Texas Instruments TPS5124EVM-001, Руководство пользователя, Страница: 9 / 23

SLUU182 − March 2004
9 High-Performance Dual Synchronous Buck Conversion Using the TPS5124
4.4.1 Case One: D1, D2 < 0.5.
The ripple current through the input capacitor is shown in Figure 2 and can be calculated using
equation (6).
I
incapRMS
:
+
1
T
ȧȱȲ
ŕ
D1
@
T
0
ǒ
I
OUT1
*
I
AVG
Ǔ 2
dt
)
ŕ
T
2
D1
@
T
ǒ
I
AVG
Ǔ 2
dt
)
ŕ
T
2
)
D2
@
T
T
2
ǒ
I
OUT2
*
I
AVG
Ǔ 2
dt
)
ŕ
T
T
2
)
D2
@
T
ǒ
I
AVG
Ǔ 2
dt
ȧȳȴ
Ǹ
I
incapRMS
:
+
D1
ǒ
I
OUT1
Ǔ 2
)
D2
ǒ
I
OUT2
Ǔ 2
*
ǒ
I
AVG
Ǔ 2
Ǹ
where:
I
AVG
is the average input current.
I
AVG
+
I
OUT1
D1
)
I
OUT2
D2
4.4.2 Case Two: D2 < 0.5 < D1.
The ripple current through the input capacitor is shown in Figure 3 and can be calculated using
equation (8).
I
incapRMS
:
+
1
T
ƪ
ŕ T
2
0
ǒ
I
OUT1
*
I
AVG
Ǔ 2
dt
)
ŕ
D1
@
T
T
2
ǒ
I
OUT1
)
I
OUT2
*
I
AVG
Ǔ 2
dt
)
ŕ T
2
)
D2
@
T
D1
@
T
ǒ
I
OUT2
*
I
AVG
Ǔ 2
dt
)
ŕ
T
T
2
)
D2
@
T
ǒ
I
AVG
Ǔ 2
dt
ƫ
Ǹ
I
incapRMS
:
+
D1
ǒ
I
OUT1
Ǔ 2
)
D2
ǒ
I
OUT2
Ǔ 2
)
(2
D1
*
1) I
OUT1
I
OUT2
*
ǒ
I
AVG
Ǔ 2
Ǹ
This EVM meets “Case One” criteria. The maximum input ripple current is 6.7 A at V
IN
=12 V.
Two 150-
µ
F, 20-V special polymer capacitors from Panasonic (part number is EEFWA1D151P)
are used. It can handle 3.7 A of ripple current each. The ESR value of each capacitor is 26 m
Ω
.
So the input ripple voltage is calculated using equation (10) and is approximately 88 mV
RMS
I
RIPPLE
+
I
incapRMS
ESR
(5)
(6)
(7)
(8)
(9)
(10)