TIDUC65 April   2022 TPSI3050 , TPSI3050-Q1 , TPSI3052 , TPSI3052-Q1

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
    3. 2.3 Design Considerations
      1. 2.3.1 Overcurrent Protection (OCP)
        1. 2.3.1.1 Immediate Overcurrent Protection
        2. 2.3.1.2 Adjustable Delay Overcurrent Protection
      2. 2.3.2 Overtemperature Protection (OTP)
        1. 2.3.2.1 TMP392
        2. 2.3.2.2 ISO7310
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
      3. 4.1.3 Altium Project
      4. 4.1.4 Assembly Drawings
      5. 4.1.5 Gerber Files
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Author

2.3.1.2 Adjustable Delay Overcurrent Protection

In some cases, it might be preferable to allow an overcurrent condition for a certain period of time. A example of this could be a highly capacitive circuit in which a temporary inrush current is allowed to charge the capacitor. AMC23C14 has an adjustable threshold which is created through a current source of 100 uA and a reference resistor. Equation 7 and Equation 8 show how to calculate for the reference resistor (RREF). This range is used to allow for 100-ms of inrush currents between 2 A to 5 A. For this reference design a typical resistor of 1.2 kΩ was selected.

Equation 7. V T R I P   =   I M A X   ×   R S E N S E     =   2   A   ×   60   m Ω   =   120   m V
Equation 8. R R E F   =   V T R I P   -   V H Y S I R E F   =   120   m V   -   4   m V 100   μ A   =   1 . 16   k Ω

Discharging Path 1 in Figure 2-4 shows that when the system is operating under normal conditions (current load less than 2 A), OUT1 is High-Z and C22 is charged to 5 V (VDD2). When the current is greater than 2 A, OUT1 is Low-Z and C22 begins to discharge through R17. The time to discharge C22 should be at least 100 ms before reaching the maximum switching threshold for SN74HCS09 of 2.2 V as shown in Equation 9.

Equation 9. t d i s c h a r g e   =   -   R 17   ×   C 22   ×   ln V C 22 V S O U R C E   t d i s c h a r g e   =   -   560   k Ω   ×   220   n F   ×   ln 2 . 2   V 5   V   =   101 . 15   m s

When the load is disconnected, the circuit attempts to reconnect the load automatically. When the voltage across the capacitor is charged above the positive switching threshold (VT+) of the AND gate, then EN is asserted high and the load connected. The AND gate (SN74HCS09) guarantees by design a minimum hysteresis of 0.4 V. With this hysteresis value, the following equations shows that at least 38 ms will passed before the load is reconnected. Is it important to select R14 to allow for a large charging time for C22 for the system not to reconnect the load immediately.

Equation 10. t c h a r g e   =   -   R 14 + R 17   ×   C 22   ×   ln V S O U R C E - ( V I N I T I A L + V H Y S )   V S O U R C E - V I N I T I A L   t c h a r g e   =   -   ( 560   k Ω   + 560   k Ω )     ×   220   n F   ×   ln 5   V   -   2 . 2   V   -   0 . 4   V 5   V   -   2 . 2   V   =   37 . 98   m s

It is recommended to add R15 and C20 to filter any high frequency noises. High frequency noises can disturb the behavior of the isolated comparator.

Figure 2-4 Overcurrent Protection