Datasheet

LT8612
14
8612fa
For more information www.linear.com/LT8612
APPLICATIONS INFORMATION
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8612 may operate with higher ripple
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 LT8612 circuit with a ceramic
capacitor of X7R or X5R type placed as close as pos-
sible to the V
IN
and PGND pins. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 10μF to 22μF ceramic capacitor
is adequate to bypass the LT8612 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 sig-
nificant 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 rip-
ple at the LT8612 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10μF
capacitor is capable of this task, but only if it is
placed close to the
LT8612 (see the PCB Layout section).
A second precaution regarding the ceramic input capaci
-
tor concerns the maximum input voltage rating of the
LT8612. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
tank circuit. If the LT8612 circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8612’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 LT8612 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 LT8612’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 out-
put ripple and good transient response. Transient perfor-
mance can be improved with a higher value output capaci-
tor 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 problems
when used with the LT8612 due to their piezoelectric
nature. When in Burst Mode operation, the LT8612’s
switching frequency depends on the load current, and
at very light loads the LT8612 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8612 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
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.