SLVAET3 October   2021 TPS8802

 

  1.   Trademarks
  2. 1Introduction
  3. 2System Architecture
    1. 2.1 Battery Voltage
    2. 2.2 VCC Supply
      1. 2.2.1 Connecting VCC to VBST
      2. 2.2.2 Connecting VCC to VBAT Through a Switch
    3. 2.3 MCU Supply
      1. 2.3.1 MCU Connected to VBAT
      2. 2.3.2 MCU Connected to MCU LDO
      3. 2.3.3 MCU with VCC Connected to VBAT Through a Switch
    4. 2.4 Photoelectric Smoke Sensor LED Supply
      1. 2.4.1 LED Connected to VBAT
      2. 2.4.2 LED Connected to PLDO
      3. 2.4.3 LED Connected to LEDLDO
    5. 2.5 Example Schematics
      1. 2.5.1 Smoke and CO Schematics
      2. 2.5.2 Smoke-Only Schematics
  4. 3Current Consumption
    1. 3.1 Standby Current
      1. 3.1.1 TPS8802 Standby Current
      2. 3.1.2 Microcontroller Standby Current
    2. 3.2 Measurement Current
      1. 3.2.1 Smoke Measurement Current
      2. 3.2.2 CO Measurement Current
      3. 3.2.3 Battery Test Current
      4. 3.2.4 User Alarm Test Current
    3. 3.3 Other Current Consumption
      1. 3.3.1 Boost Charge Current
      2. 3.3.2 Initialization Current
  5. 4System Power Calculation and Measurements
    1. 4.1 Power Calculation Spreadsheet
      1. 4.1.1 Power Consumption Overview Page
      2. 4.1.2 Detailed Calculation Pages
    2. 4.2 Power Consumption Measurements
      1. 4.2.1 Power Measurement Method
      2. 4.2.2 Smoke and CO System Measurements
      3. 4.2.3 Smoke-Only System Measurements
  6. 5Summary
  7. 6References

3.1.2 Microcontroller Standby Current

The microcontroller standby and active currents contribute to the total system power consumption. While there is no specific requirement for the microcontroller, a microcontroller in the range of 1 μA standby and 1 mA active current is generally sufficient to not dominate the total power consumption. It is critical that the microcontroller enters its standby mode in between smoke measurements. Some microcontrollers are able to enter an ultra-low power mode, less than 100 nA, if the microcontroller clocks are disabled.

The TPS8802 sleep timer allows the ultra-low power mode to be entered in between measurements. This can be done by writing the sleep time duration to the TPS8802 registers SLPTMR1 and SLPTMR2, then writing 1 to SLP_EN to start the sleep timer. The MCU can enter the ultra-low power mode while the TPS8802 keeps track of time. When the sleep timer finishes, the TPS8802 wakes up the MCU by sending a pin interrupt to the microcontroller.

The microcontroller supply voltage range has a large influence on the microcontroller current. If the boost converter is required to power the microcontroller, all of the microcontroller current is scaled by the boost converter efficiency and input-to-output voltage ratio. If an LDO is required to power the microcontroller, the LDO standby current adds to the total system current. For this reason it is most efficient to use a microcontroller that can be directly powered by the battery. The options for powering the microcontroller are discussed in Section 2.3.