SLVAFJ1 February   2023 TPS2121

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 Design Example Specification
  4. 2Design Considerations
    1. 2.1 Highest Voltage Operation Scheme (VCOMP)
    2. 2.2 Output Voltage Dip and Switchover
    3. 2.3 Overvoltage Calculation
  5. 3Test Results
    1. 3.1 Seamless Switchover from Main Power (IN1) to Backup Power (IN2)
    2. 3.2 Seamless Switchover from Backup Power (IN2) to Main Power (IN1)
  6. 4Summary
  7. 5References

2.1 Highest Voltage Operation Scheme (VCOMP)

If both PR1 and CP2 are less than VREF (typ 1.06 V), the device uses an internal comparator between two inputs to determine the priority source. This mode can be easily implemented by pulling down PR1 and CP2 to ground. #GUID-5A5F8EE4-4936-4510-80B1-CCEEAC7A4D9C shows simplified application circuit for this configuration.

Figure 2-1 Simplified Design Example with VCOMP Configuration

#GUID-CDF565C6-5D4D-4876-923C-976E199809BE shows how VCOMP priority source is selected with fixed VIN1. If both of the input voltages are equal, Vcomp and hysteresis ensures that IN2 takes priority. If IN2 falls below the Vcomp and hysteresis, then IN1 will have priority. If IN2 gets reapplied, IN2 takes priority when it falls within Vcomp of IN1. #GUID-09B15B2E-0A7C-4191-91E0-C08C08FEE84C shows the case with fixed VIN2. If IN1 rises above the Vcomp and hysteresis from IN2, then IN1 takes priority. If IN1 falls below the Vcomp from IN2, IN2 takes priority over IN1.

Figure 2-2 VCOMP Priority Source Selection (Fixed VIN1)
Figure 2-3 VCOMP Priority Source Selection (Fixed VIN2)

In this design example, IN2 is used as an auxiliary power source from battery to maintain minimum VOUT as battery voltage. #GUID-09B15B2E-0A7C-4191-91E0-C08C08FEE84C shows how VCOMP priority source is selected with fixed VIN2. See #GUID-FFB831BB-4D98-41E4-A044-AA6C773B13C7 to understand how power selection works by changing the main power and switchover timing. Minimum VOUT is not affected by Vcomp and hysteresis to maintain minimum VOUT. Auxiliary power source takes priority before main power rises above Vcomp and the hysteresis voltage.

Figure 2-4 Seamless Switchover Timing Diagram

If there is no requirement for minimum VOUT, user can select IN2 as main power and IN1 as auxiliary power. Then the system can maximize the main power range for VOUT since switchover occurs when main power falls below auxiliary power due to Vcomp and hysteresis. Therefore the system can minimize battery use to save the capacity.

By pulling CP2 down to ground, this process helps eliminate the leakage current. Also it is recommended not to populate resistors for OV2 setting in this design example to remove the additional leakage current from battery. Table 2-1 shows IQ, ISBY values user can estimate. IQ is the current from enabled input under no load condition. ISBY is the current from disabled input in no load condition. TPS2121 supports typ 15 µA for ISBY. Therefore it can achieve required quiescent current (<50 µA) in this design example. For further details about specification, see the TPS212x 2.8-V to 22-V Priority Power MUX with Seamless Switchover data sheet.

Table 2-1 Specifications for IQ, ISBY
PARAMETERTEST CONDITIONSTJMINTYPMAXUNIT
INPUT SOURCE (IN1, IN2)
IQ, INxQuiescent Current (INx Powering OUT) OUT = Open-40°C to 125°C300400µA
ISBY, INxStandby Current (INx not powering OUT)VOUT = VINx 25°C01525µA
-40°C to 125°C25µA