LT8631
15
8631fa
For more information www.linear.com/LT8631
to the V
IN
pin and ground. Y5V types have poor perfor-
mance over temperature and applied voltage, and should
not
be used. 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.
A word of caution regarding the use of ceramic capacitors
at the input. A ceramic input capacitor can combine with
stray inductance to form a resonant tank circuit. If power
is applied quickly (for example, by plugging the circuit
into a live power source) this tank can ring, doubling the
input voltage and damaging the LT8631. The solution is to
either clamp the input voltage or dampen the tank circuit
by adding a lossy capacitor in parallel with the ceramic
capacitor. For details, see Application Note 88.
Output Capacitor Selection
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8631 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 LT8631's control loop. Since the LT8631 uses
current mode control, it does not require the presence of
output capacitor series resistance (ESR) for stability. Low
ESR or ceramic capacitors should be used to achieve very
low output ripple and small circuit size.
A 47µF, X5R or X7R ceramic capacitor with a voltage rating
greater than the desired output voltage is an excellent first
choice for most applications. The 47µF output capacitor
will provide low output ripple with good transient response.
Increasing the value will reduce the output voltage ripple and
improve transient response, but may increase application
cost and require more board space. Decreasing the value
may save cost and board space but will increase output
voltage ripple, degrade transient performance, and may
cause loop instability. Increasing or decreasing the output
capacitor may require increasing or decreasing the 4.7pF
feedforward capacitor placed between the V
OUT
and FB
pins to optimize transient response. See the Typical Ap-
plications section in the data sheet for suggested output
and feedforward capacitor values.
Note that even X5R and
X7R type ceramic capacitors have
a
DC bias effect which reduces their capacitance when a
DC voltage is applied. It is not uncommon for capacitors
offered in the smallest case sizes to lose more than 50%
of their capacitance when operated near their rated volt
-
age. As
a result it is sometimes necessary to use a larger
capacitance
value, larger case size, or use a higher voltage
rating in order to realize the intended capacitance value.
Consult the manufacturer’s data for the capacitor you
select to be assured of having the necessary capacitance
for the application.
Ceramic Capacitors
Ceramic capacitors are small, robust, and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8631 due to their piezoelectric nature.
When in Burst Mode operation, the LT8631's switching
frequency depends on the load current, and at very light
loads the LT8631 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8631
operates at a lower current limit during Burst Mode opera
-
tion, the noise is typically very quiet to the casual ear. If this
noise is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Low noise ceramic
capacitors are also available.
Enable Pin
The
LT8631 is in shutdown when the EN/UV pin is low
and active when the pin is high. The rising threshold of
the EN/UV comparator is 1.19V, with 17mV of hysteresis.
The EN/UV 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/UV programs the
LT8631 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, the
EN/UV threshold is used in situations where the supply is
current limited, or has a relatively high source resistance. A
switching regulator draws constant power from the source,
APPLICATIONS INFORMATION
