Datasheet

LT8640/LT8640-1
20
Rev.C
For more information www.analog.com
APPLICATIONS INFORMATION
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. For applications
requiring smaller load currents, the value of the inductor
may be lower and the LT8640/LT8640-1 may operate
with higher ripple 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.
For duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), a
minimum inductance is required to avoid subharmonic
oscillation (See Equation 10). See Application Note 19
for more details.
L
MIN
=
V
IN
(2
•
DC
−
1)
3.5• f
SW
(10)
where DC is the duty cycle ratio (V
OUT
/V
IN
) and f
SW
is the
switching frequency.
Input Capacitors
The V
IN
of the LT8640/LT8640-1 should be bypassed with
at least three ceramic capacitors for best performance. Tw o
small ceramic capacitors of 1µF should be placed close to
the part; one at the V
IN1
/GND1 pins and a second at V
IN2
/
GND2 pins. These capacitors should be 0402 or 0603 in
size. For automotive applications requiring 2 series input
capacitors, two small 0402 or 0603 may be placed at
each side of the LT8640/LT8640-1 near the V
IN1
/GND1
and V
IN2
/GND2 pins.
A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to V
IN1
or V
IN2
. See layout section for more
detail. X7R or X5R capacitors are recommended for best per
-
formance across temperature and input voltage variations.
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 ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT8640/LT8640-1 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8640/LT8640-1’s voltage
rating. This situation is easily avoided (see Linear Technol
-
ogy 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 LT8640/LT8640-1 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 LT8640/LT8640-1’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.
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