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LTC3416EFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3416
7
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OPERATIO
U
power MOSFET is turned off and the bottom power MOS-
FET is switched on until either the overvoltage condition
clears or the bottom MOSFET’s current limit is reached.
Voltage Tracking
Some microprocessors, ASIC and DSP chips need two
power supplies with different voltage levels. These sys-
tems often require voltage sequencing between the core
power supply and the I/O power supply. Without proper
sequencing, latch-up failure or excessive current draw
may occur that could result in damage to the processor’s
I/O ports or the I/O ports of supporting system devices
such as memory, FPGAs or data converters. To ensure
that the I/O loads are not driven until the core voltage is
properly biased, tracking of the core supply voltage and
the I/O supply voltage is necessary.
Voltage tracking is enabled by applying a voltage to the
TRACK pin. When the voltage on the TRACK pin is below
0.8V, the feedback voltage will regulate to this tracking
voltage. When the tracking voltage exceeds 0.8V, tracking
control over the feedback voltage is gradually released.
Full release of tracking control over the feedback voltage
is achieved when the tracking voltage exceeds 1.05V.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle, eventually reaching 100% duty cycle. The output
voltage will then be determined by the input voltage minus
the voltage drop across the internal P-channel MOSFET
and the inductor.
Low Supply Operation
The LTC3416 is designed to operate down to an input
supply voltage of 2.25V. One important consideration at
low input supply voltages is that the R
DS(ON)
of the
P-channel and N-channel power switches increases. The
user should calculate the power dissipation when the
LTC3416 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at duty cycles greater than 50%. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
the maximum inductor peak current is reduced when
slope compensation is added. In the LTC3416, however,
slope compensation recovery is implemented to keep the
maximum inductor peak current constant throughout the
range of duty cycles. This keeps the maximum output
current relatively constant regardless of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. To
prevent current runaway from occurring, a secondary
current limit is imposed on the inductor current. If the
inductor valley current exceeds 7.8A, the top power MOS-
FET will be held off and switching cycles will be skipped
until the inductor current is reduced.
LTC3416
8
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APPLICATIO S I FOR ATIO
WUUU
The basic LTC3416 application circuit is shown in Figure 1a.
External component selection is determined by the maxi-
mum load current and begins with the selection of the
operating frequency and inductor value followed by C
IN
and C
OUT
.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency of the LTC3416 is determined by
an external resistor that is connected between the R
T
pin
and ground. The value of the resistor sets the ramp current
that is used to charge and discharge an internal timing
capacitor within the oscillator and can be calculated by
using the following equation:
R
f
k
OSC
= Ω
()
Ω
308 10
10
11
.•
–
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3416 imposes a minimum
limit on the operating duty cycle. The minimum on-time is
typically 110ns. Therefore, the minimum duty cycle is
equal to 100 • 110ns • f(Hz).
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ∆I
L
increases with higher V
IN
or lower V
OUT
and decreases with higher inductance.
∆ =
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
I
V
fL
V
V
L
OUT OUT
IN
1–
Having a lower ripple current reduces the core losses in
the inductor, the ESR losses in the output capacitors and
the output voltage ripple. Highest efficiency operation is
achieved at low frequency with small ripple current. This,
however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is ∆I
L
= 0.4(I
MAX
). The largest ripple current occurs at the
highest V
IN
. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
L
V
fI
V
V
OUT
L MAX
OUT
IN MAX
=
∆
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
() ()
–1
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
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 characteristics. The choice of which style inductor
to use mainly depends on the price vs size requirements
and any radiated field/EMI requirements. New designs for
surface mount inductors are available from Coiltronics,
Coilcraft, Toko and Sumida.
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to filter the trapezoi-
dal wave current at the source of the top MOSFET. To
LTC3416
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APPLICATIO S I FOR ATIO
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prevent large voltage transients from occurring, a low ESR
input capacitor sized for the maximum RMS current
should be used. The maximum RMS current is given by:
II
V
V
V
V
RMS OUT MAX
OUT
IN
IN
OUT
=
()
–1
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 significant deviations do not
offer much relief. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life which makes it advisable to further derate the
capacitor, or choose a capacitor rated at a higher tempera-
ture than required. Several capacitors may also be paral-
leled to meet size or height requirements in the design. For
low input voltage applications, sufficient bulk input ca-
pacitance is needed to minimize transient effects during
output load changes.
The selection of C
OUT
is determined by the effective series
resistance (ESR) that is required to minimize voltage
ripple and load step transients as well as the amount of
bulk capacitance that is necessary to ensure that the
control loop is stable. Loop stability can be checked by
viewing the load transient response as described in a later
section. The output ripple, ∆V
OUT
, is determined by:
∆≤∆ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT L
OUT
1
8
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and RMS
current handling requirements. Dry tantalum, special poly-
mer, aluminum electrolytic and ceramic capacitors are all
available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only use
types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR, but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capaci-
tors have excellent low ESR characteristics but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
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. However, care must
be taken when these capacitors are used at the input and
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 an external resistive divider
according to the following equation:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
2
1
.
The resistive divider allows the V
FB
pin to sense a fraction
of the output voltage as shown in Figure 2.
Figure 2. Setting the Output Voltage
LTC3416
SGND
R1
3416 F02
R2
V
FB
V
OUT
Voltage Tracking
The LTC3416 allows the user to program how its output
voltage ramps during start-up by means of the TRACK pin.
Through this pin, the output voltage can be set up to either
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LTC3416EFE#PBF

Mfr. #:
Buy LTC3416EFE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 4A, 4MHz, Mono Sync Buck Reg w/ Track
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New from this manufacturer.
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