Datasheet
( ) ( )
( ) ( )
( )
( )
s
s
s s
O UT
s ss
s s s
t ΔI t D ΔI 1 2 D
I
I 1 D
esr t + t<
D 2 C 2 × D 12
V (t) =
I t 1 D t I 1 2 DI
D I 1
esr t + + otherwi
1 D 2 C 2 1 D 12
´ ´ ´ - ´ - ´
D ´ ¦
D
´ ´ - ´ -
´ ¦ ¦
D ´ ¦ ´ - ´ - ¦ ´ D ´ - ´D ´ ¦
D
´ ´ - ´ -
- ¦ ´ - ´ ¦ ´ ¦
¦
æ ö
æ ö
ç ÷
ç ÷
è ø
è ø
æ ö æ öæ ö
ç ÷ ç ÷
ç ÷
è ø è øè ø
se
ì
ï
ï
í
ï
ï
î
s
s
C
s
s
I
I D
t t<
D 2
I (t) =
I D I
t + otherwise
1 D 2
D ´ ¦
ì D
´ -
ï
¦
ï
í
æ ö
D ´ ¦ D
ï
´ -
ç ÷
ï
- ¦
è ø
î
t
O UT C C
0
1
V (t) = I (t) esr + I ( ) d
C
´ ´
ò
t t
COUTRMS
I
I =
12
D
2
2
CINRMS OUT
I
I = I D (1 D) + D
12
D
´ ´ - ´
UCD7242
SLUS962B –JANUARY 2010–REVISED AUGUST 2012
www.ti.com
I
OUT
10A
I
LRMS
10A
ΔI 4A
I
MAX
13.8A
Armed with this data one can now approach the inductor data sheet to select a part with a “saturation” limit
above 13.8A and current “heating” limit above 10A. Furthermore, total losses can be estimated based on the
datasheet DCR value (I
LRMS
2
DCR) and the core loss curves for a given frequency and ΔI.
INPUT CAPACITANCE
Due to the non-zero impedance of the power planes of the input voltage rail, it is necessary to add some local
capacitance near the UCD7242 to ensure that the voltage at this node is quiet and stable. The primary things to
consider are:
1. The radiated fields generated by the di/dt and dv/dt from this node
2. RMS currents capability needed in the capacitors
3. The AC voltage present and respective susceptibility of any device connected to this node
(12)
As a point of reference if ΔI=0.4 I
OUT
this places the worst case I
CINRMS
at approximately 5A. This corresponds to
a duty cycle of approximately 50%. Other duty cycles can result in a significantly lower RMS current.
A good input capacitor would be a 22μF X5R ceramic capacitor. Equally important as selecting the proper
capacitor is placing and routing that capacitor. It is crucial that the decoupling be placed as close as possible to
both the power pin (V
IN
) and ground (PGND). It is important to recognize that each power stage should have its
own local decoupling. One 22μF capacitor should be placed across each V
IN
and PGND pair. The proximity of
the capacitance to these pins will reduce the radiated fields mentioned above.
OUTPUT CAPACITANCE
The goal of the output capacitor bank is to keep the output voltage within regulation limits during steady state
and transient conditions.
The total AC RMS current flowing through the capacitor bank can be calculated as:
(13)
For a single type of output capacitor the output ripple voltage wave form can be approximated by the following
equation:
(14)
Where:
(15)
After substitution and simplification yields
(16)
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