LT8610A/LT8610AB Series
17
8610abfa
For more information www.linear.com/LT8610A
APPLICATIONS INFORMATION
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT8610A/LT8610AB circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8610A/
LT8610AB’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 LT8610A/LT8610AB 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 LT8610A/LT8610AB’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 LT8610A/LT8610AB due to their
piezoelectric nature. When in Burst Mode operation, the
LT8610A/LT8610AB’s switching frequency depends on
the load current, and at very light loads the LT8610A/
LT8610AB can excite the ceramic capacitor at audio fre
-
quencies, generating
audible noise. Since the LT8610A/
LT8610AB operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca
-
sual 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 LT8610A/LT8610AB. As
previously mentioned, a
ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank
circuit. If the LT8610A/LT8610AB circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8610A/
LT8610AB’s rating. This situation is easily avoided (see
Linear Technology Application Note 88).
Enable Pin
The LT8610A/LT8610AB 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
LT8610A/LT8610AB to regulate the output only when V
IN
is above a desired voltage (see the Block Diagram). Typi-
cally, this
threshold, V
IN(EN)
, is used in situations where
the input supply is current limited, or has a relatively high
sour
ce 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)
=
R4
+1
•1.0V
(10)
where the LT8610A/LT8610AB will remain off until V
IN
is
above V
IN(EN)
. Due to the comparator’s hysteresis, switch-
ing will not stop until the input falls slightly below V
IN(EN)
.
