LTR-S320-TB-L Infrared Phototransistor Datasheet - Side View Package - 940nm Peak Wavelength - English Technical Documentation

1. Product Overview

The LTR-S320-TB-L is a discrete infrared phototransistor designed for sensing applications in the near-infrared spectrum. It belongs to a broad family of optoelectronic components intended for use in systems requiring reliable infrared detection. The device is engineered to convert incident infrared radiation into a corresponding electrical signal at its output terminals.

The core function of this component is based on the photoelectric effect within a semiconductor junction. When infrared light of sufficient energy (corresponding to its peak sensitivity wavelength) strikes the photosensitive area, it generates electron-hole pairs. In a phototransistor, this photocurrent is internally amplified, resulting in a much larger collector current compared to a simple photodiode, making it suitable for detecting lower light levels or for use with simpler circuitry.

Its primary design goals include compatibility with modern automated assembly processes, robustness for infrared reflow soldering, and a form factor that facilitates integration into space-constrained printed circuit board (PCB) layouts.

1.1 Features

• Compliant with RoHS (Restriction of Hazardous Substances) directives and classified as a Green Product.
• Features a side-view package configuration with a dark epoxy dome lens. The side-view orientation allows the sensor to detect infrared signals parallel to the PCB plane, which is useful for edge-sensing applications or when the IR source is not perpendicular to the board.
• The dark lens material helps attenuate visible light, reducing interference from ambient light sources and improving the signal-to-noise ratio for infrared signals.
• Supplied in 8mm carrier tape on 7-inch diameter reels, optimized for high-speed, automated pick-and-place assembly equipment.
• Package and materials are designed to withstand standard infrared (IR) reflow soldering profiles used in surface-mount technology (SMT) assembly lines.
• Conforms to EIA (Electronic Industries Alliance) standard package outlines, ensuring mechanical compatibility with industry-standard footprints and handling equipment.

1.2 Applications

• Infrared Receiver Modules: Primarily used as the sensing element in receivers for remote control systems (e.g., for TVs, audio equipment, air conditioners). It detects the modulated infrared signals from a remote control.
• PCB-Mounted Infrared Sensors: Integrated directly onto PCBs for proximity sensing, object detection, or data transmission in devices like smartphones, tablets, appliances, and industrial equipment.
• Security and Alarm Systems: Can be used in beam-break sensors or reflective object sensors for intrusion detection.
• Industrial Automation: Employed in equipment for counting, positioning, or detecting the presence/absence of objects on assembly lines.

2. Technical Parameter Deep Dive

This section provides a detailed, objective interpretation of the key electrical and optical parameters that define the performance and operational limits of the LTR-S320-TB-L phototransistor.

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 and should be avoided in reliable design.

• Power Dissipation (Pd): 75 mW at an ambient temperature (Ta) of 25°C. This is the maximum amount of power the device can dissipate as heat without exceeding its thermal limits. The derating curve (Fig. 2 in the datasheet) shows how this rating decreases as ambient temperature increases.
• Collector-Emitter Voltage (V_CEO): 30 V. The maximum voltage that can be applied between the collector and emitter terminals with the base open-circuited.
• Emitter-Collector Voltage (V_ECO): 5 V. The maximum reverse voltage that can be applied between the emitter and collector.
• Operating Temperature Range: -40°C to +85°C. The ambient temperature range over which the device is specified to operate correctly.
• Storage Temperature Range: -55°C to +100°C. The temperature range for storing the device when not powered.
• Infrared Soldering Condition: Withstands a peak temperature of 260°C for a maximum of 10 seconds. This defines its compatibility with lead-free (Pb-free) reflow soldering processes.

2.2 Electrical & Optical Characteristics

These are the typical and guaranteed performance parameters measured under specific test conditions at 25°C.

• Peak Sensing Wavelength (λ_p): 940 nm. The infrared wavelength at which the phototransistor is most sensitive. It is optimally matched to the emission wavelength of common 940nm GaAs infrared emitting diodes (IREDs).
• Collector Dark Current (I_CEO): Maximum 100 nA at V_CE=20V, E_e=0 mW/cm². This is the small leakage current that flows through the collector when no infrared light is incident (dark condition). A lower dark current is generally better for sensitivity to weak signals.
• On-State Collector Current (I_C(ON)): Typical 2.0 mA, Minimum 1.0 mA at V_CE=5V and an irradiance (E_e) of 0.5 mW/cm² with a 940nm source. This parameter indicates the output current level for a given standard input light intensity. The test tolerance is ±15%.
• Collector-Emitter Saturation Voltage (V_CE(SAT)): Maximum 0.4 V at I_C=100µA, E_e=0.5 mW/cm². This is the voltage drop across the transistor when it is fully "on" (saturated) under the specified low-current condition.
• Rise Time (T_r) & Fall Time (T_f): Typical 15 µs each at V_CE=5V, I_C=1mA, R_L=1kΩ. These parameters define the switching speed of the phototransistor—how quickly the output current can rise from 10% to 90% of its final value (rise time) and fall from 90% to 10% (fall time) in response to a step change in light. This speed is suitable for standard remote control protocols (e.g., 36-40kHz carrier).

3. Performance Curve Analysis

The datasheet includes several graphs that illustrate how key parameters vary with operating conditions. Understanding these curves is crucial for robust circuit design.

3.1 Spectral Sensitivity (Fig. 5)

This curve plots the relative sensitivity of the phototransistor across a range of wavelengths. It confirms the peak sensitivity at 940nm and shows a significant roll-off at shorter (visible) and longer (far-infrared) wavelengths. The dark lens contributes to attenuating sensitivity in the visible spectrum, reducing noise from ambient light.

3.2 Relative Collector Current vs. Irradiance (Fig. 3)

This graph shows the relationship between the output collector current and the incident infrared light power density (irradiance). It is generally linear over a certain range, indicating that the output current is directly proportional to light intensity, which is desirable for analog sensing applications. The curve helps designers determine the expected output for a given light input.

3.3 Collector Dark Current vs. Temperature (Fig. 1) & Power Dissipation Derating (Fig. 2)

Figure 1 demonstrates that the dark current (I_CEO) increases exponentially with rising ambient temperature. This is a critical consideration for high-temperature applications, as increased dark current raises the noise floor and can reduce the effective sensitivity. Figure 2 shows the derating of the maximum allowable power dissipation as ambient temperature increases. Above 25°C, the device can safely handle less power, as its ability to dissipate heat to the environment is reduced.

3.4 Rise/Fall Time vs. Load Resistance (Fig. 4)

This curve illustrates a fundamental trade-off in phototransistor circuit design. The switching speed (rise/fall time) is highly dependent on the load resistor (R_L) connected to the collector. A larger R_L increases output voltage swing but also increases the RC time constant, slowing down the response time. A smaller R_L yields faster switching but a smaller output signal. Designers must choose R_L based on whether speed or signal amplitude is more critical for their application.

4. Mechanical & Packaging Information

4.1 Outline Dimensions

The device is housed in a side-view, surface-mount package. Key dimensions include the body size, lead spacing, and lens position. All critical dimensions are provided in millimeters with a standard tolerance of ±0.1mm unless otherwise specified. The side-view orientation is clearly indicated in the drawing.

4.2 Polarity Identification

The component has two leads. The datasheet drawing indicates which lead is the collector and which is the emitter. Correct polarity must be observed during PCB assembly. Typically, the longer lead (if present in tape packaging) or a marked corner on the tape indicates the collector.

4.3 Suggested Soldering Pad Layout (Section 6)

A recommended land pattern (footprint) for the PCB is provided. This includes the pad dimensions, spacing, and shape to ensure a reliable solder joint after reflow. The recommendation includes using a metal stencil with a thickness of 0.1mm (4 mils) or 0.12mm (5 mils) for solder paste application.

5. Soldering & Assembly Guidelines

5.1 Reflow Soldering Profile

A detailed infrared reflow profile is recommended for lead-free (Pb-free) assembly processes. Key parameters include:

• Pre-heat: Ramp to 150-200°C.
• Soak/Pre-heat Time: Up to 120 seconds maximum.
• Peak Temperature: Maximum 260°C.
• Time Above Liquidus (TAL): The time within 5°C of the peak temperature should not exceed 10 seconds. The device should not be subjected to more than two reflow cycles under these conditions.

The profile is based on JEDEC standards to ensure reliable soldering without damaging the component's epoxy package or internal structure.

5.2 Hand Soldering

If hand soldering is necessary, a soldering iron with a temperature not exceeding 300°C should be used. The contact time for each lead should be limited to a maximum of 3 seconds per solder joint.

5.3 Storage & Handling

• Sealed Package: Devices are shipped in moisture-barrier bags with desiccant. They should be stored at ≤30°C and ≤60% RH. Once the sealed bag is opened, the components are considered moisture-sensitive.
• Floor Life: After opening the original packaging, it is recommended to complete the IR reflow process within one week (168 hours).
• Extended Storage/Baking: For storage beyond one week after opening, components should be kept in a sealed container with desiccant. If exposed beyond this time, a bake at 60°C for at least 20 hours is required before soldering to remove absorbed moisture and prevent "popcorning" (package cracking) during reflow.

5.4 Cleaning

Isopropyl alcohol or similar alcohol-based solvents are recommended for cleaning flux residues, if required. Harsh or aggressive chemical cleaners should be avoided.

6. Packaging & Ordering Information

6.1 Tape and Reel Specifications

The component is supplied on standard 7-inch (178mm) diameter reels. Key packaging details include:

• Carrier Tape Width: 8mm.
• Quantity per Reel: 3000 pieces.
• Minimum Order Quantity (MOQ): 500 pieces for remainder quantities.
• Pocket Coverage: Empty component pockets are sealed with cover tape.
• Missing Components: A maximum of two consecutive missing components is allowed per the packaging standard.
• The packaging conforms to ANSI/EIA-481-1-A specifications.

7. Application Design Considerations

7.1 Drive Circuit Configuration

The phototransistor is a current-output device. The most common circuit configuration is to connect it in a common-emitter setup:

• The emitter is connected to ground.
• The collector is connected to the positive supply voltage (V_CC) through a load resistor (R_L).
• The output signal is taken from the collector node. When light strikes the sensor, the transistor turns on, pulling the collector voltage low (towards V_CE(SAT)). When dark, the transistor is off, and the collector voltage is high (pulled up to V_CC by R_L).

The value of R_L is critical and involves a trade-off between output voltage swing, response speed (see Fig. 4), and power consumption. A typical starting value is 1kΩ to 10kΩ.

7.2 Improving Signal-to-Noise Ratio (SNR)

• Optical Filtering: The built-in dark lens provides some filtering. For environments with strong ambient light, an additional external infrared bandpass filter centered at 940nm can be used to block unwanted light.
• Electrical Filtering: Since many IR remote controls use a modulated carrier frequency (e.g., 38kHz), incorporating a bandpass filter tuned to this frequency in the subsequent amplifier stage can dramatically improve SNR by rejecting DC ambient light and low-frequency noise.
• Shielding: Mechanically shielding the sensor from direct exposure to ambient light sources (e.g., sunlight, lamps) can reduce noise.

7.3 Pairing with an IR Emitter

For reflective or proximity sensing applications, pair the LTR-S320-TB-L with an infrared LED that emits at or near 940nm. Ensure the drive current for the emitter is sufficient to produce the required reflected signal at the detector. Pulsing the emitter and synchronously detecting the phototransistor's output can help distinguish the signal from ambient light.

8. Technical Comparison & Differentiation

Compared to a standard photodiode, the LTR-S320-TB-L phototransistor offers inherent current gain (beta/h_FE), providing a much larger output signal for the same light input. This simplifies circuit design as it often requires less subsequent amplification. However, this gain comes at the cost of slower response times (microseconds vs. nanoseconds for photodiodes) and higher dark current. The side-view package differentiates it from top-view sensors, offering design flexibility for sensing along the edge of a PCB. Its compatibility with automated SMT assembly and standard reflow profiles makes it a cost-effective choice for high-volume manufacturing compared to through-hole alternatives.

9. Frequently Asked Questions (FAQs)

9.1 What is the purpose of the dark lens?

The dark epoxy lens acts as a visible light filter. It attenuates light in the visible spectrum while allowing infrared wavelengths (around 940nm) to pass through. This reduces the sensor's sensitivity to ambient room light, fluorescent lights, and sunlight, thereby minimizing noise and improving the reliability of detecting the intended infrared signal.

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

The choice involves a trade-off. Use Figure 4 in the datasheet as a guide. For maximum speed (fastest rise/fall times), choose a smaller R_L (e.g., 1kΩ or less). For maximum output voltage swing (higher signal amplitude), choose a larger R_L (e.g., 10kΩ or more), but this will slow down the response. Ensure the voltage drop across R_L when the transistor is on (I_C(ON) * R_L) does not exceed your supply voltage minus V_CE(SAT).

9.3 Can this sensor be used outdoors?

It can be used outdoors with careful design. Direct sunlight contains a significant amount of infrared radiation and can saturate the sensor or introduce noise. Effective optical filtering (a narrow 940nm bandpass filter), proper housing to block direct sun, and modulated signal detection techniques are essential for reliable outdoor operation.

9.4 Why is baking required before soldering if the bag is opened for more than a week?

The plastic epoxy package can absorb moisture from the air. During the high-temperature reflow soldering process, this trapped moisture can rapidly vaporize, creating high internal pressure. This can cause the package to crack or delaminate, a failure known as "popcorning." Baking at 60°C drives out this absorbed moisture, making the component safe for reflow.

10. Practical Design Example

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

1. Goal: Detect when an object is within ~5cm of the sensor.
2. Components: LTR-S320-TB-L phototransistor, 940nm IR LED, microcontroller (MCU).
3. Circuit: The phototransistor is connected with R_L = 4.7kΩ to V_CC (3.3V). Its collector output connects to an MCU's analog-to-digital converter (ADC) pin. The IR LED is placed next to the phototransistor and is driven by an MCU output pin through a current-limiting resistor (e.g., 20mA).
4. Operation: The MCU pulses the IR LED at a specific frequency (e.g., 1kHz) for a short burst. It then reads the ADC value from the phototransistor. When no object is present, the reflected signal is low. When an object is within range, infrared light reflects back to the phototransistor, causing a measurable increase in the ADC reading. A threshold is set in the MCU software to detect proximity.
5. Considerations: The sensor must be shielded from ambient IR sources. The pulse-and-measure technique helps distinguish the signal from ambient light. The value of R_L is chosen for a good voltage swing at the expected reflected light level while maintaining reasonable speed.