13
Rev. C
APPLICATIONS INFORMATION
significant inductance due to long wires or cables, addi-
tional 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 rip-
ple at the LT8607 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 LT8607 (see the PCB Layout section).
A second precaution regarding the ceramic input capaci-
tor concerns the maximum input voltage rating of the
LT8607. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
tank circuit. If the LT8607 circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8607’s voltage rating.
This situation is easily avoided (see Analog Devices
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 LT8607 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 sta-
bilize the LT8607’s control loop. Ceramic capacitors have
very low equivalent series resistance (ESR) and provide
the best ripple performance. A good starting value is:
C
OUT
=
100
V
OUT
• f
SW
where f
SW
is in MHz, and C
OUT
is the recommended output
capacitance in µF. 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 capaci-
tor 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 capaci-
tance 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 LT8607 due to their piezoelectric
nature. When in Burst Mode operation, the LT8607’s
switching frequency depends on the load current, and at
very light loads the LT8607 can excite the ceramic capacitor
at audio frequencies, generating audible noise. Since the
LT8607 operates at a lower current limit during Burst Mode
operation, 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.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8607. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT8607 circuit is plugged into
a live supply, the input voltage can ring to twice its nomi-
nal value, possibly exceeding the LT8607’s rating. This
situation is easily avoided (see Analog Devices Application
Note 88).
Enable Pin
The LT8607 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.05V, with 50mV 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
LT8607 to regulate the output only when V
IN
is above
a desired voltage (see Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
