Kurtees
Application Report
SLVA477A – December 2011 – Revised August 2012
Basic Calculation of a Buck Converter's Power Stage
Brigitte Hauke ....................................................................................... Low Power DC/DC Applications ABSTRACT This application report gives the formulas to calculate the power stage of a buck converter built with an integrated circuit having a integrated switch and operating in continuous conduction mode. It is not intended to give details on the functionality of a buck converter or how to compensate a converter. For additional information, see the references at the end of this document. Appendix A contains the formulas without description.
1
Basic Configuration of a Buck Converter
Figure 1 shows the basic configuration of a buck converter where the switch is integrated in the selected integrated circuit ( IC). Some converters have the diode replaced by a second switch integrated into the converter (synchronous converters). If this is the case, all equations in this document apply besides the power dissipation equation of the diode.
IIN SW L IOUT
VIN
CIN
D
COUT
VOUT
Figure 1. Buck Converter Power Stage
1.1
Necessary Parameters of the Power Stage
The following four parameters are needed to calculate the power stage: 1. Input voltage range: VIN(min) and VIN(max) 2. Nominal output voltage: VOUT 3. Maximum output current: IOUT(max) 4. Integrated circuit used to build the buck converter. This is necessary because some parameters for the calculations must be derived from the data sheet. If these parameters are known, the power stage can be calculated.
2
Calculate the Maximum Switch Current
The first step to calculate the switch current is to determine the duty cycle, D, for the minimum input voltage. The minimum input voltage is used because this leads to the maximum switch current. ´ η V Maximum Duty Cycle: D = OUT VIN (max) VIN(min) = minimum input voltage VOUT = output voltage
SLVA477A – December 2011 – Revised August 2012
Basic Calculation of a Buck Converter's Power Stage
Brigitte Hauke ....................................................................................... Low Power DC/DC Applications ABSTRACT This application report gives the formulas to calculate the power stage of a buck converter built with an integrated circuit having a integrated switch and operating in continuous conduction mode. It is not intended to give details on the functionality of a buck converter or how to compensate a converter. For additional information, see the references at the end of this document. Appendix A contains the formulas without description.
1
Basic Configuration of a Buck Converter
Figure 1 shows the basic configuration of a buck converter where the switch is integrated in the selected integrated circuit ( IC). Some converters have the diode replaced by a second switch integrated into the converter (synchronous converters). If this is the case, all equations in this document apply besides the power dissipation equation of the diode.
IIN SW L IOUT
VIN
CIN
D
COUT
VOUT
Figure 1. Buck Converter Power Stage
1.1
Necessary Parameters of the Power Stage
The following four parameters are needed to calculate the power stage: 1. Input voltage range: VIN(min) and VIN(max) 2. Nominal output voltage: VOUT 3. Maximum output current: IOUT(max) 4. Integrated circuit used to build the buck converter. This is necessary because some parameters for the calculations must be derived from the data sheet. If these parameters are known, the power stage can be calculated.
2
Calculate the Maximum Switch Current
The first step to calculate the switch current is to determine the duty cycle, D, for the minimum input voltage. The minimum input voltage is used because this leads to the maximum switch current. ´ η V Maximum Duty Cycle: D = OUT VIN (max) VIN(min) = minimum input voltage VOUT = output voltage