LTC3546 Linear Technology, LTC3546 Datasheet - Page 16

• Manufacturer Part Number: LTC3546
• Description: 3A/1A or 2A/2A Configurable Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC3546
the maximum RMS current. The RMS current calculation
is different if the part is used in-phase or out-of-phase.
For in-phase, there are two different equations:
V
V
Where:
When D1 = D2, then the equation simplifi es to:
or
where the maximum average output currents I1 and
I2 equals the peak current minus half the peak-to-peak
ripple current:
These formula have a maximum at V
I
used to determine the worst-case I
For out-of-phase (PHASE pin is at ground), the ripple
current can be lower than the in-phase.
In the out-of-phase case, the maximum I
cur when V
16
RMS
OUT1
OUT2
I
I
D
I
I
I1=I
I2 =I
RMS
RMS
RMS
RMS
1
2 1 2
2 1 2
= (I1 + I2)/2. This simple worst-case is commonly
=
> V
> V
• • •
• • •
LIM1
LIM2
I I D
I I D
V
=
=
=
=
OUT
V
OUT2
OUT1
(
(
IN
I
I
1 2
1 2
OUT1
–
–
+
+
1
ΔI
I
I
ΔI
:
:
and D
2
2 1
1 1
L1
2
) (
)
L2
= V
(
(
–
–
D
V
OUT2
2
D
D
OUT
=
1–
2
1
)
)
V
. The maximum typically occurs
+
+
D
(
OUT
V
V
V
I
I
)
IN
2
2
IN
IN
2
2
2
(
(
–
D
D
V
2
2
OUT
–
–
RMS
D
D
2
2
)
IN
2
2
.
RMS
)
)
= 2V
+
+
I
I
1
1
does not oc-
2
2
( (
( (
OUT
D
D
1
1
, where
–
–
D
D
1
1
2
2
)
)
when V
V
ripple is about 75% of the worst-case ripple in the in-phase
mode. Note, that when V
the ripple is at its minimum.
Note that capacitor manufacturer’s ripple current ratings
are often based on only 2000 hours lifetime. This makes
it advisable to further derate the capacitor, or choose a
capacitor rated at a higher temperature than required.
Several capacitors may also be paralleled to meet the
size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
V
ceramic capacitor solution.
Output Capacitor (C
The selection of C
ESR to minimize voltage ripple and load step transients.
Typically, once the ESR requirement is satisfi ed, the
capacitance is adequate for fi ltering. The output ripple
(ΔV
where f
and ΔI
is highest at maximum input voltage since ΔI
with input voltage.
Once the ESR requirements for C
RMS current rating generally far exceeds the I
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
The OS-CON semiconductor dielectric capacitor avail-
able from Sanyo has the lowest ESR(size) product of any
aluminum electrolytic at a somewhat higher price. Special
polymer capacitors, such as Sanyo POSCAP , offer very
low ESR, but have a lower capacitance density than other
types. Tantalum capacitors have the highest capacitance
OUT1
IN
for high frequency decoupling, when not using an all
OUT
V
. As a good rule of thumb, the amount of worst-case
OUT
L
O
) is determined by:
OUT1
= ripple current in the inductor. The output ripple
= operating frequency, C
– V
I
L
IN
ESR+
OUT1
/2 = V
OUT1
and C
8• f
OUT2
OUT1
and C
O
OUT2
•C
1
= V
and when V
OUT2
OUT
OUT
is driven by the required
OUT2
OUT
) Selection
= output capacitance
have been met, the
= V
IN
OUT2
/2 and I
L
RIPPLE(P-P)
increases
– V
IN
1
= I
/2 =
3546fb
2
,