Ams Tmd2635 Low Power Proximity Detection User Manual

TMD2635 Low Power Proximity Detection
User Manual

Application Note
AN000646
Minimizing Power Consumption with the TMD2635, TMD2636 and TMD2637

Introduction

Information
This document applies to the arms TMD2635, TMD2636, and TMD2637 devices. References to these devices will be abbreviated to “TMD2635/6/7” throughout.
The ams TMD2635/6/7 was developed for proximity sensing applications requiring small size, low power, and short-range operation. Example applications include battery-powered wearable products such as true wireless stereo earbuds, glasses, watches, etc.
Low power system designers need to strike a balance between competing requirements for optimal performance and minimal power consumption. The TMD2635/6/7 offers features that support low power operation but the application’s required proximity response will ultimately limit the degree to which these features can be leveraged. This application note will describe the settings of the TMD2635/6/7 that affect power consumption and demonstrate how to calculate power consumption based on the sensor’s configuration.

Information
For guidance on optimizing the sensor’s proximity response, refer to as application note AN000556 “Proximity Detection – Optimizing Proximity Parameters.”
Configuring the TMD2635/6/7 for low power operation requires an understanding of current consumed during all operating states and how to control the duration of those states. Knowing these two things, the “duty cycle” of the sensor can be adjusted. Duty cycle is defined as the fraction of one operating period in which the sensor is active; therefore a “low” duty cycle means that the sensor spends most of its time in a low power state. Ideally, the sensor should operate at full power only as long as is necessary to meet the application’s proximity response requirements.
Duty cycle control of the TMD2635/6/7 can be achieved in a couple of ways. The first and most basic is through the configuration of the sensor’s proximity cycle timing.

Proximity Cycle Timing & States

When the TMD2635/6/7is fully operating, it loops in a ‘proximity cycle’ whose timing is controlled by configuration register settings. A single proximity sample (Figure 1) consists of multiple user-programmable pulses (quantity “n”) which enable the  IR light source (emitter) when high. In the case of the TMD2635/6/7, the IR emitter is called a “vertical-cavity surface-emitting laser” (VCSEL) and the IR detector is a photodiode. PRATE is the duration of the entire proximity sample.
Figure 1: Pulse Timing for a Single  proximity Sample 

The proximity cycle state diagram (Figure 2) shows the sequence of operation and denotes the average amount of IDD current consumed during each state. The proximity ample pulsing shown above occurs entirely during the ACTIVE state. This state consumes the most power by far, drawing over 11× more average IDD current than IDLE states and 485× more than the SLEEP state.
IDD current is sourced from a 1.8 V supply and is consumed by the TMD2635/6/7’s digital circuitry.
During the ACTIVE state only, there is another current that is consumed by the TMD2635/6/7’s VCSEL IR emitter. This current is sourced from a VCSEL-dedicated 3.3 V supply and flows only during the high time of each drive pulse. The average value of this current is in the 3.5–15 mA¹ range. Although the drive pulses are narrow, the VCSEL becomes a more significant power consumer as the number and the width of these pulses increase. It is important that the 3.3 V power supply is capable of supporting pulses up to 15 mA in amplitude and 32 µs in duration.
IDD and VCSEL current combine during the ACTIVE state to create a major portion of the sensor’s overall power load to the system. Pulse length and the number of pulses should be minimized when configuring the sensor for the required proximity response.

¹VCSEL drive current is factory-trimmed to achieve a specific proximity response at a given PLDRIVE setting. The actual drive current can be as much as ± 50% of the value listed in the PLDRIVE bit field. This means the nominal 7–10 mA range can actually be 3.5–15 mA.

Figure 2: Proximity Cycle Simplified State Diagram

Configuration Register Settings

There is a specific set of register settings that directly affect both the timing and the power consumption of the proximity cycle.
The following bit fields configure the pulse train of a single proximity sample:

• PULSE (number of pulses, 1–64). Power consumption is directly proportional to the number of pulses.
• PPULSE_LEN (pulse length, 1–32 µs). Power consumption is directly proportional to the pulse length.
• PRATE (proximity sample duration, 88–22528 µs). Longer PRATE can reduce power consumption by decreasing the duty cycle of the proximity loop.

The following bit fields add optional wait time between proximity samples:

• PW TIMES (proximity wait time, 2.78–712 ms). Longer wait times can reduce power consumption by decreasing the duty cycle of the proximity loop.
• PWLONG (wait time 12× extension, 33.36–8544 ms). Longer wait times can reduce power consumption by decreasing the duty cycle of the proximity loop.

The following bit field averages multiple proximity samples into a single result:

• PROX_AVG (hardware averaging, 2–128 samples). Power consumption is directly proportional to the number of hardware averaging repetitions.

Note that the PMAVG bit field (proximity moving average, 2, 4, or 8 values) adds delay only at loop start-up and thereafter it has no effect on duty cycle or power consumption.
Although the following bit field does not affect loop timing, it does affect power consumption:

• PLDRIVE (VCSEL drive current, 7–10 mA). Power consumption is directly proportional to VCSEL drive strength

Once the optimal proximity response has been established, potential power savings can be explored by experimenting with reductions in duty cycle and drive current.

Application Software Control

The second level of duty cycle control exists at the application software level. The host processor can be programmed to cycle the sensor’s active/inactive states in either a “polled” or an “interrupt-driven” fashion.
A polling cycle consists of waking up the sensor, sampling data then putting the sensor back to sleep.
An interrupt-driven cycle consists of waking up the sensor, reading its previous sample then letting it free-run. When the next data event occurs, the sensor signals an interrupt to the host and automatically goes into a low power state.
The interrupt cycle described above employs the TMD2635/6/7 “Sleep after Interrupt” (SAI) feature. Referring to Figure 2, when the IDLE state is entered following an SAI-enabled interrupt, the oscillator is deactivated to reduce power consumption. The device can only be reactivated by clearing interrupt flags in the STATUS register.
TMD2635/6/7 interrupt events are programmable. Refer to the latest datasheets for a description of the following features that configure interrupt trigger conditions:

• Proximity High and Low Thresholds – Generate an interrupt when PDATA falls outside of the threshold range and the persistence filter value is reached. See the PILTL, PILTH, PIHTL, and PIHTH register descriptions in the TMD2635/6/7 datasheets.
• Persistence Filter – Counts consecutive occurrences of PDATA values that fall outside of the threshold range. See the PERS register description in the TMD2635/6/7 datasheets.
• Proximity Interrupt Mode (PIM) – Qualifies interrupt assertion based on the threshold crossing’s level or state change. See the INTENAB register description and Figure 43 in the TMD2635/6/7 datasheets.

Timing in the interrupt-driven scenario is less deterministic than in a polled scenario. Event-driven duty cycles depend on host response time and the changing stimulus in the sensor’s field of view that trigger interrupts based on the register configuration described above. This timing variability makes accurate power calculation difficult unless simplifying assumptions are made. Bench characterization is the best way to accurately assess power consumption under such dynamic conditions.
Assuming that the host will service interrupts promptly, the sensor will spend a majority of its time in states that draw idle current (30 µA average) while free-running, waiting for the next interrupt trigger event to drive it into a low power state. This interrupt-driven approach will not be able to conserve power as well as a polled design that purposely maximizes the time spent in SLEEP state drawing 0.7 µA average current.

Power Calculation Examples

Proximity Cycle Power Calculations
The TMD2635/6/7 datasheets provide all of the information needed to calculate the average current consumed during the sensor’s proximity cycle. For example, consider a sensor configured as follows:
Figure 3: Device Configuration Proximity Cycle

Based on the sensor configuration above, duty cycle time calculations can be made.
Figure 4: Loop Calculations Proximity Cycle

Applying the duty cycle time calculations above to the appropriate power supplies will determine the average current drawn from each source.
Figure 5: Power Calculations Proximity Cycle

The calculations demonstrated above are made based on the proximity cycle of the sensor while it is fully operational and free-running.

Polling Cycle Power Calculations

The following example calculates the average power consumed by a hypothetical polling cycle that consists of waking up the sensor, collecting a single data sample from 1 proximity cycle than putting the sensor back to sleep. Consider the following sensor configuration with software-controlled duty cycle timing:
Figure 6: Device Configuration Polling Cycle

Based on the sensor configuration above, duty cycle time calculations can be made.
Figure 7: Time Calculations Polling Cycle

In the polling cycle described above, the host interaction with the sensor generates bus activity that consumes system current through the I²C bus pull-up resistors. This power can be accounted for in the sensor’s total power calculation.
To analyze I²C bus activity, consider the following assumptions about the steps taken by the polling software routine, including the number of 9-bit bytes transferred during each transaction

1. Write the ENABLE register to wake up the sensor (3 bytes)
2. Wait at least (Wakeup Time + Active Time) for a proximity cycle to occur
3. Read the PDATAL and PDATAH registers (5 bytes)
4. Write the ENABLE register to put the sensor to sleep (3 bytes)

This short routine generates 11 bytes of bus activity.
Consider the following I²C bus parameters:

Figure 8: I²C Bus Parameters

Applying the duty cycle time calculations above to the appropriate power supplies will determine the average current drawn from each source. Using the I²C bus parameters above, bus current and power consumed during each polling cycle can be estimated.
More details on I²C read and write transactions can be found in the TMD2635/6/7 datasheets.

Figure 9: Power Calculations Polling Cycles

Using the full bit period in the current calculation accounts for both the SCL and SDA lines assuming that they both maintain a 50% duty cycle and that half of the data bits are low during each bus transaction. Actual bus power will vary with data value

Summary

Implementing proximity detection in a low-power system should begin by optimizing the sensor response according to the system detect/release requirements. Power savings can then be explored by experimenting with changes in the proximity cycle timing, emitter drives current, and duty cycling at the application software level. A polled sensor design will provide more predictable duty cycle control that can achieve lower average power consumption than an interrupt-driven approach. Calculation of TMD2635/6/7 average power consumption can be made using the techniques outlined in this document.

References

For further information, please refer to the following documents:

• TMD2635 Datasheet
• TMD2636 Datasheet
• TMD2637 Datasheet
• AN000556 Proximity Detection – Optimizing Proximity Parameters

Revision Information

• Page and figure numbers for the previous version may differ from page and figure numbers in the current revision.
• Correction of typographical errors is not explicitly mentioned.

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Application Note • PUBLIC
AN000646 • v3-00 • 2021-Mar-26