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LTC3409EDD#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3409
10
3409fc
APPLICATIONS INFORMATION
The basic LTC3409 application circuit is shown on the fi rst
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C
IN
and C
OUT
.
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 1μH to 10μH. Its value is chosen based
on the desired ripple current. Large value inductors
lower ripple current and small value inductors result in
higher ripple currents. Higher V
IN
or V
OUT
also increases
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI
L
= 240mA
(40% of 600mA).
I
L
=
1
f•L
V
OUT
1–
V
OUT
V
IN
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications (600mA +
120mA). For better effi ciency, choose a low DC resistance
inductor. The inductor value also has an effect on Burst
Mode operation. The transition to low current operation be-
gins when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher ΔI
L
) will cause this
to occur at lower load currents, which can cause a dip in
effi ciency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. 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 and any radiated fi eld/EMI requirements
than on what the LTC3409 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3409 applications.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
C
IN
Required I
RMS
I
OUT(MAX)
V
OUT
V
IN
–V
OUT
()
⎡
⎣
⎤
⎦
1/ 2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is common-
ly used for design because even signifi cant deviations do
not offer much relief. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life. This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Always consult the manufacturer if there is any
question. The selection of C
OUT
is driven by the required
effective series resistance (ESR). Typically, once the ESR
requirement for C
OUT
has been met, the RMS current
rating generally far exceeds the I
RIPPLE(P-P)
requirement.
The output ripple DV
OUT
is determined by:
V
OUT
= I
L
ESR +
1
8•f•C
OUT
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm
3
)
Sumida
CDRH2D18/LD
2.2
3.3
0.041
0.054
0.85
0.75
3.2 × 3.2 × 2.0
Sumida
CDRH2D11
1.5
2.2
0.068
0.170
0.90
0.78
3.2 × 3.2 × 1.2
Sumida
CMD4D11
2.2
3.3
0.116
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0
2.2
0.060
0.097
1.00
0.79
2.5 × 3.2 × 2.0
Toko
D312F
2.2
3.3
0.060
0.260
1.08
0.92
2.5 × 3.2 × 2.0
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2
LTC3409
11
3409fc
APPLICATIONS INFORMATION
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. For a fi xed output
voltage, the output ripple is highest at maximum input
voltage since ΔI
L
increases with input voltage. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount confi gurations. In the case of tantalum,
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 mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specifi c recommendations.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now avail-
able in smaller case sizes. Their high ripple current, high
voltage rating and low ESR make them ideal for switching
regulator applications. Because the LTC3409’s control loop
does not depend on the output capacitor’s ESR for stable
operation, ceramic capacitors can be used to achieve very
low output ripple and small circuit size.
However, care must be taken when these capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V
IN
. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at V
IN
, large enough
to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V
OUT
= 0.613V 1+
R1
R2
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
V
FB
V
OUT
R1
R2
3409 F01
GND
LTC3409
Figure 1
Effi ciency Considerations
The effi ciency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
Effi ciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses in LTC3409 circuits: V
IN
quiescent current and
I
2
R losses. The V
IN
quiescent current loss dominates
the effi ciency loss at very low load currents whereas the
I
2
R loss dominates the effi ciency loss at medium to high
load currents. In a typical effi ciency plot, the effi ciency
curve at very low load currents can be misleading since
the actual power lost is of no consequence as illustrated
in Figure 2.
LTC3409
12
3409fc
1. The V
IN
quiescent current is due to two components:
the DC bias current as given in the Electrical Charac-
teristics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate
is switched from high to low to high again, a packet
of charge, dQ, moves from V
IN
to ground. The result-
ing dQ/dt is the current out of V
IN
that is typically
larger than the DC bias current. In continuous mode,
I
GATECHG
= f(Q
T
+ Q
B
) where Q
T
and Q
B
are the gate
charges of the internal top and bottom switches. Both
the DC bias and gate charge losses are proportional to
V
IN
and thus their effects will be more pronounced at
higher supply voltages.
2. I
2
R losses are calculated from the resistances of the
internal switches, R
SW
, and external inductor R
L
. In
continuous mode, the average output current fl owing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R
DS(ON)
and the duty cycle
(DC) as follows:
R
SW
= (R
DS(ON)TOP
)(DC) + (R
DS(ON)BOT
)(1 – DC)
The R
DS(ON)
for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics.
Thus, to obtain I
2
R losses, simply add R
SW
to R
L
and
multiply the result by the square of the average output
current.
Other losses including C
IN
and C
OUT
ESR dissipative losses
and inductor core losses generally account for less than
2% total additional loss.
Thermal Considerations
In most applications the LTC3409 does not dissipate much
heat due to its high effi ciency. But, in applications where the
LTC3409 is running at high ambient temperature with low
supply voltage and high duty cycles, such as in dropout,
the heat dissipated may exceed the maximum junction
temperature of the part. If the junction temperature reaches
approximately 150°C, both power switches will be turned
off and the SW node will become high impedance.
To avoid the LTC3409 from exceeding the maximum
junction temperature, the user will need to do a thermal
analysis. The goal of the thermal analysis is to determine
whether the operating conditions exceed the maximum
junction temperature of the part. The temperature rise is
given by:
T
R
= (P
D
)(θ
JA
)
where P
D
is the power dissipated by the regulator and θ
JA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T
J
, is given by:
T
J
= T
A
+ T
R
where T
A
is the ambient temperature.
As an example, consider the LTC3409 in dropout at an
input voltage of 1.6V, a load current of 600mA and an
ambient temperature of 75°C. From the typical perfor-
mance graph of switch resistance, the R
DS(ON)
of the
P-channel switch at 75°C is approximately 0.48Ω. There-
fore, power dissipated by the part is:
P
D
= I
LOAD
2
• R
DS(ON)
= 172.8mW
For the DD8 package, the θ
JA
is 43°C/W. Thus, the junction
temperature of the regulator is:
T
J
= 75°C + (0.1728)(43) = 82.4°C
which is well below the maximum junction temperature
of 125°C.
Figure 2
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC3409EDD#TRPBF

Mfr. #:
Buy LTC3409EDD#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 600mA, 1.5/2.25MHz Sync Step-down in DFN
Lifecycle:
New from this manufacturer.
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