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3. COMPONENT SELECTION
L +
V
O
V
IN(max)
ǒ
V
IN(max)
* V
O
Ǔ
f
SW
DI
(17)
I
LOAD_RMS
+ I
LOAD
2
)
DI
2
12
Ǹ
+ 15.03 A
(18)
I
PK
+ I
LOAD
)
DI
2
2
+ 16.65 A
(19)
C
O
u
L I
STEP
2
2 V
UNDER
D
MAX
ǒ
V
IN
* V
O
Ǔ
(20)
C
O
u
L I
STEP
2
2 V
OVER
V
O
(21)
ESR t
V
RIPPLE
DI
(22)
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
3. 1 Power Train Components
Designers familiar with the buck converter can skip to section 3.2 Component Selection for TPS40075.
3.1.1 Output Inductor, L
O
The output inductor is one of the most important components to select. It stores the energy necessary to keep
the output regulated when the switch MOSFET is turned off. The value of the output inductor dictates the peak
and RMS currents in the converter. These currents are important when selecting other components. Equation 17
can be used to calculate a value for L.
Δ I is the allowable ripple in the inductor. Selecting Δ I also sets the output current when the converter goes into
discontinuous mode (DCM) operation. Since this converter utilizes MOSFETs for the rectifier, DCM is not a major
concern. Select Δ I to be between 20% and 30% of maximum I
LOAD
. For this design, Δ I of 3 A was selected. The
calculated L is 1.1 μ H. A standard inductor with value of 1.0 μ H was chosen. This increases Δ I by about 10% to
3.3 A.
With this Δ I value, calculate the RMS and peak current flowing in L
O
. Note this peak current is also seen by the
switching MOSFET and synchronous rectifier.
3.1.2 Output Capacitor, C
O
, ELCO and MLCC
Several parameters must be considered when selecting the output capacitor. The capacitance value should be
selected based on the output overshoot, V
OVER
, and undershoot, V
UNDER
, during a transient load, I
STEP
, on the
converter. The equivalent series resistance (ESR) is chosen to allow the converter meet the output ripple
specification, V
RIPPLE
. The voltage rating must be greater than the maximum output voltage. Other parameters to
consider are: equivalent series inductance which is important in fast transient load situations. Also size and
technology can be factors when choosing the output capacitor. In this design a large capacitance electrolytic type
capacitor, C
O
ELCO, is used to meet the overshoot and under shoot specifications. Its ESR is chosen to meet
the output ripple specification. While a smaller multiple layer ceramic capacitor, C
O
MLCC, is used to filter high
frequency noise.
The minimum required capacitance and maximum ESR can be calculated using the equations below.
Using Equation 20 through Equation 22 , the capacitance for C
O
should be greater than 495 μ F and its ESR
should be less than 9.1m Ω . The 1000 μ F/25 V capacitor from Rubycon's MBZ or Panasonic's series EEU-FL was
chosen. Its ESR is 19 m Ω , so two in parallel are used. The slightly higher ESR is offset by the four times
increase in capacitance. A 2.2 μ F/16 V MLCC is also added in parallel to reduce high frequency noise.
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