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LT8697IUDD#TRPBF

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LT8697
19
8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
Using RCBL as an Output Current Monitor
The primary function of the RCBL pin is to set the cable
drop compensation as discussed in the cable drop com
-
pensation section earlier. However, the RCBL pin produces
an output voltage that is proportional to the output load
current. The RCBL pin can therefore be used as an output
load monitor. The voltage on the RCBL pin obeys the fol
-
lowing relation to USB load current:
V
CBL
=I
LOAD
•R
SENSE
• 20.55
V
CBL
is valid when the LT8697 is switching.
Since the RCBL pin current is part of the cable drop com-
pensation control
loop,
excessive capacitive loading on the
RCBL pin can cause USB output voltage overshoot during
load steps. Keep the capacitive loading on the RCBL pin
below 100pF or isolate the load capacitance with 100kΩ
in series between the RCBL pin and the input it is driving,
as shown in Figure 7.
Table 3. SW Frequency vs R
T
Value
f
SW
(MHz) R
T
(kΩ)
0.3 140
0.4 102
0.5 80.6
0.6 66.5
0.7 56.2
0.8 47.5
1.0 37.4
1.2 30.1
1.4 25.5
1.6 21.5
1.8 18.7
2.0 16.5
2.2 14.7
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller
inductor and capacitor values may be used. The disad
-
vantages are
lower efficiency and a reduced input voltage
range with constant frequency operation.
The
highest switching frequency (f
SW(MAX
)) for a given
application can be calculated as follows:
f
SW(MAX)
=
5V + V
SW(BOT)
t
ON(MIN)
• V
IN
− V
SW(TOP)
+ V
SW(BOT)
( )
where V
IN
is the typical input voltage, V
SW(TOP)
and
V
SW(BOT)
are the internal switch drops (~0.3V and ~0.15V
respectively, at maximum load) and t
ON(MIN)
is the mini-
mum top switch on-time (see the Electrical Characteristics
section).
This equation shows that a slower switching
frequency is necessary to accommodate a high V
IN
/V
OUT
ratio.
For transient operation, V
IN
may go as high as the abso-
lute maximum
rating of 42V regardless of the R
T
value.
However, the LT8697 will reduce switching frequency as
necessary to maintain control of inductor current to as
-
sure safe operation.
Figure 7. Using the RCBL Pin as Output Current Monitor
100k ADC
8697 F07
R
CBL
RCBL
Setting the Switching Frequency
The LT8697 uses a constant frequency PWM architecture
that can be programmed to switch from 300kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
T
value for a desired switching
frequency is in Table 3. The R
T
resistor required for a
desired switching frequency can be calculated using the
following equation:
R
T
=
43
f
SW
−5.2
where R
T
is in kΩ and f
SW
is the desired switching fre-
quency in MHz.
LT8697
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8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
The LT8697 can operate at very high duty cycle, thus
maintaining the output voltage in regulation with the input
voltage only several hundred mV higher. This dropout volt
-
age depends on load current and the R
DS(ON)
of the top
switch. However, the LT8697 skips off-times in very high
duty cycle conditions, reducing the switching frequency
below that programmed by R
T
. In this dropout mode, the
maximum allowable on-time is about 18µs. If this 18µs
on-time threshold is reached, the LT8697 enforces a 400ns
off-time to keep the BST capacitor charged at light loads.
This behavior limits the maximum duty cycle to 97.5%,
but guarantees good dropout performance across all loads
and any start-up condition.
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:
f
SW(MAX)
=
1
t
OFF(MIN)
V
IN(MIN)
− 5.8 − V
SW(TOP)
V
IN(MIN)
+ V
SW(BOT)
− V
SW(TOP)
where V
IN(MIN)
is the minimum input voltage without
skipped cycles, 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 frequency (set by R
T
), 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 LT8697 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 LT8697 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 as follows:
L =
5.8V + V
SW(BOT)
f
SW
where f
SW
is the switching frequency in MHz, 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
) rat-
ing of the inductor must be higher than the load current
plus 1/2 of the inductor ripple current:
I
L(PEAK)
= I
OUT(MAX)
+
∆I
L
2
where ΔI
L
is the inductor ripple current as calculated
below and I
OUT(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 conditions, the inductor RMS
current rating requirement is greater to avoid overheat
-
ing 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 LT8697 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 4.8A at low duty cycles and
decreases linearly to 4A 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
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆I
L
=
5V
L • f
SW
• 1−
5V
V
IN(MAX)
where f
SW
is the switching frequency of the LT8697 and
L is the value of the inductor. Therefore, the maximum
output current that the LT8697 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
LT8697
21
8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
switching frequency and maximum input voltage used in
the desired application. Note that the LT8697 peak switch
current decreases in the 125°C to 150°C H-grade junction
temperature range. The maximum output current that the
LT8697 can deliver at 150°C junction temperature and
maximum duty cycle may be less than 2.5A depending
on the inductor value.
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 LT8697 may operate with higher ripple
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency.
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 L
MIN
is required to avoid
sub-harmonic oscillation:
L
MIN
=
5.8V +V
SW(BOT)
f
SW
• 0.8
For robust operation over a wide V
IN
and V
OUT
range, use
at least an inductor value as specified above.
Input Capacitor
Bypass the input of the LT8697 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 LT8697 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 LT8697 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 LT8697 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8697.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir
-
cuit. If the LT8697 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8697’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 LT8697 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 LT8697’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. 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 this may cause loop instability if the output
capacitor is too small. Since cable drop compensation
slews the voltage across the output capacitor in response
to transient load steps, a smaller output capacitor can give
faster response time. 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.
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LT8697IUDD#TRPBF

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Buy LT8697IUDD#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators USB 5V 2.5A Output, 42V Inpuut Synchronous Step-Down Regulator with Cable Drop Compensation
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