LTC3417A LINER [Linear Technology], LTC3417A Datasheet - Page 11

• Manufacturer Part Number: LTC3417A
• Description: Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
When D1 = D2 then the equation simplifies to:
or
where the maximum average output currents I
equal the respective peak currents minus half the peak-to-
peak ripple currents:
These formula have a maximum at V
I
used to determine the highest I
For “out of phase” operation, the ripple current can be
lower than the “in phase” current.
In the “out of phase” case, the maximum I
occur when V
when V
V
ripple is about 75% of the worst case ripple in the “in
phase” mode. Also note that when V
and I
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
high frequency decoupling, when not using an all ceramic
capacitor solution.
RMS
OUT1
I
I
I
I
RMS
RMS
1
2
=
1
=
= (I
. As a good rule of thumb, the amount of worst case
= I
I
I
LIM
OUT1
LIM
=
=
1
2
, the ripple is zero.
(
(
1
+ I
2
I
I
1
1
–
–
+
+
– V
OUT1
2
∆
)/2. This simple worst case is commonly
I
I
∆
2
2
2
I
2
L
I
IN
) (
)
L
1
= V
2
/2 = V
U
D
V
OUT2
OUT
1–
D
(
OUT2
. The maximum typically occurs
U
V
)
V
IN
IN
–
or when V
RMS
V
OUT
W
.
OUT1
)
IN
= 2V
= V
OUT2
RMS
OUT2
OUT
U
– V
does not
1
, where
= V
and I
IN
IN
/2 =
IN
for
/2
2
Output Capacitor (C
The selection of C
ESR to minimize voltage ripple and load step transients.
Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple
(∆V
where f
and ∆I
is highest at maximum input voltage, since ∆I
with input voltage. With ∆I
ripple will be less than 100mV at maximum V
1MHz with:
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. Alumi-
num electrolytic, special polymer, ceramic and dry tanta-
lum capacitors are all available in surface mount pack-
ages. The OS-CON semiconductor dielectric capacitor
available 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 capaci-
tance density, but it has a larger ESR and it is critical that
the capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface tantalums, available in case heights ranging from
2mm to 4mm. Aluminum electrolytic capacitors have a
significantly larger ESR, and are often used in extremely
cost-sensitive applications provided that consideration is
ESR
∆
OUT
V
OUT
L
COUT
O
) is determined by:
= ripple current in the inductor. The output ripple
= operating frequency, C
≈ ∆
< 150mΩ
I ESR
L
⎛
⎜
⎝
OUT1
OUT1
COUT
and C
and C
L
+
OUT2
= 0.35I
8 •
OUT2
OUT
is driven by the required
f
OUT
O
LOAD(MAX)
1
•
) Selection
= output capacitance
C
have been met, the
LTC3417A
OUT
⎞
⎟
⎠
, the output
L
IN
RIPPLE(P-P)
increases
and f
11
3417afa
O
=