mode before installation.

5. Soldering & Assembly Guidelines

To ensure device integrity during assembly, the following conditions must be observed:

• Reflow Soldering: The leads can withstand a temperature of 260°C for a maximum of 5 seconds. This measurement is taken 1.6mm (0.063") from the package body. Standard wave or reflow profiles must be adjusted to comply with this limit to prevent damage to the internal semiconductor die or the epoxy package.
• Hand Soldering: If hand soldering is necessary, use a temperature-controlled iron and minimize the contact time to less than 3 seconds per lead. Use a heat sink clip on the lead between the joint and the package body if possible.
• Cleaning: Use only approved cleaning solvents that are compatible with the dark green epoxy material. Avoid ultrasonic cleaning unless its compatibility and power/time settings are verified, as it can damage the package or internal bonds.
• Storage Conditions: Store in a dry, anti-static environment within the specified storage temperature range of -55°C to +100°C. The original moisture barrier bag should be used if long-term storage is anticipated.

6. Application Suggestions & Design Considerations

6.1 Typical Application Circuits

The LTR-536AD can be used in two primary configurations:

1. Switch Mode (Digital Output): The phototransistor is connected in series with a pull-up resistor between the supply voltage (V_CC) and ground. The output is taken from the collector node. When IR light falls on the sensor, it turns on, pulling the output voltage low. When dark, it turns off, and the pull-up resistor pulls the output high. The value of the pull-up resistor determines the switching speed and current consumption (a smaller resistor gives faster switching but higher power).
2. Linear Mode (Analog Output): Similar configuration, but the phototransistor is biased in its active region using a fixed base current (often zero, relying solely on photocurrent) and a collector resistor. The voltage at the collector varies linearly with the intensity of the incident IR light. This mode is used for analog sensing, like distance measurement or light level detection.

6.2 Critical Design Considerations

• Source Matching: Always pair the LTR-536AD with an IR emitter (LED) that has a peak wavelength close to 940nm and aligns with the phototransistor's spectral sensitivity peak (900nm) for maximum efficiency.
• Ambient Light Rejection: While the dark green package helps, for operation in bright environments, additional optical filtering (a dedicated IR-pass filter) or modulation/demodulation techniques (pulsing the IR source and synchronously detecting the signal) may be necessary to reject ambient light noise.
• Biasing for Speed: To achieve the fastest possible response time (50ns typ.), operate the device with a reverse voltage (V_CE) of around 10V and use a small load resistor (e.g., 1kΩ as in the test condition). This minimizes the RC time constant formed by the junction capacitance (C_T) and the load resistance (R_L).
• Temperature Compensation: For precision applications over a wide temperature range, consider circuit techniques to compensate for the variation in dark current and sensitivity. This could involve using a matched phototransistor in a dark reference channel or implementing temperature-dependent gain adjustment in the signal conditioning circuitry.

7. Technical Comparison & Differentiation

The LTR-536AD differentiates itself in the phototransistor market through its specialized package. Compared to standard clear or water-clear epoxy phototransistors, its key advantage is the built-in visible light cutoff. This eliminates the need for an external IR filter in many applications, reducing component count, cost, and assembly complexity. Its combination of relatively fast switching speed (50ns), low capacitance (25pF), and good sensitivity (2µA typ. at 0.1mW/cm²) makes it a balanced choice for both analog sensing and moderate-speed digital IR communication links.

8. Frequently Asked Questions (Based on Technical Parameters)

8.1 Can I use this with a red LED (650nm)?

Answer: No, it is not recommended. The Relative Spectral Sensitivity curve (Fig.5) shows very low responsivity at 650nm (visible red). The dark green package actively blocks this wavelength. For detecting red light, a phototransistor with a clear package and a peak sensitivity in the visible range should be selected.

8.2 Why is my output signal noisy in a warm environment?

Answer: Refer to Figure 4 (Dark Current vs. Temperature). Dark current increases exponentially with temperature. If your circuit is designed to detect a weak IR signal, the thermally generated dark current can become significant at elevated temperatures, appearing as noise or a DC offset. Solutions include cooling the sensor, using a modulated light source with synchronous detection, or selecting a circuit topology that subtracts the dark current.

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

Answer: It involves a trade-off between speed, sensitivity, and power.
For Speed (Digital Switching): Choose a small R_L (e.g., 1kΩ to 4.7kΩ). This gives a small RC time constant (C_T * R_L) for fast edges but draws more current.
For High Voltage Swing (Analog Sensing): Choose a larger R_L (e.g., 10kΩ to 100kΩ). This provides a larger output voltage change for a given change in light but slows down the response time.
Always ensure the voltage drop across R_L when the phototransistor is fully on does not cause the collector-emitter voltage to fall below the saturation level, and that the power dissipation in the phototransistor remains below the derated limit for your operating temperature.

9. Practical Use Case Example

Application: Non-Contact Object Detection in an Industrial Counter.
Implementation: An IR LED (940nm) and the LTR-536AD are mounted on opposite sides of a conveyor belt (through-beam configuration). The LED is pulsed at 10kHz using a driver circuit. The phototransistor is connected in switch mode with a 4.7kΩ pull-up resistor to 5V. Its output is fed into a microcontroller's input capture pin. Under normal conditions (no object), the pulsed IR light reaches the sensor, causing the output to pulse at 10kHz. The microcontroller firmware detects this frequency. When an object passes through the beam, it blocks the light, and the phototransistor's output goes and stays high (or low, depending on logic). The microcontroller detects the absence of the 10kHz signal and increments a counter. The dark green package of the LTR-536AD prevents ambient fluorescent or incandescent light in the factory from falsely triggering the counter.

10. Operating Principle Introduction

A phototransistor is fundamentally a bipolar junction transistor (BJT) where the base current is generated by light instead of being supplied electrically. In the LTR-536AD (NPN type), incident photons with energy greater than the bandgap of silicon (corresponding to wavelengths shorter than ~1100nm) 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 photocurrent. This photocurrent acts exactly like a base current injected into the transistor. Due to the transistor's current gain (beta, β), the collector current is much larger than the initial photocurrent (I_C = β * I_photo). This internal amplification is what gives phototransistors their high sensitivity compared to photodiodes. The dark green epoxy absorbs most visible light photons, allowing primarily infrared photons to reach the silicon chip, thus making the device selectively sensitive to IR.

11. Technology Trends

The field of optoelectronics continues to evolve. While discrete through-hole phototransistors like the LTR-536AD remain vital for many applications, trends include:
Integration: Increasing integration of the photodetector with analog front-end circuitry (amplifiers, filters) and digital logic (comparators, logic outputs) into single-chip solutions or modules.
Surface-Mount Technology (SMT): A strong shift towards smaller SMT packages for automated assembly and reduced board space, though often at a trade-off with sensitivity due to smaller active areas.
Specialization: Development of devices with even more specific spectral responses, faster speeds for optical data communication, and enhanced resilience to harsh environments (higher temperature, humidity).
The core principle of the phototransistor remains unchanged, but its implementations are becoming more application-specific and integrated.