LTR-516AD Phototransistor Datasheet - Dark Green Package - 30V Reverse Voltage - 150mW Power Dissipation - English Technical Document

1. Product Overview

The LTR-516AD is a high-performance silicon NPN phototransistor designed for detecting infrared radiation. Its core function is to convert incident infrared light into an electrical current. A key feature of this component is its special dark green plastic package, which is engineered to filter out most of the visible light spectrum. This makes it particularly suitable for applications where the sensor must respond primarily to infrared signals, minimizing interference from ambient visible light. The device offers a combination of high photosensitivity, low junction capacitance, and fast switching times, positioning it as an ideal choice for various infrared sensing and communication systems.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

The device is rated for operation within specific environmental and electrical limits to ensure reliability and prevent damage. The maximum power dissipation is 150 mW at an ambient temperature (T_A) of 25°C. It can withstand a reverse voltage (V_R) of up to 30 V. The operational temperature range is from -40°C to +85°C, while it can be stored in temperatures ranging from -55°C to +100°C. For assembly, the leads can be soldered at 260°C for a maximum duration of 5 seconds, with the soldering point located at least 1.6mm from the package body.

2.2 Electrical & Optical Characteristics

All electrical and optical parameters are specified at T_A = 25°C. The reverse breakdown voltage (V_(BR)R) is typically 30V at a reverse current (I_R) of 100µA. The reverse dark current (I_D(R)), which is the leakage current when no light is incident, has a maximum value of 30 nA at V_R = 10V. Under an irradiance (E_e) of 0.5 mW/cm² from a 940nm source, the phototransistor generates an open-circuit voltage (V_OC) of 350 mV. Its dynamic performance is characterized by rise and fall times (T_r, T_f) of 50 nanoseconds each when tested with V_R=10V, a 940nm pulse, and a 1 kΩ load resistor. The short-circuit current (I_S), a key measure of sensitivity, is 2 µA (typical) under V_R=5V, λ=940nm, and E_e=0.1 mW/cm². The total junction capacitance (C_T) is 25 pF maximum at V_R=3V and 1 MHz. The wavelength of peak spectral sensitivity (λ_SMAX) is 900 nm.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that are crucial for circuit design. Figure 1 plots the dark current (I_D) against reverse voltage (V_R), showing the device's leakage behavior in darkness. Figure 2 illustrates how the junction capacitance (C_T) decreases with increasing reverse voltage, which is important for high-frequency applications. Figure 3 shows the variation of photocurrent with ambient temperature, indicating how the sensor's output might drift with temperature changes. Figure 4 similarly plots dark current against temperature. Figure 5 is the relative spectral sensitivity curve, which graphically confirms the peak response at 900nm and the effectiveness of the dark green package in attenuating sensitivity in the visible light range. Figure 6 shows the linear relationship between photocurrent (I_p) and infrared irradiance (E_e). Figure 7 is a polar diagram showing the angular dependence of sensitivity. Figure 8 details how the maximum allowable total power dissipation derates as the ambient temperature increases above 25°C.

4. Mechanical & Package Information

The LTR-516AD is housed in a special dark green plastic package. Key dimensional notes include: all dimensions are in millimeters, with a general tolerance of ±0.25mm unless specified otherwise. The maximum protrusion of resin under the flange is 1.5mm. Lead spacing is measured at the point where the leads exit the package body. The package is designed for through-hole mounting. The dark green coloring is integral to its function, acting as a visible light filter to enhance the signal-to-noise ratio for infrared detection.

5. Soldering & Assembly Guidelines

For reliable soldering, it is critical to adhere to the specified conditions. The leads should be soldered at a temperature of 260°C for a maximum of 5 seconds. The soldering point must be at least 1.6mm (0.063 inches) away from the plastic package body to prevent thermal damage to the semiconductor die and the plastic encapsulation. Standard wave soldering or hand soldering techniques can be used, provided the temperature and time limits are strictly observed. Prolonged exposure to temperatures above the specified limit can degrade performance or cause permanent failure.

6. Application Suggestions

6.1 Typical Application Scenarios

The LTR-516AD is well-suited for a variety of infrared-based applications. These include object detection and proximity sensing in automation and security systems, slot sensors in printers and vending machines, touchless switches, and infrared data communication links (like older IRDA interfaces). Its fast switching time makes it applicable in systems requiring rapid pulse detection.

6.2 Design Considerations

When designing with this phototransistor, several factors must be considered. First, the operating point should be chosen considering the required sensitivity and speed; a higher reverse voltage generally reduces capacitance and improves speed but increases dark current. The load resistor (R_L) value is a critical design choice: a larger R_L provides higher voltage output but slows down the response time (increases the RC time constant). The dark green package reduces interference from ambient visible light, but the designer should still consider the infrared background in the application environment. For stable operation over temperature, the variations shown in Figures 3 and 4 should be accounted for, possibly through temperature compensation in the signal conditioning circuitry.

7. Technical Comparison & Differentiation

The primary differentiating feature of the LTR-516AD is its dedicated dark green package for visible light suppression, which is not found in all standard phototransistors. This gives it a significant advantage in environments with fluctuating visible light. Its combination of parameters—a relatively high short-circuit current (2 µA typical), low capacitance (25 pF max), and fast switching times (50 ns)—makes it a balanced component suitable for both sensitive and moderately high-speed applications. Compared to photodiodes, phototransistors like the LTR-516AD provide internal gain, resulting in higher output current for the same light input, simplifying subsequent amplifier stages.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the dark green package?
A: The dark green plastic acts as a built-in optical filter. It significantly attenuates wavelengths in the visible spectrum while allowing infrared light (especially around 900-940nm) to pass through. This minimizes the sensor's response to ambient room light, sunlight, or other visible sources, making it more reliable for detecting dedicated infrared signals.

Q: How do I interpret the "Short Circuit Current (I_S)" parameter?
A: I_S is measured with the collector and emitter shorted (V_CE = 0V). It represents the photogenerated current per unit of irradiance under specific test conditions (940nm, 0.1 mW/cm²). In your circuit, the actual output current will be less than I_S when a load resistor or a bias voltage is applied, but I_S is a key figure for comparing the basic sensitivity of different devices.

Q: Why are rise and fall times important?
A: These parameters (T_r and T_f) define how quickly the phototransistor can respond to changes in light intensity. A value of 50 ns means the device can theoretically handle signal frequencies up to several megahertz, making it suitable for pulsed IR systems, data transmission, or high-speed counting applications.

Q: How does temperature affect performance?
A: As shown in the curves, both the dark current (noise) and the photocurrent (signal) increase with temperature. The dark current increase can be significant, potentially raising the noise floor. Designers must ensure that the signal conditioning circuit can handle this variation, especially if the device operates over the full -40°C to +85°C range.

9. Practical Design Case

Consider designing a simple infrared object detection circuit. The LTR-516AD is paired with an infrared LED emitter. The phototransistor is connected in a common-emitter configuration: the collector is connected to a supply voltage (e.g., 5V) through a load resistor R_L, and the emitter is grounded. When no object is present, the IR light from the LED reaches the phototransistor, causing it to conduct and pulling the collector voltage (V_OUT) low. When an object interrupts the beam, the phototransistor turns off, and V_OUT goes high. The value of R_L must be chosen based on the desired output voltage swing and speed. For a 5V supply and a typical I_S of 2µA, an R_L of 10 kΩ would give a voltage drop of about 20 mV when illuminated, which is quite small. Therefore, an operational amplifier comparator stage would typically be added after the phototransistor to provide a clean digital output. The dark green package helps reject ambient light, making the system robust for use in various lighting conditions.

10. Operating Principle

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-516AD (an NPN type), incident photons with energy greater than the bandgap of silicon create electron-hole pairs in the base-collector junction region. These photogenerated carriers are swept by the electric field, effectively creating a base current. This base current is then amplified by the transistor's current gain (beta, β), resulting in a much larger collector current. The device is typically operated with the base terminal left open or disconnected, and a reverse bias is applied across the collector-base junction to widen the depletion region, improving sensitivity and speed.

11. Industry Trends

The field of optical sensing continues to evolve. There is a trend towards integration, where the photodetector, amplifier, and digital logic are combined into a single chip (e.g., integrated ambient light sensors, proximity sensors). Surface-mount device (SMD) packages are becoming more prevalent than through-hole types for automated assembly. There is also ongoing development in materials and designs to improve sensitivity, reduce noise (dark current), and extend the spectral range. However, discrete components like the LTR-516AD remain vital for applications requiring specific performance characteristics, custom optical paths, or high-voltage handling that may not be available in integrated solutions. The principle of using filtered packages for specific spectral responses remains a common and effective design practice.