LTR-323DB Phototransistor Datasheet - 5mm Package - 30V Reverse Voltage - 940nm Wavelength - English Technical Document

1. Product Overview

The LTR-323DB is a silicon NPN planar phototransistor designed for infrared detection. Its primary function is to convert incident infrared light into an electrical current. The device features a built-in lens that enhances its optical sensitivity, making it suitable for applications requiring reliable detection of IR signals. Key positioning points include its fast response time and low junction capacitance, which are critical for high-frequency or pulsed light sensing.

The core advantages of this component lie in its performance specifications. It offers a high cut-off frequency enabled by the fast switching characteristics. The device is engineered for stability across a wide operating temperature range, from -40°C to +85°C. Its primary target markets include industrial automation, consumer electronics for remote control systems, safety and security equipment, and various opto-isolation circuits where precise and rapid light detection is necessary.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

The absolute maximum ratings define the stress limits beyond which permanent damage to the device may occur. These are not operating conditions.

• Power Dissipation (P_D): 150 mW. This is the maximum allowable power the device can dissipate as heat at an ambient temperature (T_A) of 25°C. Exceeding this limit risks thermal runaway and failure.
• Reverse Voltage (V_R): 30 V. This is the maximum voltage that can be applied in reverse bias across the collector-emitter junction. The breakdown voltage (V_(BR)R) is typically equal to or greater than this value.
• Operating Temperature Range (T_A): -40°C to +85°C. The device is guaranteed to meet its electrical specifications within this ambient temperature range.
• Storage Temperature Range (T_stg): -55°C to +100°C. The component can be stored without applied power within these limits without degradation.
• Lead Soldering Temperature: 260°C for 5 seconds, measured 1.6mm from the package body. This defines the reflow or hand-soldering profile to prevent package cracking or internal damage.

2.2 Electrical & Optical Characteristics

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

• Reverse Breakdown Voltage, V_(BR)R: Min. 30 V (I_R = 100µA, E_e=0). Confirms the device can withstand the stated maximum reverse voltage.
• Reverse Dark Current, I_D(R): Max. 30 nA (V_R=10V, E_e=0). This is the leakage current when no light is incident. A low value is critical for signal-to-noise ratio in low-light detection.
• Open Circuit Voltage, V_OC: Typ. 350 mV (λ=940nm, E_e=0.5 mW/cm²). The voltage generated across the open-circuited device under illumination, indicative of its photovoltaic capability.
• Rise Time (T_r) & Fall Time (T_f): Max. 50 nsec each (V_R=10V, λ=940nm, R_L=1kΩ). These fast switching times enable the detection of high-frequency modulated IR signals, a key feature for remote control and data transmission.
• Short Circuit Current, I_S: Min. 8 µA, Typ. 13 µA (V_R=5V, λ=940nm, E_e=0.1 mW/cm²). The photocurrent when the output is shorted. This parameter directly relates to sensitivity.
• Total Capacitance, C_T: Max. 25 pF (V_R=3V, f=1MHz, E_e=0). Low junction capacitance contributes to the high cut-off frequency and fast response.
• Peak Sensitivity Wavelength, λ_SMAX: Typ. 900 nm. The device is most sensitive to infrared light near this wavelength, making it ideal for pairing with 940nm IR LEDs.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that illustrate performance under varying conditions.

3.1 Dark Current vs. Reverse Voltage (Fig. 1)

This curve shows the relationship between reverse dark current (I_D) and applied reverse voltage (V_R) in complete darkness. The current remains very low (in the pA to low nA range) until it approaches the breakdown region. This confirms the device's excellent off-state characteristics, minimizing false triggering from noise.

3.2 Capacitance vs. Reverse Voltage (Fig. 2)

This graph depicts how the junction capacitance (C_T) decreases as the reverse bias voltage increases. This is a typical behavior of a PN junction. Operating at a higher reverse voltage (within limits) can reduce capacitance, further improving high-frequency response.

3.3 Photocurrent & Dark Current vs. Ambient Temperature (Fig. 3 & 4)

Figure 3 shows how photocurrent varies with temperature. Photocurrent typically has a positive temperature coefficient, meaning it may increase slightly with temperature for a constant irradiance. Figure 4 shows that dark current (I_D) increases exponentially with temperature. This is a critical design consideration: at high temperatures, the rising dark current can become a significant noise source, potentially masking weak optical signals.

3.4 Relative Spectral Sensitivity (Fig. 5)

This is perhaps the most important optical curve. It plots the device's normalized responsivity across the light spectrum. The LTR-323DB shows peak sensitivity around 900nm and useful response from approximately 800nm to 1050nm. It is virtually insensitive to visible light, making it immune to ambient light interference in many environments.

3.5 Photocurrent vs. Irradiance (Fig. 6)

This curve demonstrates the linear relationship between incident light power (irradiance E_e) and the generated photocurrent (I_P) at a specific wavelength (940nm). The linearity is good over several decades of irradiance, which is essential for analog sensing applications where the light intensity carries information.

3.6 Sensitivity Diagram & Power Derating (Fig. 7 & 8)

Figure 7 illustrates the angular sensitivity pattern, which is shaped by the built-in lens. It shows the effective field of view. Figure 8 is the power derating curve, showing how the maximum allowable power dissipation decreases as the ambient temperature rises above 25°C. This graph is essential for thermal management in the application design.

4. Mechanical & Package Information

4.1 Package Dimensions

The LTR-323DB comes in a standard 5mm radial leaded package. Key dimensions include:

• Package diameter is approximately 5mm.
• Lead spacing is measured where leads emerge from the package body.
• A maximum resin protrusion of 1.5mm under the flange is allowed.
• All dimensional tolerances are typically ±0.25mm unless otherwise specified.

Polarity Identification: The longer lead is typically the collector, and the shorter lead is the emitter. The package may also have a flat side or other marking near the cathode (emitter) lead. Always verify polarity before installation to prevent damage.

5. Soldering & Assembly Guidelines

Proper handling is crucial for reliability.

• Reflow Soldering: Follow the specified profile: peak temperature of 260°C for a maximum of 5 seconds, measured 1.6mm (0.063") from the package body. Use a controlled thermal profile to avoid thermal shock.
• Hand Soldering: Apply heat to the lead, not the package body. Limit soldering time per lead to less than 3 seconds with a soldering iron tip temperature below 350°C.
• Cleaning: Use mild cleaning agents compatible with epoxy resin. Avoid ultrasonic cleaning as it may damage the internal die or wire bonds.
• Storage Conditions: Store in a dry, anti-static environment within the specified storage temperature range (-55°C to +100°C). Moisture-sensitive devices should be kept in sealed bags with desiccant.

6. Application Suggestions

6.1 Typical Application Scenarios

• Infrared Remote Control Receivers: Its fast switching time (50ns) makes it ideal for decoding signals from TV, audio, and appliance remotes using 38kHz or 40kHz modulation.
• Object Detection & Counting: Used in break-beam sensors for automation, vending machines, and security gates.
• Optical Encoders: Detecting slots on a rotating disk for speed or position sensing.
• Opto-isolators: Providing electrical isolation between circuits while transmitting a signal via light.
• Light Barriers & Safety Curtains: In industrial safety systems.

6.2 Design Considerations

• Bias Circuit: The phototransistor can be used in two common configurations: photoconductive mode (reverse-biased, faster response) or photovoltaic mode (zero bias, no dark current). For speed, use a reverse bias (e.g., 5V-10V) with a load resistor (R_L). The value of R_L trades off between output voltage swing and bandwidth (due to RC time constant with C_T).
• Ambient Light Rejection: Since the device is sensitive to 900nm IR, it can be affected by sunlight or incandescent bulbs which contain IR. Use a physical IR-pass filter (blocking visible light) or modulated light sources with synchronous detection in critical applications.
• Temperature Compensation: For precision analog sensing over a wide temperature range, consider circuitry to compensate for the variation in dark current and photocurrent with temperature.
• Lens Alignment: The built-in lens has a specific viewing angle. Ensure proper optical alignment with the IR source for maximum signal strength.

7. Technical Comparison & Differentiation

Compared to a standard photodiode, a phototransistor like the LTR-323DB provides internal current gain (h_FE of the bipolar transistor), resulting in much higher output current for the same light input. This eliminates the need for an external transimpedance amplifier in many simple detection circuits. Compared to other phototransistors, the LTR-323DB's key differentiators are its fast switching time (50ns) and low capacitance (25pF max), which together enable a higher useful bandwidth. The integrated lens also provides higher sensitivity and directivity than devices with a flat window.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the difference between short circuit current (I_S) and photocurrent in the curves?

A: I_S is a specific parameter measured under short-circuit conditions (V_R=5V simulates a low-impedance load). The photocurrent (I_P) in the curves is the general output current, which depends on the load resistor and bias voltage. For a small load resistor, I_P ≈ I_S.

Q: Can I use this with a 850nm IR LED?

A: Yes, but with reduced sensitivity. Refer to Figure 5. The relative sensitivity at 850nm is lower than at 900nm. You may need a stronger IR source or optical gain to achieve the same output signal.

Q: Why does dark current increase with temperature, and why does it matter?

A: Dark current is caused by thermally generated charge carriers in the semiconductor junction. As temperature rises, more carriers are generated, increasing the current. This current is indistinguishable from photocurrent, so it acts as noise. In high-temperature or low-light-level applications, this noise can limit the minimum detectable signal.

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

A: It's a trade-off. A larger R_L gives a larger output voltage swing for a given photocurrent (V_out = I_P * R_L) but slows down the response due to the time constant τ = R_L * C_T. For fast response (e.g., remote control), use a smaller R_L (e.g., 1kΩ as in the test condition). For maximum voltage output in slower applications, use a larger R_L, but ensure the voltage drop across the transistor does not exceed its ratings.

9. Practical Application Case Study

Case: Designing a Proximity Sensor for a Mobile Device.

The LTR-323DB can be used with a co-located 940nm IR LED to detect the presence of an object (like a user's ear during a phone call). The design would pulse the IR LED and measure the phototransistor's output. When an object is near, reflected IR light increases the photocurrent. Key design steps:

1. Circuit Configuration: Operate the phototransistor in photoconductive mode with a 5V reverse bias and a load resistor (e.g., 10kΩ). The output is taken from the collector.
2. Modulation & Demodulation: Pulse the IR LED at a specific frequency (e.g., 10kHz). Use a synchronous detection circuit or a microcontroller's ADC to measure only the signal at that frequency. This rejects ambient light (which is typically DC or 50/60Hz).
3. Threshold Setting: Calibrate the system to establish a baseline output with no object and a threshold value indicating proximity. The difference between Figure 3 (photocurrent) and Figure 4 (dark current) curves informs the expected signal range across temperatures.
4. Optical Design: Use a small barrier between the LED and phototransistor to minimize direct coupling and maximize sensitivity to reflected light. The lens of the LTR-323DB helps focus on the nearby field.

This case highlights the use of fast switching (for pulsed operation), sensitivity (to detect weak reflections), and the importance of managing temperature-dependent dark current.

10. Operating Principle

A phototransistor is fundamentally a bipolar junction transistor (BJT) where the base current is generated by light instead of an electrical connection. In the LTR-323DB NPN structure:

1. Infrared photons with energy greater than the silicon bandgap enter the base-collector depletion region.
2. These photons generate electron-hole pairs.
3. The electric field in the reverse-biased collector-base junction sweeps these carriers, creating a photocurrent.
4. This photocurrent acts as the base current (I_B) for the transistor.
5. The transistor then amplifies this current, producing a much larger collector current (I_C = h_FE * I_B). This is the output signal.

The integrated lens concentrates incoming light onto the active semiconductor area, increasing the number of photons absorbed and thus improving sensitivity. The fast switching time is achieved through careful design of the semiconductor geometry and doping profiles to minimize carrier transit times and junction capacitance.

11. Technology Trends

The field of infrared detection continues to evolve. Trends relevant to devices like the LTR-323DB include:

• Integration: Moving towards integrated solutions that combine the photodetector, amplifier, and signal conditioning circuitry (e.g., in a single IC). This simplifies design and improves noise immunity.
• Miniaturization: Development of phototransistors in smaller surface-mount packages (SMD) like 1206, 0805, or even chip-scale packages to meet the demands of compact consumer electronics.
• Enhanced Performance: Ongoing research aims to further reduce capacitance and dark current while maintaining or increasing sensitivity, enabling higher data rates in optical communication and more precise low-light sensing.
• Wavelength Specificity: Development of detectors with sharper spectral filtering integrated into the package to improve rejection of unwanted ambient light sources.

Despite these trends, discrete radial-leaded phototransistors like the LTR-323DB remain highly relevant due to their simplicity, reliability, low cost, and ease of use in a vast array of established applications.