TIDUEW8 August   2022

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Schematic Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 THS3491 Current Feedback Amplifier Specifications
    3. 2.3 System Design Theory
      1. 2.3.1 Theory of Operation
        1. 2.3.1.1 Concept of Power Supply Range Extension
      2. 2.3.2 Stability Considerations
        1. 2.3.2.1 Inclusion of Series Isolation Resistance (RS)
      3. 2.3.3 Power Dissipation
        1. 2.3.3.1 DC Internal Power Dissipation of Driver Amplifier for a Purely Resistive Output Load
        2. 2.3.3.2 AC Average Internal Power Dissipation of Driver Amplifier for a Purely Resistive Output Load
        3. 2.3.3.3 Internal Average Power Dissipation of Driver Amplifier for RC Output Load
      4. 2.3.4 Thermal Performance
        1. 2.3.4.1 Linear Safe Operating Area (SOA)
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware
    2. 3.2 Test Setup
    3. 3.3 Test Results
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  10. 5Related Documentation
    1. 5.1 Trademarks

2.3.3.1 DC Internal Power Dissipation of Driver Amplifier for a Purely Resistive Output Load

The internal power dissipation of an amplifier consists of two parts: the quiescent power that biases internal op-amp circuitry independent of loading, and the power dissipated in the output transistors when the op amp is loaded. For a high output power amplifier such as THS3491, the majority of power dissipation, and therefore temperature rise, occurs in the output transistors of the device.

The quiescent power dissipation (PQ) shown in Equation 5 is the internal amplifier power dissipation when the amplifier output is open, or when there is no current drive in or out of the amplifier.

Equation 5. P Q W = V c c - V e e × I Q

where:

  • Vcc = The potential at the positive power supply terminal
  • Vee = The potential at the negative power supply terminal
  • IQ = The total quiescent current drawn from the power supply of the device

From Figure 2-5, it is apparent that although Vcc and Vee are variable for the driver amplifier (U1), the potential across the supply rails (Vcc – Vee) is constant and equal to the supply potential of amplifiers U2 and U3. Considering that (Vcc – Vee) = Vs+ = –Vs– = Vs , Equation 5 is written as Equation 6.

Equation 6. P Q W =   V S I Q

The THS3491 has a class AB output stage as shown in Figure 2-8, in which only one of the output transistors is turned on for high or low output voltage depending upon the source or sink current. For a ground-referenced load, the amplifier sources current for positive output voltages, and sinks current for negative output voltages. Equation 7 shows the DC power dissipated in the output transistors (POUT(DC)), for a load (RL) referenced to ground, when the amplifier is sourcing current. This is the voltage drop across the sourcing (NPN) output transistor (Vcc – VOUT) multiplied by the output current drive (IOUT).

Equation 7. P S o u r c e D C W = V c c - V O U T × I O U T = V c c - V O U T V O U T R L

where:

  • VOUT = Output voltage of the amplifier
  • IOUT = Output current drive of the amplifier
  • RL = Total Resistive load at the output of the amplifier. In Figure 2-1, RL is equal to the combination of the series output resistor (RS) and the load resistor (RLOAD).

Equation 8 shows the DC power dissipation in the output transistors when the amplifier is sinking current for a ground-referenced load. This is similar to Equation 7, except that the voltage drop across the sinking (PNP) output transistor is considered.

Equation 8. P S i n k D C W = V e e - V O U T × I O U T = V e e - V O U T V O U T R L
Figure 2-8 THS3491 Class AB Output Structure

The combination of Equation 6 with Equation 7 and Equation 8 produces Equation 9 and Equation 10, the total internal DC power dissipation of a single amplifier when sourcing and sinking current, respectively.

Equation 9. P A m p S o u r c e D C W = V S I Q + V c c - V O U T V O U T R L
Equation 10. P A m p S i n k D C W = V S I Q + V e e - V O U T V O U T R L
Figure 2-9 Internal DC Power Dissipation of Driver Amplifier