LT3690 LINER [Linear Technology], LT3690 Datasheet - Page 14

• Manufacturer Part Number: LT3690
• Description: 42V, 2.5A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LT8610
APPLICATIONS INFORMATION
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
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitor
Bypass the input of the LT8610 circuit with a ceramic ca-
pacitor of X7R or X5R type placed as close as possible to
the V
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8610 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 significant 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
ripple at the LT8610 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8610 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8610.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir-
cuit. If the LT8610 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8610’s voltage rating. This situation is
easily avoided (see Linear Technology Application Note 88).
14
IN
and PGND pins. Y5V types have poor performance
OUT
/V
IN
> 0.5),
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8610 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 LT8610’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
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 LT8610 due to their piezoelectric nature.
When in Burst Mode operation, the LT8610’s switching
frequency depends on the load current, and at very light
loads the LT8610 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT8610
operates at a lower current limit during Burst Mode op-
eration, 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.
OUT
and FB. Increasing the output capacitance will also
8610p