SNVS788H January   2012  – August 2021

PRODUCTION DATA

1. Features
2. Applications
3. Description
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Applications
1. 8.2.1 Application Circuit: 12.5 V to 95 V Input and 10 V, 100-mA Output Buck Converter
1. 8.2.1.1 Design Requirements
2. 8.2.1.2 Detailed Design Procedure
3. 8.2.1.3 Application Curves
2. 8.2.2 Application Circuit: 20 V to 95 V Input and 10 V, 100 mA Output Isolated Fly-Buck Converter
1. 8.2.2.1 Design Requirements
2. 8.2.2.2 Detailed Design Procedure
3. 8.2.2.3 Application Curves
9. Power Supply Recommendations
10. 10Layout
11. 11Device and Documentation Support
1. 11.1 Device Support
2. 11.2 Documentation Support
4. 11.4 Support Resources
6. 11.6 Electrostatic Discharge Caution
7. 11.7 Glossary
12. 12Mechanical, Packaging, and Orderable Information

Package Options

• DDA|8
• NGU|8
• DDA|8
8.2.2.2.6 Primary Output Capacitor

In a conventional buck converter, the output ripple voltage is calculated as shown in Equation 23.

Equation 23.

To limit the primary output ripple voltage ΔVOUT1 to approximately 50 mV, an output capacitor COUT1 of 0.33 µF is required.

Figure 8-6 shows the primary winding current waveform (IL1) of a Fly-Buck converter. The reflected secondary winding current adds to the primary winding current during the buck switch off-time. Because of this increased current, the output voltage ripple is not the same as in conventional buck converter. The output capacitor value calculated in Equation 23 must be used as the starting point. Optimization of output capacitance over the entire line and load range must be done experimentally. If the majority of the load current is drawn from the secondary isolated output, a better approximation of the primary output voltage ripple is given by Equation 24.

Equation 24.
Figure 8-6 Current Waveforms for COUT1 Ripple Calculation

A standard 1-µF, 25-V capacitor is selected for this design. If lower output voltage ripple is required, a higher value must be selected for COUT1, COUT2, or both.