OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LTC3406AIS5#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3406A
7
3406afa
Main Control Loop
The LTC3406A uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top power
MOSFET is turned on each cycle when the oscillator sets
the RS latch, and turned off when the current comparator,
I
COMP
, resets the RS latch. The peak inductor current at
which I
COMP
resets the RS latch, is controlled by the output
of error amplifi er EA. When the load current increases,
it causes a slight decrease in the feedback voltage, FB,
relative to the 0.6V reference, which in turn, causes the
EA amplifi er’s output voltage to increase until the average
inductor current matches the new load current. While the
top MOSFET is off, the bottom MOSFET is turned on until
either the inductor current starts to reverse, as indicated
by the current reversal comparator I
RCMP
, or the beginning
of the next clock cycle.
The main control loop is shut down by grounding RUN,
resetting the internal soft-start. Re-enabling the main
control loop by pulling RUN high activates the internal
soft-start, which slowly ramps the output voltage over
approximately 0.9ms until it reaches regulation.
Burst Mode Operation
The LTC3406A is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand.
In Burst Mode operation, the peak current of the inductor
is set to approximately 100mA regardless of the output
load. Each burst event can last from a few cycles at light
loads to almost continuously cycling with short sleep
intervals at moderate loads. In between these burst events,
the power MOSFETs and any unneeded circuitry are turned
off, reducing the quiescent current to 20μA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifi er’s output rises above the sleep threshold signaling
the BURST comparator to trip and turn the top MOSFET
on. This process repeats at a rate that is dependent on
the load demand.
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 LTC3406A is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
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 LTC3406A uses a patented
scheme that counteracts this compensating ramp, which
allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
OPERATION
(Refer to Functional Diagram)
LTC3406A
8
3406afa
The basic LTC3406A application circuit is shown on the
front page. External component selection is driven by the
load requirement and begins with the selection of L fol-
lowed 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 cur-
rent as shown in Equation 1. A reasonable starting point for
setting ripple current is ∆I
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 opera-
tion. The transition to low current operation begins when
the inductor current peaks fall to approximately 100mA.
Lower inductor values (higher ∆I
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
APPLICATIONS INFORMATION
style inductor to use often depends more on the price vs
size requirements and any radiated fi eld/EMI requirements
than on what the LTC3406A requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406A 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
OMAX
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
commonly used for design because even signifi cant de-
viations 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 tem-
perature than required. Always consult the manufacturer
if there is any question.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm
3
)
Sumida
CDRH3D16
1.5
2.2
3.3
4.7
0.043
0.075
0.110
0.162
1.55
1.20
1.10
0.90
3.8 × 3.8 × 1.8
Sumida
CMD4D06
2.2
3.3
4.7
0.116
0.174
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2
Murata
LQH32CN
1.0
2.2
4.7
0.060
0.097
0.150
1.00
0.79
0.65
2.5 × 3.2 × 2.0
LTC3406A
9
3406afa
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
∆
V
OUT
is
determined by:
V
OUT
I
L
ESR +
1
8fC
OUT
where f = operating frequency, C
OUT
= output capacitance
and
∆
I
L
= ripple current in the inductor. For a fi xed 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 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 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
LTC3406A’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
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
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
V
OUT
= 0.6V 1+
R2
R1
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
APPLICATIONS INFORMATION
V
FB
GND
LTC3406A
0.6V ≤ V
OUT
≤ 5.5V
R2
R1
3406A F01
Figure 1. Setting the LTC3406A Output Voltage
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.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC3406AIS5#TRMPBF

Mfr. #:
Buy LTC3406AIS5#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1.5MHz, 600mA Sync Buck Reg in SOT
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet