LTR-3208 Phototransistor Datasheet - Package Dimensions - Collector-Emitter Voltage 30V - Power Dissipation 100mW - English Technical Document

1. Product Overview

The LTR-3208 is a silicon NPN phototransistor designed for infrared detection applications. It is housed in a low-cost plastic package featuring an integrated lens optimized for high sensitivity. This component is engineered to convert incident infrared light into a corresponding electrical current at its collector terminal, making it suitable for various sensing and detection systems where reliable and cost-effective light detection is required.

1.1 Core Advantages

The device offers several key benefits for designers. Its primary feature is a wide operating range for the collector current, providing flexibility in circuit design across different signal levels. The incorporation of a lens directly into the package enhances its sensitivity to incoming infrared radiation, improving signal-to-noise ratio and detection range. Furthermore, the utilization of a standard plastic package contributes to a low overall component cost, making it an attractive option for high-volume or cost-sensitive applications.

1.2 Target Market and Applications

This phototransistor is targeted at the broad optoelectronics market, serving applications that require non-contact sensing. Typical use cases include object detection, position sensing, slot interrupters (e.g., in printers and encoders), touchless switches, and industrial automation systems. Its reliability and simple interface (typically requiring a pull-up resistor and a supply voltage) make it a common choice for both consumer electronics and industrial control systems.

2. In-Depth Technical Parameter Analysis

The electrical and optical performance of the LTR-3208 is characterized under standard ambient temperature conditions (25°C). Understanding these parameters is critical for proper circuit design and ensuring reliable operation within the device's specified limits.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. Operation under or at these limits is not guaranteed. The maximum power dissipation is 100 mW, which dictates the thermal design of the application. The collector-emitter voltage rating (V_CEO) is 30V, while the emitter-collector voltage rating (V_ECO) is 5V, indicating the device's asymmetry. The operating temperature range is from -40°C to +85°C, and it can be stored in environments from -55°C to +100°C. For soldering, leads can withstand 260°C for 5 seconds when measured 1.6mm from the package body.

2.2 Electrical and Optical Characteristics

The key operational parameters define the device's performance under specific test conditions. The Collector-Emitter Breakdown Voltage (V_(BR)CEO) is typically 30V at 1mA collector current with no illumination. The Collector-Emitter Saturation Voltage (V_CE(SAT)) is very low, ranging from 0.1V (min) to 0.4V (max) when the device is driven with 100μA collector current under an irradiance of 1 mW/cm². This low saturation voltage is desirable for switching applications. The switching speed is characterized by Rise Time (T_r) and Fall Time (T_f), specified as 10 μs and 15 μs respectively under test conditions of V_CC=5V, I_C=1mA, and R_L=1kΩ. The Collector Dark Current (I_CEO), which is the leakage current with no light, has a maximum value of 100 nA at V_CE=10V.

2.3 On-State Collector Current and Binning

A critical parameter is the On-State Collector Current (I_C(ON)), which is the current output when the device is illuminated. This parameter is binned, meaning devices are sorted into performance groups. The test condition is V_CE = 5V with an irradiance of 1 mW/cm² at a wavelength of 940nm. The bins are as follows: Bin C: 0.8 to 2.4 mA; Bin D: 1.6 to 4.8 mA; Bin E: 3.2 to 9.6 mA; Bin F: 6.4 mA (minimum). This binning allows designers to select a device with a sensitivity range appropriate for their specific application, ensuring consistent system performance.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that illustrate how key parameters vary with environmental and operational factors. These graphs are essential for understanding device behavior beyond the single-point specifications given in the tables.

3.1 Temperature Dependence

Figure 1 shows the relationship between Collector Dark Current (I_CEO) and Ambient Temperature (T_a). The dark current increases exponentially with temperature, which is a fundamental property of semiconductor junctions. Designers must account for this increased leakage in high-temperature environments, as it can affect the off-state signal level and noise floor. Figure 2 depicts the derating of the maximum allowable Collector Power Dissipation (P_C) as ambient temperature rises. The 100 mW rating is only valid at or below 25°C; above this temperature, the maximum power must be reduced linearly to prevent thermal overstress.

3.2 Dynamic and Responsive Characteristics

Figure 3 illustrates how the Rise and Fall Times (T_r, T_f) are affected by the Load Resistance (R_L). The switching times increase with larger load resistances. This is a crucial consideration for designing high-speed detection circuits, where a smaller load resistor may be necessary to achieve the desired bandwidth, albeit at the cost of higher current consumption. Figure 4 shows the Relative Collector Current as a function of Irradiance (E_e). The relationship is generally linear in the operating region, confirming that the output current is directly proportional to the incident light power, which is ideal for analog sensing applications.

3.3 Spectral Response

Figures 5 and 6 are related to the device's spectral sensitivity. Figure 5 is a polar diagram showing the angular dependence of sensitivity, indicating how the output varies with the angle of incident light relative to the device's axis. This is important for alignment in optical systems. Figure 6, the Spectral Distribution curve, shows that the LTR-3208 is most sensitive to infrared light, with peak responsivity occurring at a specific wavelength (implied to be in the near-infrared region, typical for silicon phototransistors). It has negligible response to visible light, making it immune to ambient room lighting in many cases.

4. Mechanical and Packaging Information

4.1 Package Dimensions

The LTR-3208 uses a standard plastic package with three leads. The package includes a molded lens on top to focus incoming light onto the sensitive semiconductor area. Critical dimensions include the body size, lead spacing, and the protrusion of resin under the flange, which is specified as a maximum of 1.5mm. The lead spacing is measured at the point where the leads exit the package body. All dimensions are provided in millimeters with a standard tolerance of ±0.25mm unless otherwise noted. The physical outline and dimensions are essential for PCB footprint design and ensuring proper fit within the assembly.

4.2 Polarity Identification and Pinout

The device has three pins: Collector, Emitter, and Base (often left unconnected or used for biasing in some configurations). The typical pinout for a phototransistor in this package is: when viewing the device from the top (lens side) with the flat side or notch facing a specific direction, the pins from left to right are usually Emitter, Collector, and Base. However, designers must always verify the pinout from the mechanical drawing in the datasheet to avoid connection errors. The package may also have a marking or indentation to identify pin 1.

5. Soldering and Assembly Guidelines

5.1 Reflow Soldering Parameters

While specific reflow profile details are not provided in this excerpt, the Absolute Maximum Ratings give a critical constraint: the leads can withstand a soldering temperature of 260°C for a maximum of 5 seconds when measured 1.6mm from the package body. This implies that standard lead-free reflow profiles (which often peak around 245-260°C) are acceptable, but the time above liquidus must be controlled to prevent package damage. It is recommended to follow JEDEC or IPC standards for plastic-encapsulated device soldering.

5.2 Handling and Storage Precautions

The device should be handled with standard ESD (Electrostatic Discharge) precautions, as the semiconductor junction can be damaged by static electricity. Storage should be within the specified temperature range of -55°C to +100°C in a low-humidity environment. The lens should be kept clean and free from scratches, contaminants, or epoxy bleed during assembly, as these can significantly affect optical performance and sensitivity.

6. Application Suggestions

6.1 Typical Application Circuits

The most common circuit configuration is the \"switch mode.\" The phototransistor's collector is connected to a positive supply voltage (V_CC) through a pull-up resistor (R_L). The emitter is connected to ground. The output signal is taken from the collector node. When no light is present, the device is off, and the output is pulled high to V_CC. When sufficient infrared light strikes the device, it turns on, pulling the output voltage low towards V_CE(SAT). The value of R_L determines the output swing, current consumption, and switching speed, as shown in the performance curves.

6.2 Design Considerations

Key design factors include: Biasing: Ensure the operating V_CE is within the maximum rating (30V). Load Resistor Selection: Choose R_L based on required switching speed (see Fig. 3), output voltage swing, and power consumption. A smaller R_L gives faster speed but higher current. Optical Alignment: Consider the angular sensitivity diagram (Fig. 5) when designing the optical path between the IR emitter and detector. Ambient Light Immunity: While the device is primarily sensitive to IR, strong ambient IR sources (like sunlight or incandescent bulbs) can cause false triggering. Using a modulated IR signal and synchronous detection can greatly improve noise immunity. Temperature Effects: Account for the increase in dark current with temperature, which may require a threshold adjustment in the detection circuit.

7. Technical Comparison and Differentiation

Compared to a simple photodiode, a phototransistor provides internal gain, resulting in a much larger output current for the same light input, often eliminating the need for an additional amplifier stage. Compared to other phototransistors, the LTR-3208's differentiation lies in its specific combination of package (with integrated lens for higher sensitivity), its defined current bins allowing for sensitivity selection, and its balanced electrical ratings (30V V_CEO, 100mW P_D). The low V_CE(SAT) is also a favorable characteristic for clean digital switching.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the different bins (C, D, E, F) for I_C(ON)?
A: Binning sorts devices by their sensitivity. Bin F devices have the highest minimum output current (most sensitive), while Bin C devices have the lowest. This allows you to choose a part that matches your system's required signal level, ensuring consistency and potentially simplifying circuit design by providing a predictable signal range.

Q: Can I use this sensor in sunlight?
A: Direct sunlight contains a significant amount of infrared radiation and will likely saturate the sensor, causing a constant \"on\" state. For outdoor use or in brightly lit environments, optical filtering (an IR-pass filter that blocks visible light) and/or signal modulation techniques are strongly recommended to distinguish the intended IR signal from ambient IR noise.

Q: How do I interpret the Rise and Fall Times?
A: These specify the speed at which the output can change state. A 10μs rise time means it takes approximately 10 microseconds for the output to go from 10% to 90% of its final value when light is applied. This limits the maximum frequency of modulated light that can be accurately detected. For simple object detection, this speed is more than adequate. For high-speed communication, it may be a limiting factor.

9. Practical Use Case Example

Scenario: Paper Detection in a Printer. An LTR-3208 (from an appropriate sensitivity bin) and an infrared LED are placed on opposite sides of the paper path, aligned so the paper breaks the beam. The phototransistor is configured in a switch circuit with a 10kΩ pull-up resistor to 5V. When no paper is present, the IR light hits the sensor, turning it on and pulling the output pin to a low voltage (~0.2V). When paper passes through, it blocks the light, turning the phototransistor off and allowing the output pin to be pulled high to 5V. This digital signal is fed to a microcontroller to track paper presence and edge detection. The lens on the LTR-3208 helps focus the IR beam, improving reliability and allowing for a slightly greater gap between the emitter and detector.

10. Operating Principle

A phototransistor is a bipolar junction transistor where the base region is exposed to light. Incident photons with energy greater than the semiconductor's bandgap generate electron-hole pairs in the base-collector junction. These photogenerated carriers are equivalent to a base current. Due to the transistor's current amplification (beta or h_FE), this small photocurrent is multiplied, resulting in a much larger collector current. The device essentially combines the light detection of a photodiode with the current gain of a transistor in a single package. The integrated lens serves to concentrate more light onto the active semiconductor area, increasing the effective \"base current\" and thus the output signal.

11. Technology Trends

The general trend in discrete optoelectronic components like phototransistors is towards miniaturization, higher integration, and improved performance. This includes the development of surface-mount packages with smaller footprints and lower profiles to meet the demands of modern, dense PCB designs. There is also a move towards devices with better defined and more consistent performance parameters, reducing the need for calibration in end applications. In some advanced applications, phototransistors are being integrated with on-chip amplification and signal conditioning circuits to create more complete \"sensor-in-a-package\" solutions, though discrete components like the LTR-3208 remain highly relevant for their simplicity, reliability, and cost-effectiveness in a vast array of standard sensing tasks.