LTR-546AD Infrared Phototransistor Datasheet - Dark Green Package - 30V Reverse Voltage - 150mW Power Dissipation - English Technical Document

1. Product Overview

The LTR-546AD is a high-performance silicon NPN phototransistor designed specifically for the detection of infrared radiation. Its core function is to convert incident infrared light into an electrical current. The device is housed in a special dark green plastic package, which is engineered to attenuate visible light, thereby enhancing its sensitivity and signal-to-noise ratio in infrared-specific applications. This makes it an ideal choice for systems where discrimination between visible and infrared light is critical.

The primary target markets for this component include industrial automation (e.g., object detection, counting, and position sensing), consumer electronics (e.g., remote control receivers, proximity sensors), security systems (e.g., beam break sensors), and various communication systems utilizing infrared data links.

2. In-Depth Technical Parameter Analysis

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.

• 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 30V, aligning with this rating.
• Operating Temperature Range: -40°C to +85°C. The device is guaranteed to function within this ambient temperature range.
• Storage Temperature Range: -55°C to +100°C. The device can be stored without applied power within this wider range.
• 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 at T_A=25°C and define the device's performance.

• Reverse Dark Current (I_D(R)): Max 30 nA at V_R=10V, E_e=0 mW/cm². This is the leakage current flowing through the phototransistor in complete darkness. A low value is essential for high sensitivity, as it represents the noise floor of the detector.
• Open Circuit Voltage (V_OC): Typ. 350 mV at λ=940nm, E_e=0.5 mW/cm². This is the voltage generated across the open-circuited phototransistor when illuminated. It is a photovoltaic effect parameter.
• Short Circuit Current (I_S): Min 1.7 μA, Typ. 2 μA at V_R=5V, λ=940nm, E_e=0.1 mW/cm². This is the photocurrent generated when the output is short-circuited, directly proportional to the irradiance.
• Rise/Fall Time (T_r, T_f): 50 nsec each at V_R=10V, λ=940nm, R_L=1KΩ. These parameters define the switching speed of the phototransistor, crucial for high-frequency modulation and data transmission applications.
• Total Capacitance (C_T): 25 pF at V_R=3V, f=1MHz. A low junction capacitance contributes to the high cut-off frequency and fast switching times by reducing the RC time constant of the circuit.
• Peak Sensitivity Wavelength (λ_SMAX): 900 nm. The device is most sensitive to infrared light at this wavelength. It is optimally paired with infrared emitters (like LEDs) operating at 940nm, as indicated in other test conditions.

3. Performance Curve Analysis

The datasheet provides several key graphs illustrating performance under varying conditions.

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

This curve shows that the reverse dark current (I_D) remains very low (in the pA to low nA range) for reverse voltages up to approximately 15-20V. Beyond this point, it begins to increase more sharply as it approaches the breakdown region. For reliable operation, the applied reverse voltage should be kept well below the breakdown voltage to minimize dark current and associated noise.

3.2 Capacitance vs. Reverse Voltage (Fig.2)

The graph demonstrates that the junction capacitance (C_t) decreases with increasing reverse bias voltage. This is a characteristic of semiconductor junctions, where a wider depletion region under higher reverse bias reduces capacitance. Designers can use a higher bias voltage (within limits) to achieve faster response times in speed-critical applications.

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

Figure 3 shows that the photocurrent (I_p) has a positive temperature coefficient; it increases slightly with rising ambient temperature for a constant irradiance. Figure 4 shows that the dark current (I_D) increases exponentially with temperature. This is a critical design consideration: while signal (photocurrent) may increase slightly with heat, the noise (dark current) increases much more dramatically, potentially degrading the signal-to-noise ratio at high temperatures.

3.4 Relative Spectral Sensitivity (Fig.5)

This is one of the most important curves. It plots the normalized responsivity of the phototransistor across a range of wavelengths from about 800nm to 1100nm. The sensitivity peaks around 900nm and has a significant bandwidth, typically covering the common IR ranges of 850nm and 940nm. The dark green package effectively blocks shorter, visible wavelengths, as shown by the low sensitivity below ~750nm.

3.5 Photocurrent vs. Irradiance (Fig.6)

This graph shows the linear relationship between the generated photocurrent (I_p) and the incident infrared irradiance (E_e). The phototransistor operates in a linear region for a wide range of irradiance levels, making it suitable for both simple on/off detection and analog light intensity measurement.

4. Mechanical & Packaging Information

4.1 Package Dimensions

The LTR-546AD uses a standard 3mm radial leaded package. Key dimensional notes from the datasheet include:

• All dimensions are in millimeters (inches).
• Standard tolerance is ±0.25mm (±0.010") unless otherwise specified.
• A maximum resin protrusion of 1.5mm (0.059") under the flange is allowed.
• Lead spacing is measured at the point where leads emerge from the package body.

The dark green epoxy resin used for the lens and body is formulated for high infrared transmittance while blocking visible light.

4.2 Polarity Identification

Phototransistors are polarized devices. The longer lead is typically the collector, and the shorter lead is the emitter. The flat side on the package rim may also indicate the emitter side. Correct polarity must be observed during circuit assembly for proper biasing and operation.

5. Soldering & Assembly Guidelines

To ensure reliability and prevent damage during the assembly process:

• Soldering: The leads can withstand a temperature of 260°C for a maximum of 5 seconds, measured at a distance of 1.6mm (0.063") from the package body. This guideline applies to wave soldering. For reflow soldering, a standard lead-free profile with a peak temperature not exceeding 260°C is recommended.
• Cleaning: Use standard electronics cleaning solvents that are compatible with epoxy plastic. Avoid ultrasonic cleaning with excessive power, which may damage the internal chip or wire bonds.
• Mechanical Stress: Avoid bending the leads at the root of the package. Use proper lead-forming tools and techniques.
• Storage: Store in a dry, anti-static environment within the specified temperature range (-55°C to +100°C) to prevent moisture absorption and electrostatic discharge (ESD) damage. While phototransistors are less ESD-sensitive than some active devices, standard ESD precautions should be followed.

6. Application Suggestions

6.1 Typical Application Circuits

The LTR-546AD can be used in two primary configurations:

1. Switch Mode (Digital Output): The phototransistor is connected in a common-emitter configuration with a pull-up resistor at the collector. When illuminated, the phototransistor turns on, pulling the collector voltage low. When dark, it turns off, and the resistor pulls the voltage high. The value of the load resistor (R_L) affects both output voltage swing and switching speed (higher R_L gives larger swing but slower speed due to higher RC constant).
2. Linear Mode (Analog Output): The phototransistor is used in a photoconductive mode with a reverse bias. The photocurrent generated is roughly proportional to light intensity and can be converted to a voltage using a transimpedance amplifier (operational amplifier with feedback resistor) for precise light measurement.

6.2 Design Considerations

• Bias Voltage: Select an operating reverse voltage (V_R) that provides a good compromise between low capacitance (for speed), acceptable dark current, and staying safely below the 30V maximum. 5V to 12V is a common range.
• Load Resistor Selection: For switching applications, choose R_L based on required switching speed (see T_r/T_f specs) and the desired logic levels. A 1kΩ to 10kΩ resistor is typical for 5V systems.
• Optical Alignment: Ensure proper alignment with the infrared source. The dark green package has a specific viewing angle; consult the sensitivity diagram (Fig.7) for angular response.
• Ambient Light Rejection: While the dark green package helps, for operation in environments with strong visible light (e.g., sunlight), additional optical filtering or modulation/demodulation techniques may be necessary to avoid false triggering.
• Temperature Compensation: For applications operating over a wide temperature range, consider the significant increase in dark current. Circuitry to compensate for this temperature-dependent offset may be required for precision analog sensing.

7. Technical Comparison & Differentiation

The LTR-546AD offers several key advantages in its category:

• Visible Light Cutoff: The specialized dark green package is a significant differentiator from clear or water-clear packaged photodetectors, providing inherent filtering for infrared-only applications without the need for an external filter.
• Speed: With rise/fall times of 50ns and low junction capacitance, it is suitable for moderately high-speed applications like IR data communication (e.g., remote control signals) compared to slower photodiodes or phototransistors.
• Sensitivity: The phototransistor structure provides internal gain, resulting in higher output current for a given light level compared to a photodiode, simplifying subsequent amplifier design.
• Trade-off: Compared to a PIN photodiode, a phototransistor like the LTR-546AD generally has higher sensitivity but slower response time and larger temperature dependence of dark current. The choice depends on the priority of the application: sensitivity vs. speed/linearity.

8. Frequently Asked Questions (Based on Technical Parameters)

Q1: What is the purpose of the dark green package?
A1: The dark green epoxy acts as a built-in optical filter. It transmits infrared light (around 900nm) efficiently while attenuating visible light. This reduces interference from ambient visible light sources, improving the signal-to-noise ratio in IR detection systems.

Q2: Can I use this with an 850nm IR LED instead of 940nm?
A2: Yes. Referring to the spectral sensitivity curve (Fig.5), the device has significant sensitivity at 850nm, though it is slightly lower than at its peak of 900nm. You will still get good performance, but the output current for a given irradiance will be somewhat less compared to using a 940nm source.

Q3: Why does the dark current increase with temperature, and why does it matter?
A3: Dark current is caused by the thermal generation of electron-hole pairs within the semiconductor junction. This process accelerates exponentially with temperature (Fig.4). In low-light or precision applications, this increasing dark current adds noise and offset to the signal, potentially masking weak optical signals or causing false triggering at high temperatures.

Q4: How do I choose the value of the load resistor (R_L)?
A4: It involves a trade-off. A larger R_L gives a larger output voltage swing (good for noise immunity) but slows down the switching speed due to the increased RC time constant (C_T * R_L). A smaller R_L gives faster speed but a smaller voltage swing. Start with the test condition value (1kΩ) and adjust based on your circuit's speed and voltage requirements.

9. Practical Application Examples

Example 1: Proximity Sensor in an Automatic Faucet
The LTR-546AD is paired with a co-located 940nm IR LED. The LED emits a beam downwards. When a hand is placed under the faucet, it reflects the IR light back to the phototransistor. The resulting increase in photocurrent is detected by a comparator circuit, which triggers the solenoid valve to open. The dark green package prevents activation from changes in visible room lighting.

Example 2: Slot-Type Object Counter
The phototransistor and an IR LED are mounted on opposite sides of a U-shaped bracket, forming a beam. Objects passing through the slot break the beam, causing the phototransistor's output to change state. The fast switching time (50ns) allows for counting very fast-moving objects. The linear photocurrent vs. irradiance relationship could also be used to estimate the size of partially transparent objects based on the amount of light attenuation.

10. Operating Principle

The LTR-546AD is an NPN bipolar phototransistor. It functions similarly to a standard bipolar transistor but uses light instead of a base current to control the collector-emitter current. The base region is exposed to light. When photons with energy greater than the semiconductor bandgap (infrared in this case) strike the base-collector junction, they generate electron-hole pairs. These photogenerated carriers are swept by the internal electric field, effectively creating a base current. This photocurrent is then amplified by the transistor's current gain (β or h_FE), resulting in a much larger collector current. This internal gain is the key advantage over a simple photodiode.

11. Technology Trends

Photodetector technology continues to evolve. Trends relevant to devices like the LTR-546AD include:

• Integration: Moving towards integrated solutions where the photodetector, amplifier, and digital logic (e.g., for ambient light rejection or proximity detection algorithms) are combined into a single chip (e.g., ALS/Proximity sensor modules).
• Miniaturization: Development of phototransistors in smaller surface-mount device (SMD) packages (e.g., chip LEDs) for space-constrained applications.
• Enhanced Performance: Ongoing research aims to improve the speed, sensitivity, and linearity of discrete phototransistors while further reducing dark current and temperature dependence.
• Application-Specific Optimization: Devices are being tailored for specific wavelength bands (e.g., for LiDAR at 905nm or 1550nm) or for operation in harsh environments with wider temperature ranges.

While integrated solutions are growing, discrete components like the LTR-546AD remain vital for cost-sensitive designs, custom optical configurations, and applications requiring specific performance characteristics not met by integrated modules.