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LT8614EUDC#PBF

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LT8614
16
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
where ∆I
L
is the inductor ripple current as calculated in
Equation 9 and I
LOAD(MAX)
is the maximum output load
for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an I
SAT
of greater than 1.3A. During long duration
overload or short-circuit conditons, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8614 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (I
LIM
) is at least 8.5A at low
duty cycles and decreases linearly to 7.2A at DC = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current.
I
OUT(MAX)
= I
LIM
–
ΔI
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
ΔI
L
=
V
OUT
L • f
SW
• 1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
(9)
where f
SW
is the switching frequency of the LT8614, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8614 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (I
OUT(MAX)
) given the
switching frequency, and maximum input voltage used in
the desired application.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8614 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e., 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch
-
ing frequency
application, if high light load efficiency is
desired, a higher inductor value should be chosen. See
curve in Typical Performance Characteristics.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A
larger value
inductor
provides a higher maximum load current and
reduces the output voltage ripple. For applications requir
-
ing smaller load currents, the value of the inductor may
be lower and the LT8614 may operate with higher ripple
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitors
The V
IN
of the LT8614 should be bypassed with at least
three ceramic capacitors for best performance. Tw o small
ceramic capacitors of 1µF should be placed close to the
part; one at the V
IN1
/GND1 pins and a second at V
IN2
/GND2
pins. These capacitors should be 0402 or 0603 in size. For
automotive applications requiring 2 series input capaci
-
tors, two small 0402 or 0603 may be placed at each side
of the LT8614 near the V
IN1
/GND1 and V
IN2
/GND2 pins.
A third, larger ceramic
capacitor of 2.2µF or larger should
be placed close to V
IN1
or V
IN2
. See layout section for
more detail. X7R or X5R capacitors are recommended for
best performance across temperature and input voltage
variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
LT8614
17
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir-
cuit. If
the LT8614 circuit is plugged into a live supply, the
input
voltage can ring to twice its nominal value, possibly
exceeding the LT8614’s voltage rating. This situation is
easily avoided (see Linear Technology Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8614 to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8614’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. For good starting
values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feedforward capacitor placed between
V
OUT
and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor
can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capacitance
under the relevant operating conditions of voltage bias and
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8614 due to their piezoelectric nature.
When in Burst Mode operation, the LT8614’s switching
frequency depends on the load current, and at very light
loads the LT8614 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8614
operates at a lower current limit during Burst Mode op
-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output. Low noise ceramic
capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input
voltage rating
of the LT8614. As
previously mentioned, a
ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank
circuit. If the LT8614 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8614’s rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Enable Pin
The LT8614 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to V
IN
if the shutdown feature is not used, or
tied to a logic level if shutdown control is required.
Adding a resistor divider from V
IN
to EN programs the
LT8614 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch
low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
V
IN(EN)
=
R3
R4
+ 1
⎛
⎝
⎜
⎞
⎠
⎟
•1.0V
(10)
where the LT8614 will remain off until V
IN
is above V
IN(EN)
.
Due to the comparator’s hysteresis, switching will not stop
until the input falls slightly below V
IN(EN)
.
LT8614
18
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
When operating in Burst Mode operation for light load
currents, the current through the V
IN(EN)
resistor network
can easily be greater than the supply current consumed
by the LT8614. Therefore, the V
IN(EN)
resistors should be
large to minimize their effect on efficiency at low loads.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from V
IN
that powers the drivers and the internal
bias circuitry. The INTV
CC
can supply enough current for
the LT8614’s circuitry and must be bypassed to ground
with a minimum of 1μF ceramic capacitor. Good bypassing
is necessary to supply the high transient currents required
by the power MOSFET gate drivers. To improve efficiency
the internal LDO can also draw current from the BIAS
pin when the BIAS pin is at 3.1V or higher. Typically the
BIAS pin can be tied to the output of the LT8614, or can
be tied to an external supply of 3.3V or above. If BIAS is
connected to a supply other than V
OUT
, be sure to bypass
with a local ceramic capacitor. If the BIAS pin is below
3.0V, the internal LDO will consume current from
V
IN
.
Applications with high input voltage and high switching
frequency where the internal LDO pulls current from V
IN
will increase die temperature because of the higher power
dissipation across the LDO. Do not connect an external
load to the INTV
CC
pin.
Output Voltage Tracking and Soft-Start
T
he LT8614 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
pulls up the TR/SS pin to INTV
CC
. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent
current surge
on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS can
be externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
An active pull-
down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, V
IN
voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8614’s output voltage is within the ±9%
window of the regulation point, which is a V
FB
voltage in
the range of 0.883V to 1.057V (typical), the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 1.2% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV
CC
has fallen too
low, V
IN
is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC pin
below 0.4V (this can be ground or a logic low output). To
synchronize the LT8614 oscillator to an external frequency
connect a square
wave (with 20% to 80% duty cycle)
to
the SYNC pin. The square wave amplitude should have val-
leys that
are below 0.4V and peaks above 2.4V (up to 6V).
The
LT8614 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8614
may be synchronized over a 200kHz to 3MHz range. The
R
T
resistor should be chosen to set the LT8614 switching
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the R
T
should be selected for 500kHz.
The slope compensation is set by the R
T
value, while the
minimum slope compensation required to avoid subhar-
monic oscillations
is established by the inductor size,
input voltage, and output voltage. Since the synchroniza-
tion frequency
will not change the slopes of the inductor
current waveform, if the inductor is large enough to avoid
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LT8614EUDC#PBF

Mfr. #:
Buy LT8614EUDC#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 4A Synchronous Step-Down SILENT SWITCHER with 2.5 A Quiescent Current
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