LTR-X1503 Optical Sensor Datasheet - Integrated ALS & PS - I2C Interface - 3.0-3.6V - English Technical Document

1. Product Overview

The LTR-X1503 is a highly integrated, low-voltage optical sensor combining an ambient light sensor (ALS) and a proximity sensor (PS) with a built-in infrared emitter into a single, miniature, chipled, lead-free surface-mount package. This integration simplifies design and saves board space in compact electronic devices.

The core advantage of this sensor lies in its dual functionality. The ALS provides a linear photometric response over a wide dynamic range, making it suitable for applications ranging from very dark to extremely bright ambient lighting conditions. Simultaneously, the built-in proximity sensor can detect the presence or absence of an object at a user-configurable distance, enabling features like display blanking during phone calls or touchscreen deactivation.

The device is primarily targeted at mobile, computing, and consumer electronics markets. Its ultra-small form factor, low power consumption with sleep mode capability, and I2C digital interface make it ideal for smartphones, tablets, laptops, wearables, and IoT devices where efficient power management and space are critical constraints.

1.1 Core Features & Advantages

• Dual Sensing in One Package: Integrates both Ambient Light Sensing (ALS) and Proximity Sensing (PS), reducing component count and PCB footprint.
• Digital I2C Interface: Supports Standard mode (100kHz) and Fast mode (400kHz) for easy communication with host microcontrollers.
• Ultra-Low Power Operation: Features active and standby modes. Typical active supply current is 160 uA for both sensors, while standby current drops to just 1 uA, significantly extending battery life.
• Programmable Interrupt Function: The PS includes an interrupt system with programmable upper and lower thresholds and hysteresis. This eliminates the need for the host processor to continuously poll the sensor, improving overall system efficiency and power savings.
• High Performance ALS: Offers 16-bit effective resolution, a linear response across a wide range, and a spectral response close to the human eye. It includes automatic rejection for 50Hz/60Hz lighting flicker to ensure stable readings under artificial lighting.
• Robust Proximity Sensing: Includes a built-in LED driver, high ambient light suppression capability (up to 10 klux), 16-bit resolution, and crosstalk cancellation algorithms for reliable object detection.
• Factory Calibration: One-time factory trimming minimizes unit-to-unit variation, ensuring consistent performance and easing manufacturing calibration requirements for end customers.
• Wide Operating Range: Operates from 3.0V to 3.6V and across a temperature range of -40°C to +85°C, with a built-in temperature compensation circuit for stable operation.

2. Technical Specifications Deep Dive

2.1 Absolute Maximum Ratings

Stresses beyond these limits may cause permanent damage to the device.

• Supply Voltage (VDD): 3.6 V
• Digital I/O Pins (SCL, SDA, INT): -0.5 V to 3.6 V
• LED Anode Voltage (V_LED): -0.5 V to 4.6 V
• LED Driver Pin Voltage (V_LDR): -0.5 V to 3.6 V
• Storage Temperature: -40°C to 100°C
• ESD Protection (HBM): 2000 V

2.2 Recommended Operating Conditions

For normal device operation.

• Supply Voltage (VDD): 3.0 V to 3.6 V
• LED Supply Voltage (V_LED): 2.8 V to 4.0 V
• Operating Temperature: -40°C to 85°C
• I2C High-Level Input: 1.5 V to VDD
• I2C Low-Level Input: 0 V to 0.4 V

2.3 Electrical & Optical Specifications

Specifications are typically given at VDD = 1.8V and Ta = 25°C.

2.3.1 Power Characteristics

• Supply Current (Both ALS & PS Active): 160 uA (Typical, with 100ms measurement repeat rate).
• ALS Active Current: 160 uA (Typical).
• PS Active Current: 57 uA (Typical, with 8 pulses, 100% duty, 32us pulse width).
• Standby Current: 1 uA (Typical).
• Wake-up Time from Standby: 0.25 ms (Typical).

2.3.2 Ambient Light Sensor (ALS) Characteristics

• Resolution: Programmable to 13, 14, 15, or 16 bits effective.
• Lux Accuracy: ±10% (Typical, under white LED illumination).
• Dark Level Count: 0 to 5 counts (at 0 Lux, 16-bit resolution, 512x gain, 100ms integration).
• Integration Time: Programmable from 0.2 ms to 200 ms.
• Flicker Noise Rejection: ±5% error for 50Hz/60Hz lighting.
• Spectral Response: Close to the photopic response of the human eye.

2.3.3 Proximity Sensor (PS) Characteristics

• Resolution: 16 bits effective.
• Sensitivity Peak Wavelength: 940 nm (Typical, for the integrated IR emitter).
• Detection Distance: Up to 20 cm (Typical, configurable based on pulse number, gain, and current settings).
• LED Pulse Current: Programmable, up to 186 mA (Typical).
• LED Pulse Width: Programmable: 8, 16, 32, or 64 us.
• Number of LED Pulses: Programmable from 1 to 256 pulses per measurement.
• Ambient Light Suppression: Up to 10 klux (direct sunlight). A fail-safe feature prevents false triggers above this level.

3. Performance Curve Analysis

3.1 ALS Spectral Response

The sensor's ambient light photodiode is designed with a filter to match the CIE photopic luminosity function, which defines the standard human eye response to light. This ensures that the lux readings reported by the sensor accurately represent the brightness as perceived by a person, rather than just the raw radiant energy. This is crucial for automatic display brightness control that feels natural to the user.

3.2 PS Performance vs. Distance

The proximity sensor's performance is characterized by the reflected signal strength as a function of distance to a standard reflective object (typically 88% reflectance). The relationship is non-linear and follows an inverse square law. The graph shows that with typical settings (e.g., VDD=1.8V, 104mA LED current, 16 pulses), a clear and measurable signal is obtained, allowing reliable detection thresholds to be set for specific application distances (e.g., 5cm for phone ear detection).

3.3 ALS Angular Response

The sensor's angular response graphs (for X and Y axes) show how the measured light intensity varies with the angle of incidence. A perfectly cosine (Lambertian) response is ideal for most ambient light sensing applications. The LTR-X1503 exhibits a response close to this ideal, ensuring accurate readings regardless of the primary light source's direction relative to the sensor. Deviations from the ideal cosine response at extreme angles (> ±60 degrees) are typical for most sensors due to package and optical design constraints.

4. Mechanical & Package Information

The LTR-X1503 is housed in an ultra-small 8-pin chipled surface-mount package. The exact outline dimensions are provided in the datasheet's dimensional drawing, which includes top, side, and bottom views with critical dimensions such as package length, width, height, lead pitch, and pad sizes. This information is essential for PCB footprint design and ensuring proper mechanical fit within the end product.

4.1 Pin Configuration and Function

• Pin 1 (VDD): Power supply input (3.0V - 3.6V).
• Pin 2 (SCL): I2C serial clock input.
• Pin 3 (GND): Ground connection.
• Pin 4 (LEDA): Anode connection for the integrated infrared LED. Must be connected to the LED supply rail (V_LED).
• Pin 5 (LDR): LED driver connection. This pin should be left floating (NC) as the driver is internal.
• Pin 6 (NC): No internal connection. Can be left unconnected or tied to ground.
• Pin 7 (INT): Active-low interrupt output pin. This open-drain output asserts low when a proximity event (object detect/remove) occurs based on the programmed thresholds.
• Pin 8 (SDA): I2C serial data input/output (open-drain).

5. Application Circuit & Design Guidelines

5.1 Recommended Application Circuit

A typical application circuit includes the sensor, necessary decoupling capacitors, and I2C pull-up resistors.

• Power Decoupling: A 1uF ceramic capacitor (C1) should be placed as close as possible between VDD and GND. An additional 0.1uF capacitor (C2) can be added for high-frequency noise suppression.
• LED Supply Decoupling: A 1uF capacitor (C3) is recommended between the LEDA pin (and V_LED rail) and GND.
• I2C Pull-up Resistors: Resistors (Rp1, Rp2) with values between 1 kΩ and 10 kΩ are required on the SCL and SDA lines. The exact value depends on the bus capacitance and desired rise time; lower values provide stronger pull-up but increase current consumption. A similar pull-up may be needed on the INT line if used.

5.2 Power Sequencing

Critical Requirement: Proper power sequencing must be followed to prevent potential latch-up or damage.

• Power-On: VDD (main logic supply) must be powered on before V_LED (LED supply).
• Power-Off: V_LED must be powered off before VDD.

6. Soldering & Assembly Guidelines

The component is a surface-mount device (SMD) designed for reflow soldering processes common in high-volume electronics manufacturing.

6.1 Reflow Soldering Profile

While the specific datasheet may not detail a profile, a standard lead-free (RoHS compliant) reflow profile is applicable. This typically involves:

• Preheat/Ramp: A gradual ramp (1-3°C/second) to ~150-200°C to activate flux and minimize thermal shock.
• Soak Zone: A plateau at 150-200°C for 60-120 seconds to ensure uniform temperature across the board and evaporate volatiles.
• Reflow Zone: A rapid rise to the peak temperature. The peak temperature should not exceed the maximum package rating (likely 260°C for a short time, e.g., 10-30 seconds above 245°C).
• Cooling: A controlled cool-down phase.

Consult the package's moisture sensitivity level (MSL) and follow appropriate baking and handling procedures if the device has been exposed to ambient humidity beyond its rated threshold.

6.2 Storage Conditions

Devices should be stored in their original moisture-barrier bags with desiccant in a controlled environment (typically <40°C and <90% relative humidity) to prevent oxidation and moisture absorption.

7. Packaging & Ordering Information

The LTR-X1503 is supplied in a tape-and-reel format suitable for automated pick-and-place assembly machines.

• Part Number: LTR-X1503
• Package Type: 8-pin chipled package.
• Packaging: Tape and Reel.
• Standard Quantity per Reel: 3,000 pieces.

8. Application Suggestions

8.1 Typical Application Scenarios

• Smartphones/Tablets: Automatic screen brightness adjustment (ALS) and screen blanking/touch deactivation during calls when the device is held to the ear (PS).
• Laptops & Monitors: Dynamic backlight adjustment for power savings and viewing comfort based on ambient light.
• Wearable Devices: Wake-on-gesture or display activation when the user looks at the device (PS), and brightness management.
• Consumer Electronics: Automatic on/off control in appliances, touchless switches, and presence detection.

8.2 Design Considerations & Best Practices

• Optical Path: Ensure a clear, unobstructed optical path to the environment for the ALS. For the PS, design the window or opening to allow the IR light to exit and reflected light to return efficiently. Avoid placing the sensor behind dark or IR-absorbing materials.
• IR Contamination: The proximity sensor uses 940nm IR light. Sunlight and some artificial lights contain IR components. The sensor's high ambient light suppression and crosstalk cancellation help, but positioning away from direct, strong IR sources improves performance.
• I2C Bus Management: Utilize the interrupt feature to put the host MCU to sleep, waking it only when a proximity event occurs. Poll the ALS at a moderate rate (e.g., once per second) unless rapid brightness changes need tracking.
• Threshold Calibration: The PS detection threshold must be calibrated in the final product enclosure to account for cover glass thickness, reflectivity, and internal reflections (crosstalk). This is typically done during manufacturing.

9. Technical Comparison & Differentiation

The LTR-X1503 competes in a market with other integrated ALS/PS solutions. Its key differentiators likely include:

• High Level of Integration: Combining the IR emitter within the same package as the sensors is a significant advantage, reducing the bill of materials (BOM) and simplifying optical alignment compared to solutions requiring a discrete IR LED.
• Performance: Features like 16-bit resolution for both sensors, high ambient light rejection (10 klux), and programmable measurement parameters offer design flexibility and robust performance.
• Power Efficiency: Competitive low active and standby currents are critical for battery-powered devices.
• Digital Interface: The I2C interface is a standard, widely supported bus, making integration straightforward.

10. Frequently Asked Questions (Based on Technical Parameters)

10.1 How do I set the detection distance for the proximity sensor?

The detection distance is not a single fixed parameter but a result of several configurable settings: LED pulse current, pulse width, number of pulses, and the receiver gain. By increasing the LED current, pulse count, or gain, the reflected signal strength increases, allowing detection of objects at a greater distance or with lower reflectivity. The specific threshold for \"detection\" is set by the user in the interrupt threshold registers by characterizing the PS data count at the desired distance in the final product.

10.2 Why is power sequencing between VDD and V_LED important?

Improper sequencing can cause a large inrush current to flow through the internal ESD protection structures or logic circuits, potentially leading to latch-up—a high-current state that can damage the device. Following the specified sequence (VDD then V_LED on; V_LED then VDD off) ensures internal transistors are properly biased before the higher-voltage LED supply is applied or removed.

10.3 What does \"crosstalk cancellation\" mean for the PS?

Crosstalk refers to internal reflection within the device module or its cover where IR light from the emitter directly reaches the PS photodiode without reflecting off an external object. This creates a background offset that can cause false triggers or reduce sensitivity. The LTR-X1503 incorporates algorithms (often involving a baseline measurement with the LED off) to measure and subtract this crosstalk component from the final PS data, improving the accuracy of object detection.

10.4 How does the ALS achieve 50/60Hz flicker rejection?

Incandescent and fluorescent lights powered by AC mains fluctuate in intensity at 100Hz or 120Hz (twice the line frequency). If the sensor's integration time is a multiple of the flicker period (e.g., 10ms, 20ms, 100ms), it averages over complete light cycles, canceling out the variation and providing a stable lux reading. The sensor's integration time is programmable to be a multiple of these periods to enable this rejection.

11. Design and Usage Case Study

11.1 Implementing Power-Saving Display Control in a Smartwatch

Scenario: A smartwatch needs to maximize battery life. The display should be bright outdoors, dim indoors, and turn off completely when not viewed (e.g., when the user's arm is down).

Implementation with LTR-X1503:

1. ALS Role: The ALS is configured with a 16-bit resolution and a 100ms integration time (for flicker rejection). The host MCU reads the ALS data every second via I2C. A lookup table or algorithm maps the lux value to a corresponding PWM duty cycle for the display backlight, providing smooth automatic brightness adjustment.
2. PS Role: The PS is configured with an appropriate pulse current and count for the expected watch-to-face distance (e.g., ~30cm). The interrupt thresholds are set: a lower threshold for \"object removed\" (watch not looked at) and an upper threshold for \"object detected\" (watch raised to view). The INT pin is connected to a wake-up-capable GPIO on the MCU.
3. Power Saving Workflow:
   • When the user lowers their arm, the PS count drops below the lower threshold, triggering an interrupt.
   • The MCU wakes from sleep, reads the interrupt status, and commands the display to enter a low-power off state.
   • The MCU can then put itself and the sensor (except perhaps a low-power PS monitoring mode) back to sleep.
   • When the user raises their arm to view the watch, the PS detects the object, triggers an interrupt, wakes the MCU, which then fully powers the display and ALS, showing the correct time at an appropriate brightness.

This combination significantly reduces average system power compared to a display that is always on or only time-controlled.

12. Operating Principle Introduction

12.1 Ambient Light Sensing Principle

The ALS function is based on a photodiode, a semiconductor device that generates a small current proportional to the intensity of light falling on it. In the LTR-X1503, this photodiode is covered by a filter that mimics the human eye's sensitivity across the visible spectrum. The generated photocurrent is very small (picoamps to nanoamps). An integrated transimpedance amplifier converts this current to a voltage, which is then digitized by a high-resolution Analog-to-Digital Converter (ADC). The digital value is processed and made available via the I2C registers, representing the illuminance in counts that can be converted to lux units using a calibrated formula.

12.2 Proximity Sensing Principle

The PS operates on the principle of active infrared reflection. The integrated infrared LED emits short pulses of 940nm light, which is invisible to the human eye. A separate, dedicated photodiode (different from the ALS diode) acts as the receiver. When an object is within range, some of the emitted IR light reflects off the object and returns to the receiver photodiode. The sensor measures the amount of reflected light received during and after each LED pulse. By comparing this signal to the ambient IR level (measured when the LED is off), and after crosstalk cancellation, the sensor calculates a proximity data count. A higher count indicates a closer or more reflective object. This count is compared to the user-programmed thresholds to trigger interrupts.

13. Technology Trends

The market for integrated optical sensors like the LTR-X1503 is driven by several clear trends in the electronics industry:

• Miniaturization: Continuous demand for smaller package sizes (like chipled) to fit into ever-slimmer devices with larger displays and batteries.
• Increased Integration: The trend is moving beyond combining ALS and PS. Future sensors may integrate additional environmental sensors (color, gesture, time-of-flight), further reducing system complexity.
• Intelligence at the Edge: Sensors are gaining more on-chip processing capabilities. Instead of just providing raw data, future versions might perform lux calculation, proximity state machine logic, and gesture recognition internally, sending only high-level event notifications to the host processor, further saving system power.
• Improved Performance: Expectations for accuracy, dynamic range, and power consumption continue to rise. Advances in semiconductor processes and optical design enable lower noise, higher resolution ADCs, and more efficient LEDs.
• Standardization & Software Support: Robust and standardized software drivers (e.g., for Android, Linux) are becoming as important as hardware performance, reducing time-to-market for device manufacturers.