LTR-516AB Infrared Phototransistor Datasheet - Package Dimensions - Reverse Voltage 30V - Wavelength 940nm - English Technical Documentation

1. Product Overview

The LTR-516AB is a silicon NPN phototransistor specifically designed for infrared (IR) detection applications. Its core function is to convert incident infrared light into an electrical current. A key feature is its special dark blue plastic epoxy package, which acts as a visible light filter. This design significantly reduces the sensor's sensitivity to ambient visible light, making it highly suitable for applications that rely purely on infrared signals, such as remote control systems, object detection, and IR data transmission.

The device offers a combination of high photosensitivity and fast response times, enabling reliable detection of modulated IR signals. Its low junction capacitance contributes to a high cut-off frequency, which is beneficial for high-speed switching applications.

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. The LTR-516AB can withstand a maximum reverse voltage (V_R) of 30V. Its maximum power dissipation is 150 mW at an ambient temperature (T_A) of 25°C. The device is rated for operation within a temperature range of -40°C to +85°C and can be stored in environments from -55°C to +100°C. For soldering, the leads can tolerate 260°C for up to 5 seconds when measured 1.6mm from the package body.

2.2 Electro-Optical Characteristics

These parameters are measured under specific test conditions at T_A=25°C and define the device's performance.

• Reverse Breakdown Voltage (V_(BR)R): Minimum 30V at I_R=100µA. This is the voltage at which the junction breaks down.
• Reverse Dark Current (I_D(R)): Maximum 30 nA at V_R=10V. This is the small leakage current that flows when no light is incident on the device.
• Open Circuit Voltage (V_OC): Typical 350 mV when illuminated with 940nm IR light at an irradiance (E_e) of 0.5 mW/cm². This is the voltage generated across the open terminals.
• Short Circuit Current (I_S): Typical 2 µA (min 1.7 µA) at V_R=5V, λ=940nm, and E_e=0.1 mW/cm². This represents the photocurrent generated when the output is shorted.
• Rise/Fall Time (T_r, T_f): Maximum 50 ns each. These parameters define the switching speed of the phototransistor when driven by a pulsed light source, with a load resistor (R_L) of 1 kΩ and V_R=10V.
• Total Capacitance (C_T): Maximum 25 pF at V_R=3V and f=1 MHz. Low capacitance is crucial for high-frequency operation.
• Peak Sensitivity Wavelength (λ_SMAX): Approximately 900 nm. The device is most sensitive to infrared light near this wavelength.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that illustrate device behavior under varying conditions.

3.1 Dark Current vs. Reverse Voltage

Figure 1 shows the relationship between reverse dark current (I_D) and applied reverse voltage (V_R). The dark current remains very low (in the pA to low nA range) across the specified voltage range, which is essential for maintaining a good signal-to-noise ratio in low-light detection.

3.2 Capacitance vs. Reverse Voltage

Figure 2 depicts how the junction capacitance (C_t) decreases as the reverse bias voltage increases. This is a typical characteristic of PN junctions. Operating at a higher reverse bias can reduce capacitance, thereby improving the high-frequency response.

3.3 Temperature Dependence

Figure 3 shows that the photocurrent (I_P) has a positive temperature coefficient; it generally increases with ambient temperature for a constant irradiance level. Figure 4 illustrates that the dark current (I_D) increases exponentially with temperature. Designers must account for these variations in applications with wide operating temperature ranges.

3.4 Spectral Response

Figure 5 is a critical graph showing the relative spectral sensitivity versus wavelength. The response peaks around 900 nm and extends from roughly 700 nm to 1100 nm, covering the near-infrared spectrum. The dark blue package effectively attenuates sensitivity below approximately 700 nm (visible light).

3.5 Photocurrent vs. Irradiance

Figure 6 demonstrates the linear relationship between generated photocurrent (I_P) and incident infrared irradiance (E_e) at 940 nm. This linearity is important for analog sensing applications.

3.6 Derating Curve

Figure 8 presents the total power dissipation derating curve versus ambient temperature. The maximum allowable power dissipation decreases linearly as the ambient temperature rises above 25°C. This curve is vital for ensuring reliable operation and preventing thermal runaway.

4. Mechanical and Package Information

4.1 Package Dimensions

The LTR-516AB comes in a standard 3mm radial leaded package. Key dimensions include a body diameter, lead spacing, and overall length. The dark blue epoxy resin is molded into a lens shape. A small flange is present on the package body, with a note that protruded resin under this flange has a maximum height of 1.5mm. Lead spacing is measured at the point where the leads emerge from the package. All dimensional tolerances are ±0.25mm unless otherwise specified.

4.2 Polarity Identification

The longer lead is typically the collector, and the shorter lead is the emitter. The flat side on the package rim may also serve as a visual indicator for proper orientation. Always refer to the package diagram for definitive pin identification.

5. Soldering and Assembly Guidelines

The device is suitable for wave soldering or hand soldering processes. The absolute maximum rating specifies that the leads can withstand 260°C for 5 seconds when measured 1.6mm (.063\") from the package body. It is recommended to use a soldering iron with temperature control and to minimize the total heat exposure time to prevent damage to the epoxy package or the internal semiconductor die. Avoid applying mechanical stress to the leads during and after soldering.

6. Application Suggestions

6.1 Typical Application Circuits

The LTR-516AB is commonly used in a simple common-emitter configuration. The collector is connected to a positive supply voltage (V_CC) through a load resistor (R_L). The emitter is connected to ground. When IR light falls on the phototransistor, it turns on, causing a voltage drop across R_L. This voltage signal can be fed into a comparator, microcontroller ADC, or amplifier for further processing. The value of R_L affects gain, bandwidth, and output swing; a 1 kΩ resistor is used in the rise/fall time test condition.

6.2 Design Considerations

• Biasing: Applying a reverse bias (V_R) reduces junction capacitance, improving speed, but may slightly increase dark current.
• Ambient Light Rejection: The dark blue package provides excellent rejection of visible light. However, for applications in environments with strong IR sources (e.g., sunlight, incandescent bulbs), additional optical filtering or housing design may be necessary.
• Speed vs. Sensitivity: A smaller load resistor (R_L) improves switching speed but reduces output voltage swing for a given photocurrent. Designers must balance these factors based on application needs.
• Temperature Compensation: For precision analog sensing over a wide temperature range, circuitry to compensate for the variation in dark current and photocurrent may be required.

7. Technical Comparison and Differentiation

The primary differentiating feature of the LTR-516AB is its dark blue package, which is not found on standard clear or water-clear phototransistors. This built-in filter makes it superior for IR-only applications by simplifying optical design. Compared to photodiodes, phototransistors provide internal gain, resulting in higher output current for the same light level, but typically have slower response times. The LTR-516AB's 50 ns rise/fall time positions it well for medium-speed IR communication protocols.

8. Frequently Asked Questions (FAQ)

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

A: It acts as a filter to block most visible light, allowing primarily infrared light to reach the semiconductor chip. This enhances performance in IR systems by reducing noise from ambient visible light.

Q: Can I use this sensor for detecting visible light?

A: No, its sensitivity in the visible spectrum is severely attenuated by the package filter. It is specifically designed for infrared detection.

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

A: The choice involves a trade-off. A higher R_L gives more output voltage per unit photocurrent (higher gain) but increases the RC time constant, slowing down the response. Start with the 1 kΩ value from the test condition and adjust based on your required speed and signal level.

Q: What is the difference between short circuit current (I_S) and photocurrent in a circuit?

A: I_S is a parameter measured under specific short-circuit conditions. In a practical circuit with a load resistor, the output current will be slightly less due to the internal resistance of the transistor and the applied bias.

9. Operational Principle

A phototransistor is a bipolar junction transistor (BJT) where the base-collector junction is exposed to light. Incident photons with energy greater than the semiconductor's bandgap generate electron-hole pairs in the depletion region of this junction. These carriers are swept by the electric field, creating 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 light signal controls a larger output current.

10. Development Trends

The field of optoelectronics continues to advance towards higher integration, smaller packages (like surface-mount devices), and improved performance. Trends include phototransistors and photodiodes integrated with amplification and signal conditioning circuits on a single chip (opto-ICs), reducing system complexity. There is also ongoing development in materials and packaging to enhance sensitivity, speed, and wavelength selectivity for emerging applications in sensing, LiDAR, and optical communications.