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LTC3406BES5-1.8#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
7
LTC34 06B
3406bfa
OPERATIO
U
(Refer to Functional Diagram)
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will then
be determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R
DS(ON)
of the P-channel switch increases
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3406B is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Low Supply Operation
The LTC3406B will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles >40%. However, the LTC3406B uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
SUPPLY VOLTAGE (V)
2.5
MAXIMUM OUTPUT CURRENT (mA)
1200
1000
800
600
400
200
0
3.0
3.5 4.0 4.5
3406B F02
5.0 5.5
V
OUT
= 1.8V
V
OUT
= 1.5V
V
OUT
= 2.5V
Figure 2. Maximum Output Current vs Input Voltage
8
LTC34 0 6B
3406bfa
APPLICATIO S I FOR ATIO
WUUU
The basic LTC3406B application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection 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 4.7µH. 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 ∆I
L
= 240mA (40% of 600mA).
∆ =
()( )
−
⎛
⎝
⎜
⎞
⎠
⎟
I
fL
V
V
V
L OUT
OUT
IN
1
1
(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 efficiency, choose a low DC-resis-
tance inductor.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally 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 field/EMI requirements
than on what the LTC3406B requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406B applications.
Table 1. Representative Surface Mount Inductors
PART VALUE DCR MAX DC SIZE
NUMBER (µH) (Ω MAX) CURRENT (A) W × L × H (mm
3
)
Sumida 1.5 0.043 1.55 3.8 × 3.8 × 1.8
CDRH3D16 2.2 0.075 1.20
3.3 0.110 1.10
4.7 0.162 0.90
Sumida 2.2 0.116 0.950 3.5 × 4.3 × 0.8
CMD4D06 3.3 0.174 0.770
4.7 0.216 0.750
Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2
ELT5KT 4.7 0.20 0.95
Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0
LQH3C 2.2 0.097 0.79
4.7 0.150 0.65
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:
CI
VVV
V
IN OMAX
OUT IN OUT
IN
required I
RMS
≅
−
()
[]
12/
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant 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 manufac-
turer if there is any question.
The selection of C
OUT
is driven by the required effective
series resistance (ESR).
9
LTC34 06B
3406bfa
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 ∆V
OUT
is deter-
mined by:
∆≅∆ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT L
OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I
L
increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. 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 specific recommendations.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3406B’s 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.
However, care must be taken when ceramic 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
APPLICATIO S I FOR ATIO
WUUU
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 (LTC3406B Only)
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1
2
1
.
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
Figure 3. Setting the LTC3406B Output Voltage
V
FB
GND
LTC3406B
0.6V ≤ V
OUT
≤ 5.5V
R2
R1
3406B F03
Efficiency Considerations
The efficiency 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 efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
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LTC3406BES5-1.8#TRMPBF

Mfr. #:
Buy LTC3406BES5-1.8#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 600mA, 1.5MHz Sync Step-dwn in ThinSOT w/ burst defeat mode
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