Datasheet

LT8608/LT8608B
14
Rev. D
For more information www.analog.com
APPLICATIONS INFORMATION
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir-
cuit. If the LT8608 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8608’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
LT8608 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
LT8608’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 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 LT8608 due to their piezoelectric nature.
When in Burst Mode operation, the LT8608’s switching
frequency depends on the load current, and at very light
loads the LT8608 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8608
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 LT8608. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT8608 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8608’s rating.
This situation is easily avoided (see Analog Devices Ap-
plication Note 88).
Enable Pin
The LT8608 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
LT8608 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
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
⎛
⎝
⎜
⎞
⎠
⎟
• 1V
where the LT8608 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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