Datasheet

LT8609/LT8609A/LT8609B
16
Rev. H
For more information www.analog.com
APPLICATIONS INFORMATION
and should not be used. A 4.7μF to 10μF ceramic capacitor
is adequate to bypass the LT8609/LT8609A/LT8609B 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 LT8609/LT8609A/LT8609B 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 LT8609/LT8609A/
LT8609B (see the PCB Layout section). A second precau-
tion regarding the ceramic input capacitor concerns the
maximum input voltage rating of the LT8609/LT8609A/
LT8609B. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
tank circuit. If the LT8609/LT8609A/LT8609B circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8609/
LT8609A/LT8609B’s voltage rating. This situation is eas
-
ily 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
LT8609/LT8609A/LT8609B 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 stabi
-
lize the LT8609/LT8609A/LT8609B’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
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 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 prob-
lems when used with the LT8609/LT8609A/LT8609B
due to their piezoelectric nature. When in Burst Mode
operation, the LT8609/LT8609A/LT8609B’s switch
-
ing frequency depends on the load current, and at very
light loads the LT8609/LT8609A/LT8609B can excite
the ceramic capacitor at audio frequencies, generat-
ing audible noise. Since the LT8609/LT8609A/LT8609B
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 LT8609/LT8609A/
LT8609B. As previously mentioned, a ceramic input
capacitor combined with trace or cable inductance forms
a high quality (under damped) tank circuit. If the LT8609/
LT8609A/LT8609B circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8609/LT8609A/LT8609B’s rating. This
situation is easily avoided (see Application Note 88).
Enable Pin
The LT8609/LT8609A/LT8609B 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
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.