Datasheet

LT8620
15
8620fa
For more information www.linear.com/LT8620
APPLICATIONS INFORMATION
tank circuit. If the LT8620 circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8620’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 LT8620
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 LT8620’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. 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 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.
Ceramic Capacitors
Ceramic capacitors
are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8620 due to their piezoelectric
nature. When in Burst Mode operation, the LT8620’s
switching frequency depends on the load current, and
at very light loads the LT8620 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8620 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. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8620. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms
a high quality (un-
derdamped) tank circuit. If the LT8620 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8620’s rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Enable Pin
The LT8620 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold
of the EN
comparator is 1.0V, with 40mV 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
LT8620 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
V
IN(EN)
=
R3
R4
+1






• 1.0V
(10)
where the LT8620 will remain off until V
IN
is above V
IN(EN)
.
Due to the comparator’s hysteresis, switching will not stop
until the input falls slightly below V
IN(EN)
.
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