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LT8610EMSE#PBF

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LT8610
13
8610fa
For more information www.linear.com/LT8610
APPLICATIONS INFORMATION
The LT8610 is capable of a maximum duty cycle of greater
than 99%, and the V
IN
-to-V
OUT
dropout is limited by the
R
DS(ON)
of the top switch. In this mode the LT8610 skips
switch cycles, resulting in a lower switching frequency
than programmed by RT.
For applications that cannot allow deviation from the pro
-
grammed switching
frequency at low V
IN
/V
OUT
ratios use
the following formula to set switching frequency:
V
IN(MIN)
=
V
OUT
+ V
SW(BOT)
1– f
SW
• t
OFF(MIN)
– V
SW(BOT)
+ V
SW(TOP)
(5)
where V
IN(MIN)
is the minimum input voltage without
skipped cycles, V
OUT
is the output voltage, V
SW(TOP)
and
V
SW(BOT)
are the internal switch drops (~0.3V, ~0.15V,
respectively at maximum load), f
SW
is the switching fre-
quency (set by RT),
and t
OFF(MIN)
is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8610 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8610 safely tolerates opera
-
tion with a saturated inductor through the use of a high
speed peak-current mode architecture.
A good first choice for the inductor value is:
L =
V
OUT
+ V
SW(BOT)
f
SW
(6)
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.15V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled I
SAT
)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
I
L(PEAK)
=I
LOAD(MAX)
+
1
2
∆I
L
(7)
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
routing 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 LT8610 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 3.5A at low
duty cycles and decreases linearly to 2.8A 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
–
∆I
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆I
L
=
V
OUT
L • f
SW
• 1–
V
OUT
V
IN(MAX)
(9)
where f
SW
is the switching frequency of the LT8610, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8610 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.
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 LT8610 may operate with higher ripple
LT8610
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8610fa
For more information www.linear.com/LT8610
APPLICATIONS INFORMATION
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 Capacitor
Bypass the input of the LT8610 circuit with a ceramic ca
-
pacitor of
X7R or X5R type placed as close as possible to
the
V
IN
and PGND pins. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8610 and will easily handle the ripple current.
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.
Step-down regulators draw current from the
input sup-
ply in
pulses with very fast rise and fall times. The input
capacitor
is required to reduce the resulting voltage
ripple at the LT8610 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8610 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8610.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir
-
cuit. If the LT8610 circuit is plugged into a live supply, the
input
voltage can ring to twice its nominal value, possibly
exceeding the LT8610’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 LT8610 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 LT8610’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 LT8610 due to their piezoelectric nature.
When in Burst Mode operation, the LT8610’s switching
frequency depends
on the load current, and at very light
loads the LT8610 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8610
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.
LT8610
15
8610fa
For more information www.linear.com/LT8610
APPLICATIONS INFORMATION
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8610. As
previously mentioned
, a
ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank
circuit. If the LT8610 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8610’s rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Enable Pin
The LT8610 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
LT8610 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 LT8610 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)
.
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 LT8610. 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 LT8610’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 LT8610, 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 LT8610 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.
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LT8610EMSE#PBF

Mfr. #:
Buy LT8610EMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 2.5A Synchronous Step-Down Regulator with 2.5uA Quiescent Current
Lifecycle:
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