LTR-546AB Phototransistor Datasheet - Dark Blue Package - 30V Reverse Voltage - 150mW Power Dissipation - English Technical Documentation

1. Product Overview

The LTR-546AB is a silicon NPN phototransistor designed for infrared radiation detection. Its core advantage lies in its specialized dark blue plastic package, which effectively filters out visible light, making it highly suitable for pure infrared sensing applications where ambient light interference must be minimized. This component is targeted at markets requiring reliable, fast-response infrared detection, such as proximity sensing, object detection, encoders, and remote control receivers.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

The device is rated for a maximum power dissipation of 150 mW at an ambient temperature (T_A) of 25°C. The absolute maximum reverse voltage (V_R) is 30 V, defining the upper limit for safe operation without risking breakdown. The operational temperature range is specified from -40°C to +85°C, with a wider storage temperature range of -55°C to +100°C. For assembly, the leads can withstand a soldering temperature of 260°C for 5 seconds when measured 1.6mm from the body.

2.2 Electrical & Optical Characteristics

Key performance parameters are defined at T_A=25°C. The reverse breakdown voltage (V_(BR)R) is typically 30V at a reverse current (I_R) of 100μA. The reverse dark current (I_D(R)) is very low, with a maximum of 30 nA at V_R=10V and no illumination. This low dark current is crucial for signal-to-noise ratio in low-light detection. The device exhibits a peak spectral sensitivity (λ_SMAX) at a wavelength of 900 nm, aligning it with common infrared emitter wavelengths like 940 nm. Under specific test conditions (V_R=5V, λ=940nm, E_e=0.1mW/cm²), the short-circuit current (I_S) is typically 2 μA. The switching speed is characterized by rise and fall times (T_r, T_f) of 50 nsec each, enabled by a low junction capacitance (C_T) of 25 pF maximum at V_R=3V. The open-circuit voltage (V_OC) is typically 350 mV under illumination.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that are essential for design engineers.

3.1 Dark Current vs. Reverse Voltage

Figure 1 shows the relationship between dark current (I_D) and reverse voltage (V_R). The curve demonstrates that the dark current remains at a very low picoampere level until the reverse voltage approaches the breakdown region, confirming stable operation within the recommended voltage range.

3.2 Capacitance vs. Reverse Voltage

Figure 2 illustrates how the total capacitance (C_T) decreases with increasing reverse bias. This is a typical behavior of a phototransistor's junction capacitance. Lower capacitance directly contributes to the device's high cut-off frequency and fast switching times, as seen in the 50 nsec specifications.

3.3 Photocurrent vs. Irradiance & Temperature

Figure 6 plots photocurrent (I_P) against irradiance (E_e) at 940 nm. The relationship is linear over a significant range, which is desirable for analog sensing applications. Figure 3 shows how photocurrent varies with ambient temperature, typically decreasing as temperature increases, which must be compensated for in precision designs. Figure 4 shows the dark current's positive temperature coefficient, increasing with temperature.

3.4 Spectral Sensitivity

Figure 5 is a critical graph showing the relative spectral sensitivity versus wavelength. It confirms the device's peak response at 900 nm and its significant sensitivity in the near-infrared region (approx. 800-1100 nm), while the dark blue package effectively attenuates sensitivity in the visible light spectrum.

3.5 Power Derating

Figure 8 presents the total power dissipation versus ambient temperature. It shows the allowable power dissipation decreasing linearly as the ambient temperature rises above 25°C, a standard derating curve necessary for thermal management in the application.

4. Mechanical & Package Information

The LTR-546AB uses a dark blue plastic package. Key dimensional notes include: all dimensions are in millimeters, with a general tolerance of ±0.25mm unless specified otherwise. The maximum protrusion of resin under the flange is 1.5mm. Lead spacing is measured at the point where the leads emerge from the package body. The specific package drawing (not fully detailed in the provided text) would show the exact dimensions for PCB footprint design.

5. Soldering & Assembly Guidelines

The datasheet specifies a lead soldering temperature of 260°C for a maximum duration of 5 seconds, measured 1.6mm (0.063") from the package body. This is a standard reflow or wave soldering parameter. Designers must ensure the thermal profile during assembly does not exceed this limit to prevent damage to the semiconductor junction or the plastic package. Standard ESD (Electrostatic Discharge) precautions should be observed during handling.

6. Application Suggestions

6.1 Typical Application Scenarios

The LTR-546AB is ideal for applications requiring detection of modulated or pulsed infrared light. Common uses include: infrared remote control receivers, proximity sensors in appliances or robotics, object detection in vending machines or printers, slot sensors in encoders, and break-beam sensors.

6.2 Design Considerations

Biasing: The device can be used in two common configurations: photodiode mode (with reverse bias, V_R applied) for fastest speed and linear response, or phototransistor mode (with collector-emitter bias) for higher gain. The choice depends on the required speed versus sensitivity trade-off.
Load Resistor (R_L): The value of the load resistor in the collector circuit affects both output voltage swing and bandwidth. A smaller R_L improves speed but reduces signal amplitude.
Optical Coupling: For best performance, pair the detector with an infrared emitter (IRED) at a matching wavelength, typically 940 nm. Consider using lenses, apertures, or optical filters to shape the field of view and reject unwanted ambient light, even though the dark blue package provides some filtering.
Circuit Layout: Keep the phototransistor and its associated amplifier circuitry close together to minimize parasitic capacitance and noise pickup. Bypass capacitors on the supply lines are recommended.

7. Technical Comparison & Differentiation

The primary differentiating feature of the LTR-546AB is its dark blue plastic package. Compared to clear or non-filtered packages, this provides inherent suppression of visible light, reducing noise in environments with fluctuating ambient light (e.g., indoor lighting). Its combination of low capacitance (25 pF max) and fast switching times (50 nsec) makes it suitable for higher-frequency modulated light applications compared to slower, higher-capacitance phototransistors. The 30V reverse voltage rating offers a good margin for circuit design robustness.

8. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the dark blue package?
A: It acts as a visible light filter. It transmits infrared light (to which the silicon chip is sensitive) while attenuating most of the visible spectrum. This improves the signal-to-noise ratio by reducing the detector's response to ambient room light, sunlight, or indicator LEDs.

Q: How do I interpret the "Short Circuit Current (I_S)" parameter?
A: I_S is the photocurrent generated when the voltage across the device is zero (short-circuited). It represents the maximum current the device can produce for a given irradiance level (0.1 mW/cm² in the test condition). In a practical circuit with a load resistor, the output current will be slightly less.

Q: What does "High Cut-off Frequency" imply for my design?
A: A high cut-off frequency means the device can respond to rapidly changing light signals. This is essential for applications using pulsed or modulated infrared light, such as remote controls (typically 36-40 kHz carrier) or high-speed data transmission. The 50 nsec rise/fall times support modulation frequencies into the hundreds of kilohertz.

Q: How does temperature affect performance?
A: As shown in the curves, both dark current and photocurrent are temperature-dependent. Dark current increases with temperature, potentially raising the noise floor. Photocurrent generally decreases with increasing temperature. For precision applications over a wide temperature range, temperature compensation circuitry or calibration may be necessary.

9. Practical Design Case

Case: Designing a Simple Infrared Proximity Sensor.
Objective: Detect an object within 10 cm.
Implementation: Place an infrared LED (emitting at 940 nm) and the LTR-546AB phototransistor side-by-side, facing the same direction. Drive the LED with a pulsed current (e.g., 1 kHz, 50% duty cycle) to distinguish its signal from ambient IR. Bias the phototransistor in photodiode mode with a 10V reverse bias and a 10kΩ load resistor connected to a comparator or microcontroller ADC. When an object is present, infrared light reflects off it and into the phototransistor, causing a voltage change across the load resistor. The pulsed drive allows synchronous detection in the microcontroller, further rejecting ambient light noise. The dark blue package of the LTR-546AB helps minimize false triggers from visible light sources.

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-546AB (NPN type), photons with energy greater than the bandgap of silicon (corresponding to wavelengths shorter than ~1100 nm) are absorbed in the base-collector junction region. This absorption creates electron-hole pairs. The electric field in the reverse-biased base-collector junction sweeps these carriers, generating a photocurrent. This photocurrent acts as the base current for the transistor. The transistor then amplifies this current, resulting in a collector current that is the photocurrent multiplied by the current gain (h_FE) of the transistor. This internal gain provides higher sensitivity compared to a simple photodiode, though often at the cost of slower response time. When used in photodiode mode (with only the base-collector junction biased), the internal transistor action is disabled, offering faster speed and better linearity.

11. Technology Trends

The field of optoelectronics continues to evolve. Trends relevant to components like the LTR-546AB include:
Miniaturization: Ongoing reduction in package size for integration into smaller consumer electronics and IoT devices.
Enhanced Integration: Movement towards combining the photodetector with amplification, digitization, and digital interface logic (like I2C) into single packages, simplifying system design.
Improved Wavelength Selectivity: Development of detectors with sharper spectral response curves or tunable sensitivity, often through integrated optical filters or novel semiconductor materials, for more precise color or chemical sensing.
Higher Speed & Lower Noise: Continuous improvement in materials and fabrication processes to achieve faster response times and lower dark currents, enabling higher data rates in optical communication and more sensitive detection in scientific instruments.
While discrete phototransistors like the LTR-546AB remain vital for cost-effective, high-volume applications requiring simple infrared detection, these trends are expanding the capabilities of optoelectronic sensors.