LT8697
applicaTions inForMaTion
switching frequency and maximum input voltage used in
the desired application.
concernsthemaximuminputvoltageratingoftheLT8697.
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
easilyavoided(seeLinearTechnologyApplicationNote88).
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.
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.
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 /V
>
OUT IN
is required to avoid
0.5), a minimum inductance L
sub-harmonic oscillation:
MIN
5.8V+ VSW(BOT)
LMIN
=
• 0.8
fSW
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.
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 and PGND pins. Y5V types have poor performance
IN
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypasstheLT8697andwilleasilyhandletheripplecurrent.
Notethatlargerinputcapacitanceisrequiredwhenalower
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.
When choosing a capacitor, special attention should be
giventothedatasheettocalculatetheeffectivecapacitance
undertherelevantoperatingconditionsofvoltagebiasand
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
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).
Asecondprecautionregardingtheceramicinputcapacitor
Enable Pin
The LT8697 is in shutdown when the EN/UV 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/UV
pin can be tied to V if the shutdown feature is not used,
or tied to a logic level if shutdown control is required.
IN
8697p
20
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
