LTR-3208E Phototransistor Datasheet - Package 3.0x2.8x1.5mm - Vce 30V - Collector Current up to 3.6mA - Dark Plastic Package - English Technical Document

1. Product Overview

The LTR-3208E is a discrete infrared (IR) phototransistor component designed for sensing applications in the infrared spectrum. Its primary function is to convert incident infrared light into a corresponding electrical current at its collector terminal. This device is part of a broader family of optoelectronic components intended for use in systems requiring reliable and cost-effective infrared detection.

1.1 Core Advantages and Product Positioning

The LTR-3208E is positioned as a general-purpose infrared detector suitable for cost-sensitive applications. Its key advantages stem from its specific package and electrical characteristics. The device is housed in a special dark plastic package. This material is engineered to attenuate or cut off visible light wavelengths, thereby enhancing its sensitivity and signal-to-noise ratio specifically for infrared signals, typically around 940nm. This makes it highly suitable for environments with ambient visible light where only the IR signal should be detected. Furthermore, it offers a wide operational range for its collector current, allowing it to interface with a variety of circuit designs without requiring highly precise biasing. The use of a standard plastic package contributes to its low cost, making it an attractive option for high-volume consumer electronics.

1.2 Target Market and Applications

The primary target market for the LTR-3208E includes consumer electronics and basic industrial control systems. Its design caters to applications where reliable infrared detection is needed without the extreme performance requirements (like ultra-high speed or ultra-low noise) of more specialized components. The most common application is as a detector in infrared remote control systems for televisions, audio equipment, and other home appliances. It is also applicable in simple IR wireless data transmission links, security alarm systems where an IR beam break is detected, and various proximity or object sensing scenarios. Its robustness and simplicity make it a staple in entry-level to mid-range electronic designs requiring IR sensing capability.

2. In-Depth Technical Parameter Analysis

This section provides a detailed, objective interpretation of the electrical and optical parameters specified in the datasheet, explaining their significance for circuit design.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. They are not conditions for normal operation.

• Power Dissipation (P_D): 100 mW. This is the maximum amount of power the device can dissipate as heat, primarily determined by I_C * V_CE. Exceeding this limit risks thermal runaway and failure.
• Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter terminals when the base (light input) is open. Exceeding this can cause avalanche breakdown.
• Emitter-Collector Voltage (V_ECO): 5 V. The maximum reverse voltage that can be applied between emitter and collector. This is typically much lower than V_CEO.
• Operating & Storage Temperature: -40°C to +85°C and -55°C to +100°C, respectively. These define the environmental limits for reliable operation and non-operational storage.
• Lead Soldering Temperature: 260°C for 5 seconds at 1.6mm from the package body. This is critical for wave or reflow soldering processes to prevent package damage.

2.2 Electrical & Optical Characteristics

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

• Breakdown Voltages (V_(BR)CEO, V_(BR)ECO): Typically 30V and 5V minimum, respectively. These confirm the device can withstand the voltages listed in the absolute maximum ratings.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): 0.4V max at I_C=100µA and E_e=1 mW/cm². This low voltage indicates good efficiency when the transistor is fully \"on\" (saturated), minimizing power loss.
• Rise and Fall Times (T_r, T_f): 10 µs and 15 µs typical under test conditions (V_CC=5V, I_C=1mA, R_L=1kΩ). These specify the switching speed. The LTR-3208E is not a high-speed device; it is suitable for low to moderate frequency signals like those from remote controls (typically up to a few tens of kHz).
• Collector Dark Current (I_CEO): 100 nA max at V_CE=10V in complete darkness. This is the leakage current that flows when no light is present. A lower value is better for sensitivity, as it represents the noise floor of the detector.

3. Binning System Explanation

The LTR-3208E employs a binning system for its key parameter, the On-State Collector Current (I_C(ON)). Binning is a manufacturing process where components are sorted based on measured performance into different groups (\"bins\") to ensure consistency within a batch.

3.1 Collector Current Binning

The datasheet specifies I_C(ON) under standard test conditions (V_CE=5V, E_e=1mW/cm², λ=940nm). Devices are sorted into bins labeled A through F, each with a defined minimum and typical current range.

• Bin A: 0.64 to 1.68 mA
• Bin B: 1.12 to 2.16 mA
• Bin C: 1.44 to 2.64 mA
• Bin D: 1.76 to 3.12 mA
• Bin E: 2.08 to 3.60 mA
• Bin F: 2.40 mA (Typical, Max likely similar to Bin E)

Design Implication: This binning is crucial for design. If a circuit requires a minimum photocurrent to trigger a logic level, the designer must select a bin that guarantees this current under worst-case conditions (minimum irradiance, maximum temperature). Using a device from Bin E or F provides higher signal strength, which can improve range or allow for the use of a higher-value load resistor for increased voltage swing. Conversely, for very sensitive circuits, even a Bin A device might be sufficient. The bin code is typically part of the full ordering part number.

4. Performance Curve Analysis

The datasheet includes several graphs depicting how key parameters vary with environmental and operational conditions.

4.1 Collector Dark Current vs. Ambient Temperature (Fig. 1)

This curve shows that I_CEO increases exponentially with temperature. At 85°C, the dark current can be orders of magnitude higher than at 25°C. This is a fundamental semiconductor behavior. For applications operating at elevated temperatures, this increased leakage current raises the noise floor, potentially reducing the sensitivity or requiring compensation in the signal processing circuit (e.g., a higher detection threshold).

4.2 Collector Power Dissipation vs. Ambient Temperature (Fig. 2)

This graph illustrates the concept of \"derating.\" As the ambient temperature (T_A) increases, the maximum allowable power dissipation (P_C) decreases linearly. At T_A=85°C, the maximum power dissipation is significantly less than the 100mW rating at 25°C. Designers must calculate the actual power (I_C * V_CE) in their application and ensure it falls below the derated curve at the maximum expected operating temperature to avoid thermal overload.

4.3 Rise and Fall Time vs. Load Resistance (Fig. 3)

This curve demonstrates a classic trade-off in phototransistor circuit design. Rise and fall times (T_r, T_f) increase with larger load resistance (R_L). A larger R_L provides a larger output voltage swing (ΔV = I_C * R_L) but slows down the switching speed because the transistor's junction capacitance takes longer to charge and discharge through the larger resistor. Designers must choose R_L to balance the need for signal amplitude against the required bandwidth of the IR signal.

4.4 Relative Collector Current vs. Irradiance (Fig. 4)

This graph shows the relationship between incident infrared light power (irradiance E_e) and the resulting collector current (I_C). The response is generally linear over a certain range. This linearity is important for analog applications where the signal strength carries information. The slope of this line represents the responsivity of the phototransistor (mA per mW/cm²). The graph confirms that under a constant V_CE, the output current is directly proportional to the light input, which is the fundamental operating principle.

5. Mechanical and Packaging Information

5.1 Outline Dimensions and Tolerances

The device has a standard transistor-style package (likely similar to T-1 or similar). Key dimensions include the body size, lead spacing, and overall height. Tolerances are typically ±0.25mm unless specified otherwise. The lens is integrated into the package for focusing incoming IR light, enhancing sensitivity. A notable feature is the allowance for a maximum of 1.5mm of protruded resin under the flange, which is important for PCB layout and clearance.

5.2 Polarity Identification

Phototransistors have three terminals: Collector (C), Emitter (E), and the optical \"Base\" which is light. The package will have a physical marker, such as a flat side or a tab, to identify the emitter lead. The collector is usually the middle lead in a standard three-lead package. Correct polarity is essential for proper biasing and circuit operation.

6. Soldering and Assembly Guidelines

While detailed reflow profiles are not provided, the absolute maximum rating gives a critical guideline: leads can be soldered at 260°C for a maximum of 5 seconds, measured 1.6mm from the package body. This is a standard rating for plastic packages. For reflow soldering, a standard lead-free profile with a peak temperature around 260°C is acceptable, provided the time above liquidus is controlled. For hand soldering, a temperature-controlled iron should be used, and heat should be applied to the lead quickly and efficiently to avoid prolonged heating of the package itself, which could damage the internal die attach or the plastic. Storage should be in a dry, controlled environment as per the storage temperature range to prevent moisture absorption, which can cause \"popcorning\" during soldering.

7. Application Notes and Design Considerations

7.1 Typical Application Circuit

The most common circuit configuration is the \"common-emitter\" mode. 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 strikes the phototransistor, it conducts, causing a voltage drop across R_L. The output signal is taken from the collector node. The value of R_L is chosen based on the desired output voltage swing and bandwidth, as shown in the performance curves. A bypass capacitor may be added at the supply or output to filter noise.

7.2 Design Considerations

• Biasing: The phototransistor is inherently biased by the light signal. No external electrical bias is applied to the base.
• Load Resistor Selection: As analyzed, this is a critical trade-off between signal amplitude (voltage swing) and speed (rise/fall time). For remote control applications (low frequency), a resistor in the range of 1kΩ to 10kΩ is common.
• Ambient Light Rejection: The dark plastic package provides significant rejection of visible light. However, strong ambient IR sources (sunlight, incandescent bulbs) can still cause interference. Optical filtering (an additional IR-pass filter) or modulation/demodulation of the IR signal (as used in remote controls) are common techniques to improve noise immunity.
• Interfacing with Logic: The output is an analog voltage. To interface with a digital input (like a microcontroller), a comparator or a Schmitt-trigger input should be used to provide a clean digital signal with hysteresis, preventing chatter due to noise or slowly changing light levels.

8. Technical Comparison and Differentiation

The LTR-3208E's primary differentiation lies in its dark plastic package. Compared to a clear or transparent packaged phototransistor, it offers superior rejection of visible ambient light, leading to a better signal-to-noise ratio in environments with fluctuating visible light. Its performance parameters (speed, dark current) are typical for a general-purpose device, making it less suitable for very high-speed data links or ultra-low-light detection compared to specialized PIN photodiodes or avalanche photodiodes (APDs). Its advantage is simplicity, robustness, and cost-effectiveness for its intended market segment. The binning system for collector current provides designers with a guaranteed performance level, which is a key advantage over un-binned or loosely specified components.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: What does the \"E\" in LTR-3208E signify?

A: It typically indicates a specific variant or revision. In this context, it likely denotes the special dark plastic package version, as mentioned in the features.

Q: Can I use this phototransistor with a 940nm IR LED from a different manufacturer?

A: Yes, it is specifically tested at 940nm, which is the most common wavelength for consumer IR applications. Ensure the LED's output spectrum aligns well with the phototransistor's sensitivity peak (which is typically also around 940nm for this material).

Q: Why is my output signal slow or distorted at high frequencies?

A> Check the value of your load resistor (R_L). As shown in Fig. 3, a large R_L increases rise and fall times, limiting bandwidth. For faster signals, use a smaller R_L and possibly amplify the smaller voltage swing with a subsequent op-amp stage.

Q: The device gets warm during operation. Is this normal?

A> Some heating is normal due to power dissipation (P = V_CE * I_C). Refer to Fig. 2. Calculate your actual power dissipation and ensure it is below the derated curve for your ambient temperature. If it is too high, reduce the supply voltage, collector current, or improve heat sinking/airflow.

10. Practical Use Case Example

Scenario: Designing a simple IR proximity sensor for a toy.

An IR LED is pulsed at a low frequency (e.g., 1kHz). The LTR-3208E (from Bin D for good sensitivity) is placed nearby. When an object comes close, it reflects the IR pulses back to the detector. The phototransistor's collector, connected to V_CC=5V through a 4.7kΩ resistor, produces a pulsating voltage. This signal is fed into a band-pass filter amplifier tuned to 1kHz to reject ambient light noise, then into a peak detector and comparator. The comparator's output goes high when the reflected signal exceeds a threshold, indicating the presence of an object. The dark package of the LTR-3208E helps reject room lighting, and its moderate speed is perfectly adequate for the 1kHz modulation.

11. Operating Principle Introduction

A phototransistor operates on the same principle as a standard bipolar junction transistor (BJT) but with the base current generated by light instead of an electrical connection. The device is essentially a transistor where the base-collector junction acts as a photodiode. When photons with sufficient energy (infrared, in this case) strike the base-collector depletion region, they generate electron-hole pairs. This photogenerated current acts as the base current (I_B). Due to the transistor's current gain (β or h_FE), this small base current is amplified, resulting in a much larger collector current (I_C = β * I_B). This internal gain is what gives a phototransistor higher sensitivity than a simple photodiode (which has no gain), though often at the expense of slower response time and higher dark current.

12. Technology Trends and Context

Discrete infrared phototransistors like the LTR-3208E represent a mature and stable technology. Their development has focused on cost reduction, package optimization (like the light-filtering package), and consistent manufacturing through binning. The trend in infrared sensing is moving towards integration. Many modern systems use integrated solutions that combine a photodiode, transimpedance amplifier, and sometimes a digital interface (like I2C) into a single package. These integrated sensors offer better performance, lower noise, and simpler design but at a higher cost. Therefore, discrete components like the LTR-3208E continue to hold a strong position in high-volume, cost-driven applications where basic functionality is sufficient and board space allows for discrete circuitry. The demand for reliable, low-cost IR detection in IoT devices, smart home accessories, and basic industrial sensors ensures the ongoing relevance of such components.