LTR-S320-DB-L Phototransistor Datasheet - EIA Package - 940nm Peak Sensitivity - English Technical Document

1. Product Overview

The LTR-S320-DB-L is a high-performance silicon NPN phototransistor designed for infrared sensing applications. This component is optimized to detect light in the near-infrared spectrum, with peak sensitivity specifically at 940nm, making it suitable for a wide range of remote control systems, object detection, and industrial automation tasks. Its primary function is to convert incident infrared light into a corresponding electrical current.

The device is housed in a standard EIA-compliant package with a black daylight cut-off resin lens. This lens effectively filters out visible ambient light, significantly reducing noise and false triggering, thereby enhancing the signal-to-noise ratio in the presence of background illumination. The package is designed for compatibility with high-volume, automated assembly processes, including tape-and-reel feeding and infrared reflow soldering, aligning with modern manufacturing requirements.

As a RoHS-compliant and lead-free (Pb-free) "Green Product," it meets contemporary environmental standards. The combination of its spectral response, package design, and manufacturing compatibility positions it as a reliable and versatile solution for cost-sensitive and performance-driven infrared detection circuits.

2. Technical Parameters: In-Depth Objective Interpretation

All electrical and optical characteristics are specified at an ambient temperature (T_A) of 25°C, providing a standardized baseline for performance evaluation.

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 circuit design.

• Power Dissipation (P_D): 150 mW. This is the maximum allowable power the device can dissipate as heat. 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 is open (phototransistor in dark condition).
• Operating Temperature Range: -40°C to +85°C. The ambient temperature range over which the device is designed to function correctly.
• Storage Temperature Range: -55°C to +100°C. The temperature range for non-operational storage without degradation.
• Infrared Soldering Condition: 260°C peak temperature for a maximum of 10 seconds. This defines the thermal profile limit for lead-free reflow soldering processes.

2.2 Electrical & Optical Characteristics

These parameters define the device's performance under specific test conditions.

• Reverse Breakdown Voltage (V_(BR)R): Minimum 33V, typical 170V at I_R=100µA. This high value indicates a robust junction capable of withstanding significant reverse bias, which is beneficial for circuits with inductive loads or voltage spikes.
• Reverse Dark Current (I_D): Maximum 10 nA at V_R=10V. This is the leakage current when no light is incident. A low dark current is critical for achieving high sensitivity and low-noise operation, especially in low-light detection scenarios.
• Open Circuit Voltage (V_OC): Typical 390 mV when illuminated by 940nm light at an irradiance (E_e) of 0.5 mW/cm². This parameter is relevant when the device is used in photovoltaic mode (no external bias).
• Short Circuit Current (I_SC): Typical 1.8 µA under the same test conditions as V_OC (V_R=5V, λ=940nm, E_e=0.5 mW/cm²). This represents the photocurrent generated when the output is shorted.
• Rise Time (T_r) & Fall Time (T_f): Maximum 30 ns each (V_R=10V, R_L=1kΩ). These switching speed specifications are crucial for applications requiring fast pulse detection or high-frequency modulation, such as in data communication links.
• Total Capacitance (C_T): Maximum 1 pF at V_R=5V, f=1MHz. Low junction capacitance is essential for maintaining fast response times, as it limits the RC time constant of the circuit.
• Spectral Bandwidth (λ_0.5): 750 nm to 1100 nm. This defines the range of wavelengths where the device's responsivity is at least half of its peak value. It covers the common infrared region used by many IR emitters (like 850nm and 940nm LEDs).
• Peak Sensitivity Wavelength (λ_P): 940 nm. The device is spectrally matched to infrared LEDs emitting at 940nm, ensuring maximum efficiency and signal strength in such pairings.

3. Performance Curve Analysis

The datasheet references typical characteristic curves which provide visual insight into device behavior under varying conditions. While the specific graphs are not reproduced in the text, their typical implications are analyzed below.

3.1 IV (Current-Voltage) Characteristics

A family of curves plotting collector current (I_C) against collector-emitter voltage (V_CE) for different levels of incident irradiance (E_e). These curves would typically show that for a fixed irradiance, I_C increases with V_CE until it reaches a saturation region. Higher irradiance levels shift the curves upward, indicating greater photocurrent. The slope in the active region relates to the device's output conductance.

3.2 Relative Sensitivity vs. Wavelength

This curve graphically represents the spectral response, peaking at 940nm and tapering off towards 750nm and 1100nm (the λ_0.5 points). It is essential for selecting an appropriate IR emitter to pair with the detector and for assessing the impact of ambient light sources with different spectra.

3.3 Temperature Dependence

Curves likely show the variation of key parameters like dark current (I_D) and photocurrent with ambient temperature. Dark current typically increases exponentially with temperature (doubling approximately every 10°C), which can be a significant source of noise in high-temperature applications. Photocurrent may also have a slight negative temperature coefficient.

4. Mechanical & Packaging Information

4.1 Package Dimensions

The device conforms to a standard EIA package outline. All dimensions are provided in millimeters with a standard tolerance of ±0.10 mm unless otherwise specified. The package features a black, daylight-cut-off resin lens molded over the silicon chip.

4.2 Polarity Identification & Pinout

The phototransistor is a 2-pin device. The pinout is standard for such packages: the collector is typically connected to the case or the longer lead (if applicable), while the emitter is the other pin. The datasheet diagram provides the definitive identification. Correct polarity is essential for proper circuit operation.

4.3 Suggested Solder Pad Layout

A recommended land pattern (footprint) for PCB design is provided to ensure reliable solder joint formation during reflow. Adhering to these dimensions helps prevent tombstoning, misalignment, or insufficient solder fillets.

5. Soldering & Assembly Guidelines

5.1 Reflow Soldering Profile

A detailed suggestion for an infrared reflow profile suitable for lead-free (Pb-free) solder processes is provided. Key parameters include:

• Pre-heat: 150°C to 200°C.
• Pre-heat Time: Maximum 120 seconds.
• Peak Temperature: Maximum 260°C.
• Time Above Liquidus (at peak): Maximum 10 seconds.
• Maximum Number of Reflow Cycles: Two.

The profile is based on JEDEC standards to ensure package integrity. Engineers must characterize the profile for their specific PCB design, components, and solder paste.

5.2 Hand Soldering

If hand soldering is necessary, the iron tip temperature should not exceed 300°C, and the soldering time per lead should be limited to a maximum of 3 seconds. Only one hand-soldering cycle is recommended to avoid thermal stress.

5.3 Cleaning

Only specified cleaning agents should be used. Isopropyl alcohol (IPA) or ethyl alcohol are recommended. The device should be immersed at normal temperature for less than one minute. Unspecified chemical liquids may damage the package resin.

5.4 Storage Conditions

Sealed Package (Moisture Barrier Bag): Store at ≤30°C and ≤90% RH. The components are rated for use within one year from the bag seal date.

Opened Package: Store at ≤30°C and ≤60% RH. Components should be reflowed within one week (168 hours). For longer storage outside the original bag, they must be stored in a sealed container with desiccant or in a nitrogen desiccator. Components stored for more than one week should be baked at approximately 60°C for at least 20 hours before soldering to remove absorbed moisture and prevent "popcorning" during reflow.

6. Packaging & Ordering Information

6.1 Tape and Reel Specifications

The device is supplied in 8mm carrier tape on 7-inch (178mm) diameter reels, compatible with standard automatic placement equipment.

• Pieces per Reel: 3000.
• Cover Tape: Empty component pockets are sealed with a top cover tape.
• Missing Components: A maximum of two consecutive missing components ("missing lamps") is allowed per reel specification.
• Standard: Packaging follows ANSI/EIA 481-1-A-1994 specifications.

7. Application Suggestions

7.1 Typical Application Scenarios

• Infrared Remote Control Receivers: For TVs, audio systems, and set-top boxes (paired with a 940nm IR LED).
• Object/Proximity Detection: In printers, copiers, vending machines, and industrial automation for sensing paper, objects, or position.
• Smoke Detectors: In optical chamber-based designs.
• Encoders: For speed or position sensing in motor control.
• Basic Optical Isolation: In low-speed, cost-sensitive isolation circuits.

7.2 Circuit Design Considerations

Drive Method: The phototransistor is a current-output device. For consistent performance, especially when multiple devices are used in parallel, it is strongly recommended to use a current-limiting resistor in series with each phototransistor (Circuit Model A in the datasheet).

Circuit Model A (Recommended): Each phototransistor has its own series resistor connected to the supply voltage. This ensures each device operates at a defined current point, compensating for minor variations in their current-voltage (I-V) characteristics and preventing current hogging by one device.

Circuit Model B (Not Recommended for Parallel Use): Multiple phototransistors connected directly in parallel to a single shared resistor. Due to natural variances in the I-V curve of individual components, one device may draw more current than others, leading to uneven brightness or sensitivity in detection applications.

Biasing: The device is typically used in a common-emitter configuration with a pull-up resistor at the collector. The value of this load resistor (R_L) affects both the output voltage swing and the response speed (via the RC time constant formed with the device capacitance). A smaller R_L gives faster response but a smaller output voltage change.

Noise Immunity: The black daylight cut-off lens provides excellent rejection of visible light. However, for high-noise environments (e.g., with fluorescent lighting or sunlight), additional electrical filtering (e.g., a capacitor in parallel with the load resistor or a hardware/software debounce algorithm) may be necessary to reject modulated interference.

8. Technical Comparison & Differentiation

Compared to a simple photodiode, a phototransistor provides internal current gain (the transistor's beta, β), resulting in a much higher output current for the same incident light level. This makes it easier to interface directly with logic circuits or microcontrollers without requiring a subsequent amplification stage, simplifying design and reducing component count.

However, this gain comes at the cost of slower response times (typically tens to hundreds of nanoseconds for phototransistors vs. nanoseconds for photodiodes) and potentially higher capacitance. For very high-speed applications (e.g., >1 MHz modulation), a photodiode with an external transimpedance amplifier might be a better choice.

The LTR-S320-DB-L's key differentiators within the phototransistor category are its standardized EIA package for manufacturing ease, the specific 940nm spectral matching, the integrated daylight filter lens, and its qualification for lead-free reflow processes.

9. Frequently Asked Questions (Based on Technical Parameters)

9.1 What is the purpose of the "daylight cut-off" lens?

The black resin lens is doped to be opaque to visible light but transparent to infrared wavelengths around 940nm. This dramatically reduces the photocurrent generated by ambient room light, sunlight, or other visible sources, minimizing false triggers and improving the reliability of the IR signal detection.

9.2 Can I use this with an 850nm IR LED?

Yes, but with reduced efficiency. The device's spectral response curve shows significant sensitivity at 850nm (within the 750-1100nm bandwidth), but it is not at the peak (940nm). The output signal will be weaker compared to using a matched 940nm emitter. For optimal performance and maximum range, pairing with a 940nm source is recommended.

9.3 How do I calculate the appropriate series resistor value?

The resistor value depends on the desired operating current and the supply voltage (V_CC). Under a specific irradiance, the phototransistor will behave like a current source. Using Ohm's Law: R = (V_CC - V_CE(sat)) / I_C. V_CE(sat) is the saturation voltage (typically a few hundred mV at moderate currents). I_C is the desired collector current, which can be estimated from the I_SC parameter and the expected light level. Start with the typical I_SC (1.8 µA at 0.5 mW/cm²) and scale it based on your application's irradiance. Choose R to set the operating point in the desired region of the IV curve.

9.4 Why is baking required if the parts are stored outside the bag?

Plastic packages can absorb moisture from the atmosphere. During the high-temperature reflow soldering process, this trapped moisture can rapidly vaporize, creating high internal pressure. This can cause delamination of the package from the die ("popcorning") or internal cracks, leading to immediate or latent failures. Baking drives out this absorbed moisture, making the components safe for reflow.

10. Operational Principle

A phototransistor is fundamentally a bipolar junction transistor (BJT) where the base current is generated by light instead of an electrical connection. Incident photons with energy greater than the bandgap of the silicon create electron-hole pairs in the base-collector junction region. These carriers are swept by the internal electric field, generating a photocurrent that acts as the base current (I_B). This photogenerated base current is then amplified by the transistor's current gain (h_FE or β), resulting in a much larger collector current (I_C = β * I_B). The output is taken from the collector terminal, with the emitter grounded. The absence of a physical base lead is a common feature, though some phototransistors include a base connection for bias control or speed optimization.

11. Development Trends

The field of photodetection continues to evolve. Trends relevant to devices like the LTR-S320-DB-L include:

• Miniaturization: Development of phototransistors in smaller package footprints (e.g., chip-scale packages) to enable denser electronics.
• Enhanced Integration: Combining the photodetector with amplification, filtering, and digital logic on a single chip to create "smart sensors" with digital output (I2C, SPI), reducing external component count and simplifying system design.
• Improved Speed: Research into structures and materials to reduce carrier transit time and capacitance, pushing phototransistor bandwidths higher for data communication applications.
• Wavelength Specificity: Development of detectors with narrower and more precisely tuned spectral responses to improve selectivity in environments with multiple IR sources or to enable new sensing modalities.
• Focus on Reliability and Testing: As optoelectronics penetrate automotive, medical, and industrial safety applications, there is increased emphasis on rigorous qualification standards, extended temperature range operation, and failure mode analysis.

While discrete phototransistors remain vital for many applications due to their simplicity and cost-effectiveness, these trends point towards more sophisticated and application-specific solutions in the future.