
NXP Semiconductors
AN10876
Buck converter for SSL applications
NXP B.V. 2011.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
Date of release: 23 June 2011
Document identifier: AN10876
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
10. Contents
1
2
2.1
2.2
2.3
3
4
4.1
4.2
4.3
4.4
4.5
4.6
4.6.1
4.6.2
4.6.3
4.6.4
4.7
4.7.1
4.8
5
5.1
5.2
5.3
5.4
5.5
5.5.1
5.5.2
5.5.3
5.6
5.7
6
6.1
6.2
7
8
9
9.1
9.2
9.3
10
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
General philosophy of the application note . . . 4
Related documents and tools. . . . . . . . . . . . . . 4
Theory of operation. . . . . . . . . . . . . . . . . . . . . . 5
Key components design procedure. . . . . . . . . 8
Output current versus peak current . . . . . . . . . 8
Inductor dimensioning . . . . . . . . . . . . . . . . . . . 9
Valley detect . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Peak current limit . . . . . . . . . . . . . . . . . . . . . . 12
Ripple current calculation . . . . . . . . . . . . . . . . 12
Inductor design parameters . . . . . . . . . . . . . . 13
Core type selection. . . . . . . . . . . . . . . . . . . . . 14
Calculate windings . . . . . . . . . . . . . . . . . . . . . 14
Auxiliary winding count. . . . . . . . . . . . . . . . . . 15
Wire diameter selection . . . . . . . . . . . . . . . . . 15
V
cc
generation dimensioning. . . . . . . . . . . . . . 16
Buffer capacitance C6 calculation: . . . . . . . . . 17
Demagnetization detection dimensioning. . . . 18
Power calculations . . . . . . . . . . . . . . . . . . . . . 19
Resistive switch dissipation . . . . . . . . . . . . . . 19
Capacitive switch dissipation . . . . . . . . . . . . . 19
Switching losses. . . . . . . . . . . . . . . . . . . . . . . 20
Freewheel diode losses . . . . . . . . . . . . . . . . . 21
Inductor losses . . . . . . . . . . . . . . . . . . . . . . . . 21
Resistive losses . . . . . . . . . . . . . . . . . . . . . . . 21
Proximity losses . . . . . . . . . . . . . . . . . . . . . . . 22
Core losses. . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Sense resistor losses. . . . . . . . . . . . . . . . . . . 24
Total system losses. . . . . . . . . . . . . . . . . . . . . 24
Current tolerance and stability. . . . . . . . . . . . 26
Current tolerance . . . . . . . . . . . . . . . . . . . . . . 26
Current stability. . . . . . . . . . . . . . . . . . . . . . . . 26
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Abbreviations. . . . . . . . . . . . . . . . . . . . . . . . . . 27
Legal information. . . . . . . . . . . . . . . . . . . . . . . 28
Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Disclaimers. . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29