SLVU641A January   2012  – November 2021 TPS54120

 

  1.   Trademarks
  2. 1Background
  3. 2Setup
    1. 2.1 Input and Output Connections and Jumper Descriptions
    2. 2.2 Modifications
      1. 2.2.1 SW Output Voltage Setpoint
      2. 2.2.2 LDO Output Voltage Setpoint
      3. 2.2.3 Switcher Slow-Start Time
      4. 2.2.4 LDO Start-Up
    3. 2.3 Equipment Interconnect
  4. 3Operation
  5. 4Test Results
    1. 4.1 Output Voltage Ripple
    2. 4.2 Output Noise
    3. 4.3 Output Turn-On
    4. 4.4 Load Transient
    5. 4.5 Efficiency
    6. 4.6 Thermal Characteristic
  6. 5Board Layout
    1. 5.1 Layout Description
  7. 6Schematic
  8. 7Bill of Materials
  9. 8Revision History

2.2.2 LDO Output Voltage Setpoint

The output voltage of the LDO also can be set by an external resistor divider network (R1 and R2). R2 is fixed at 10kΩ. To change the LDO output voltage of the EVM, it is necessary to change the value of resistor R1. Changing the value of R1 can change the output voltage from 0.8 V to 6 V. The value of R1 for a specific output voltage can be calculated using Equation 2. Note that the LDO output should be 0.8 V below the SW output for best PSR and noise performance.

Equation 2. R1 = 10 kΩ (LDO VOUT – 0.8 V) / (0.8 V)

Table 2-2 lists the R1 values for some common output voltages. Note that the minimum VIN equals VOUT + VDO or 2.2 V, whichever is greater. The values given in Table 2-2 are standard values, not the exact value calculated using Equation 2.

Table 2-2 Sample 1% Resistor Values for Common LDO Output Voltages
LDO OUTPUT VOLTAGE (V)R1 VALUE (kΩ)
0.80 (Short)
12.49
1.24.99
1.58.87
1.812.5
2.521
3.330.9
552.3