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© 2006 National Semiconductor Corporation
Buck Calculations for Inductance
• Need to know V
IN
, V
OUT
, I
OUT
, Switch resistance, operating
frequency, and diode drop
• Next calculate D
• We calculate
I where:
• And then inductance:
DSWIN
DOUT
V+V-V
V+V
T
Ton
D
f
DVVV
I
L
OUTSWIN
'
Rule of thumb:
I
OUT
x 0.3
For a given set of performance parameters, the first component to select is the inductor value since this will
determine all the currents in the other components.
There are a number of ways to determine which inductance value is optimal for your circuit and the method
described above is our recommendation for general everyday use. It is based on using a current ripple
ratio (the ratio of the average inductor current to the total swing of the current during a switching cycle) of
0.4 for load currents up to 2A, and a ratio of 0.3 for load currents in excess of 2A. This produces a good
tradeoff between inductor size, output voltage ripple (produced by the ripple current flowing through the
capacitor ESR), and peak currents in the external components.
On occasion there may be reasons to increase or decrease the inductance value. The inductance value is
a major factor in determining stability of the control system. Decreasing the inductor value may allow for a
smaller size inductor. Increasing the value may allow for a cheaper output capacitor. Increasing the value
reduces peak currents and may allow you to achieve more output current from a cheaper switcher. Lastly,
Purchasing may be telling you to use up inventory from projects which are not selling as fast as they like.
Note: Since these equations were originally developed, capacitor technology has improved significantly.
Capacitors with very low ESR are easily available. This technology development may allow you to
decrease the inductance (increase ripple) beyond the guidelines above while still maintaining low output
ripple voltage.
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shlll2016-08-29很全面,很好用,谢谢分享.
