SNVSBZ3 June   2021

1. Features
2. Applications
3. Description
4. Revision History
5. Device Comparison Table
6. Pin Configuration and Functions
7. Specifications
8. Detailed Description
1. 8.1 Overview
2. 8.2 Functional Block Diagram
3. 8.3 Feature Description
4. 8.4 Device Functional Modes
9. Application and Implementation
1. 9.1 Application Information
2. 9.2 Typical Application
1. 9.2.1 Design Requirements
2. 9.2.2 Detailed Design Procedure
3. 9.2.3 Application Curves
10. 10Power Supply Recommendations
11. 11Layout
1. 11.1 Layout Guidelines
2. 11.2 Layout Example
12. 12Device and Documentation Support
13. 13Mechanical, Packaging, and Orderable Information

• DDA|8

#### 9.2.2.3 Output Capacitor Selection

The primary output capacitor, COUT1, can be selected using either Equation 23 or Equation 24. For this design, an output voltage ripple of 5 mV and a load transient of 0.2 V is used. From this, a ripple current of 0.34 A at 60 V input, and a peak transformer current of 0.77 A at full load is calculated. The two output capacitor equations give values of 11 μF and 5 μF. Because of the large derating of ceramic capacitors, COUT1 = 1 × 22 μF is used. Keep in mind that the equations give the minimum capacitance value and in no case should the capacitance of COUT1 be less than 2.2 μF. More output capacitance can be used to improve load transient response. Also note that when using type III ripple injection, the actual ripple voltage appearing on the output can be kept small.

Equation 23.

where

• IPK = peak transformer current from Equation 22
• ΔVO = output voltage load transient
Equation 24.

where

• ΔI = ripple current from Equation 21
• Vripple = ripple voltage on primary output

COUT2 is selected using Equation 25. In this case, a ripple voltage on the secondary otuput of 20 mV is chosen. The minimum input voltage should be used in this equation. A value of 10 μF is calculated and 1 × 22 μF for COUT2 is selected. Again, the equation gives the minimum capacitance value and in no case should the capacitance of COUT2 be less than 2.2 μF.

Equation 25.

Both output capacitors should be X7R ceramics in a 1206 or 1210 case size, and rated for at least twice the output voltage.