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LTC3417AEDHC#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LTC3417A
10
3417afc
applicaTions inForMaTion
Inductor Core Selection
Different core materials and shapes will change the size/
current relationship of an inductor. Toroid or shielded
pot cores in ferrite or permalloy materials are small and
don’t radiate much energy, but generally cost more than
powdered iron core inductors with similar electrical
characteristics. The choice of which style inductor to use
often depends more on the price vs size requirements
of any radiated field/EMI requirements than on what the
LTC3417A requires to operate. Table 1 shows some
typical surface mount inductors that work well in
LTC3417A applications.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter can
be approximated by the sum of two square waves with
duty cycles of approximately V
OUT1
/V
IN
and V
OUT2
/V
IN
. To
prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. Some capacitors have a
de-rating spec for maximum RMS current. If the capaci-
tor being used has this requirement, it is necessary to
calculate the maximum RMS current. The RMS current
calculation is different if the part is used in “in phase” or
“out of phase”.
For “in phase”, there are two different equations:
V
OUT1
> V
OUT2
:
V
OUT2
> V
OUT1
:
where:
D1=
V
OUT1
V
IN
and D2 =
V
OUT2
V
IN
Table 1
MANUFACTURER PART NUMBER VALUE (µH) MAX DC CURRENT (A) DCR DIMENSIONS L × W × H (mm)
L1 on OUT1
Toko A920CY-1R5M-D62CB
A918CY-1R5M-D62LCB
1.5
1.5
2.8
2.9
0.014
0.018
6 × 6 × 2.5
6 × 6 × 2
Coilcraft DO1608C-152ML 1.5 2.6 0.06 6.6 × 4.5 × 2.9
Sumida CDRH4D22/HP 1R5 1.5 3.9 0.031 5 × 5 × 2.4
Midcom DUP-1813-1R4R 1.4 5.5 0.033 4.3 × 4.8 × 3.5
L2 on OUT2
Toko A915AY-2R0M-D53LC 2.0 3.9 0.027 5 × 5 × 3
Coilcraft DO1608C-222ML 2.2 2.3 0.07 6.6 × 4.5 × 2.9
Sumida CDRH3D16/HP 2R2
CDRH2D18/HP 2R2
2.2
2.2
1.75
1.6
0.047
0.035
4 × 4 × 1.8
3.2 × 3.2 × 2
Midcom DUP-1813-2R2R 2.2 3.9 0.047 4.3 × 4.8 × 3.5
LTC3417A
11
3417afc
applicaTions inForMaTion
When D1 = D2 then the equation simplifies to:
I
RMS
= I
1
+I
2
( )
D 1– D
( )
or
I
RMS
= I
1
+I
2
( )
V
OUT
V
IN
– V
OUT
( )
V
IN
where the maximum average output currents I
1
and I
2
equal the respective peak currents minus half the peak-
to-peak ripple currents:
I
1
= I
LIM1
–
∆I
L1
2
I
2
= I
LIM2
–
∆I
L2
2
These formula have a maximum at V
IN
= 2V
OUT
, where
I
RMS
= (I
1
+ I
2
)/2. This simple worst case is commonly
used to determine the highest I
RMS
.
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
RMS
does not
occur when V
OUT1
= V
OUT2
. The maximum typically oc-
curs when V
OUT1
– V
IN
/2 = V
OUT2
or when V
OUT2
– V
IN
/2
= V
OUT1
. As a good rule of thumb, the amount of worst
case ripple is about 75% of the worst case ripple in the
“in phase” mode. Also note that when V
OUT1
= V
OUT2
=
V
IN
/2 and I
1
= I
2
, the ripple is zero.
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
IN
for high frequency decoupling, when not using an all
ceramic capacitor solution.
Output Capacitor (C
OUT1
and C
OUT2
) Selection
The selection of C
OUT1
and C
OUT2
is driven by the required
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
OUT
) is determined by:
∆V
OUT
≈ ∆I
L
ESR
COUT
+
1
8 • f
O
• C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f
O
= operating frequency, C
OUT
= output capacitance
and
∆
I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage, since
∆
I
L
increases
with input voltage. With
∆
I
L
= 0.35I
LOAD(MAX)
, the output
ripple will be less than 100mV at maximum V
IN
and f
O
=
1MHz with:
ESR
COUT
< 150mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
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
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
LTC3417A
12
3417afc
applicaTions inForMaTion
is given to ripple current ratings and long term reliability.
Ceramic capacitors have the lowest ESR and cost but also
have the lowest capacitance density, high voltage and
temperature coefficient and exhibit audible piezoelectric
effects. In addition, the high Q of ceramic capacitors along
with trace inductance can lead to significant ringing. Other
capacitor types include the Panasonic specialty polymer
(SP) capacitors.
In most cases, 0.1µF to 1µF of ceramic capacitors should
also be placed close to the LTC3417A in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Because the
LTC3417 control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size. When choosing the input and output
ceramic capacitors, choose the X5R or X7R dielectric
formulations. These dielectrics have the best temperature
and voltage characteristics of all the ceramics for a given
value and size.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the V
IN
pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must fulfill a charge storage re-
quirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation com-
ponents and the output capacitor size. Typically, 3 to 4
cycles are required to respond to a load step, but only in
the first cycle does the output drop linearly. The output
droop, V
DROOP
, is usually about 2 to 3 times the linear
droop of the first cycle. Thus, a good place to start is with
the output capacitor size of approximately:
C
OUT
≈ 2.5
∆I
OUT
f
O
• V
DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10µF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3417A develops a 0.8V reference voltage between
the feedback pins, V
FB1
and V
FB2
, and the signal ground
as shown in Figure 4. The output voltages are set by two
resistive dividers according to the following formulas:
V
OUT1
≈ 0.8V 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT2
≈ 0.8V 1+
R3
R4
⎛
⎝
⎜
⎞
⎠
⎟
Keeping the current small (<5µA) in these resistors
maximizes efficiency, but making the current too small
may allow stray capacitance to cause noise problems and
reduce the phase margin of the error amp loop.
To improve the frequency response, a feed-forward ca-
pacitor, C
F
, may also be used. Great care should be taken
to route the V
FB
node away from noise sources, such as
the inductor or the SW line.
Soft-Start
Soft-start reduces surge currents from V
IN
by gradu-
ally increasing the peak inductor current. Power supply
sequencing can also be accomplished by controlling the
I
TH
pin. The LTC3417A has an internal digital soft-start
for each regulator output, which steps up a clamp on
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LTC3417AEDHC#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual, Sync. 1.5A/1A, 4MHz Step-Down DC/DC in DFN
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