Datasheet

LT3680
11
Rev C
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Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value inductor provides a slightly higher maximum
load current and will reduce the output voltage ripple. If
your load is lower than 3.5A, then you can decrease the
value of the inductor and operate with higher ripple cur
-
rent. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency.
There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is okay but further reduces maximum load current.
For details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3680 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor perfor-
mance over temperature and applied voltage, and should
not be used. A 10µF to 22µF ceramic capacitor is adequate
to bypass the LT3680 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 lower 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 rip
-
ple at the LT3680 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10µF capacitor is capable of this task, but only if it
is placed close to the LT3680 and the catch diode (see
the PCB Layout section). A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3680. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3680 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3680’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safely section).
For space sensitive applications, a 4.7µF ceramic capaci
-
tor can be used for local bypassing of the LT3680
input. However, the lower input capacitance will result
in increased input current ripple and input voltage rip-
ple, and may couple noise into other circuitry. Also, the
increased voltage ripple will raise the minimum operating
voltage of the LT3680 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3680 to produce the DC output. In this role it deter
-
mines the output ripple, and low impedance at the switch-
ing frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3680’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
Vf
OUT
OUT SW
=
100
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 capacitor if the compensation network is also
adjusted to maintain the loop bandwidth. A lower value
of output capacitor can be used to save space and cost
but transient performance will suffer. See the Frequency
Compensation section to choose an appropriate compen-
sation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher volt-
age rating, may be required. High performance tanta-
lum or electrolytic capacitors can be used for the output
APPLICATIONS INFORMATION
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