Datasheet

L(ripple)
RIPPLE(C)
OUT SW
I
V
8 C f
=
´ ´
RIPPLE(ESR) L(ripple)
V I ESR= ´
IN
RIPPLE(ESL)
V ESL
V
L
´
=
( )
2
L(ripple) OUT
RIPPLE(DCM)
SW OUT L(ripple)
I I
V
2 f C I
a ´ -
=
´ ´ ´
ON(DCM)
ON(CCM)
t
t
a =
I
L
V
OUT
a x I
L(ripple)
T1
V
RIPPLE
a x T
I
OUT
UDG-11253
( )
IN(ripple) OUT
I I D 1 D= ´ ´ -
OUT
IN
V
D
V
=
TPS53313
SLUSAS8 –DECEMBER 2011 www.ti.com
(3)
(4)
(5)
When ceramic output capacitor is chosen, the ESL component is usually negligible. In the case when multiple
output capacitors are used, the total ESR and ESL should be the equivalent of the all output capacitors in
parallel.
When operating in DCM, the output ripple is dominated by the component determined by capacitance. It also
varies with load current and can be expressed as shown in Equation 6.
where
• α is the DCM on-time coefficient and can be expressed as shown in Equation 7. (6)
(7)
Figure 14. DCM Output Voltage Ripple
Input Capacitor Selection
The selection of input capacitor should be determined by the ripple current requirement. The ripple current
generated by the converter needs to be absorbed by the input capacitors as well as the input source. The RMS
ripple current from the converter can be expressed as shown in Equation 8.
where
• D is the duty cycle and can be expressed as shown in Equation 9. (8)
(9)
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