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LTC3417AEFE-1#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTC3417A-1
10
3417a1fa
A reasonable starting point for setting ripple current is
ΔI
L
= 0.35I
LOAD(MAX)
, where I
LOAD(MAX)
is the maximum
current output. The largest ripple, ΔI
L
, occurs at the maxi-
mum input voltage. To guarantee that the ripple current
stays below a specifi ed maximum, the inductor value
should be chosen according to the following equation:
L =
V
OUT
f
O
• I
L
1–
V
OUT
V
IN(MAX)
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation begins
when the peak inductor current falls below a level set by the
burst clamp. Lower inductor values result in higher ripple
current which causes this to occur at lower load currents.
This causes a dip in effi ciency in the upper range of low
current operation. In Burst Mode operation, lower inductor
values will cause the burst frequency to increase.
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 fi eld/EMI requirements than on what the
LTC3417A-1 requires to operate. Table 1 shows some
typical surface mount inductors that work well in
LTC3417A-1 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
:
I
RMS
= 2•I
1
•I
2
•D1(1–D2)+I
2
2
(D2 – D2
2
)+I
1
2
(D1– D1
2
)
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 OT1
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 D01608C-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-2ROM-D53LC 2.0 3.9 0.027 5 × 5 × 3
Coilcraft D01608C-222ML 2.2 2.3 0.07 6.6 × 4.5 × 2.9
Sumida CDRH3D16/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
APPLICATIONS INFORMATION
I
RMS
= 2•I
1
•I
2
•D2(1–D1)+I
2
2
(D2 – D2
2
)+I
1
2
(D1– D1
2
)
LTC3417A-1
11
3417a1fa
When D1 = D2 then the equation simplifi es to:
I
RMS
= I
1
+I
2
()
D1–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
–
ΔI
L1
2
I
2
=I
LIM2
–
ΔI
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 satisfi ed, the
capacitance is adequate for fi ltering. The output ripple
(ΔV
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 ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage, since ΔI
L
increases
with input voltage. With ΔI
L
= 0.35I
LOAD(MAX)
, the output
ripple will be less than 100mV at maximum V
IN
and f
O
=
1MHz with:
ESR
COUT
< 150m
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 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 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 signifi cantly larger ESR, and
are often used in extremely cost-sensitive applications
provided that consideration is given to ripple current
ratings and long term reliability. Ceramic capacitors have
the lowest ESR and cost but also have the lowest capaci-
tance density, high voltage and temperature coeffi cient
and exhibit audible piezoelectric effects. In addition, the
high Q of ceramic capacitors along with trace inductance
can lead to signifi cant ringing. Other capacitor types
include the Panasonic specialty polymer (SP) capacitors.
APPLICATIONS INFORMATION
LTC3417A-1
12
3417a1fa
In most cases, 0.1µF to 1µF of ceramic capacitors should
also be placed close to the LTC3417A-1 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 fulfi ll 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 fi rst cycle does the output drop linearly. The output
droop, V
DROOP
, is usually about 2 to 3 times the linear
droop of the fi rst cycle. Thus, a good place to start is with
the output capacitor size of approximately:
C
OUT
≈ 2.5
ΔI
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-1 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 effi ciency, 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.
Power-On Reset
The POR pin is an open-drain output which pulls low when
either regulator is out of regulation. When both output
voltages are above –6% of regulation, a timer is started
which allows the POR output to go high after 212,992
clock cycles (when FREQ is tied to V
IN
) or 294,912 clock
cycles (when FREQ is tied to ground through an external
resistor). This results in a delay of approximately 150ms
when the oscillator is set to 2MHz. When either channel
is shut down, the POR output refl ects the condition of the
running regulator.
APPLICATIONS INFORMATION
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LTC3417AEFE-1#TRPBF

Mfr. #:
Buy LTC3417AEFE-1#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual 1.5A & 1A, 4MHz Synchronous Step-Down DC/DC Converter in 3x5 DFN-16 or TSSOP-20E
Lifecycle:
New from this manufacturer.
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