LT3971A/LT3971A-5
14
3971af
Where f
SW
is the switching frequency of the LT3971A, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3971A will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (I
OUT(MAX)
)
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. If your load is lower
than the maximum load current, than you can relax 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. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
the input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximum load current. For details of maximum output cur-
rent and discontinuous operation, see Linear Technology’s
Application Note 44. Finally, for duty cycles greater than
50% (V
OUT
/V
IN
>0.5), a minimum inductance is required to
avoid sub-harmonic oscillations. See Application Note 19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the LT3971A will
be able to deliver the required output current. Note again
that these equations assume that the inductor current is
continuous. Discontinuous operation occurs when I
OUT
is less than ∆I
L
/2.
Input Capacitor
Bypass the input of the LT3971A circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 4.7F to 10F ceramic capacitor
is adequate to bypass the LT3971A and will easily handle
the ripple current. Note that larger input capacitance is
required when a lower switching frequency is used (due
to longer on-times). 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 LT3971A and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7F capacitor is capable of this task, but only if it is
placed close to the LT3971A (see the PCB Layout sec-
tion). A second precaution regarding the ceramic input
capacitor concerns the maximum input voltage rating of
the LT3971A. A ceramic input capacitor combined with
trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3971A circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT3971A’s voltage
rating. This situation is easily avoided (see the Hot Plug-
ging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT3971A to produce the DC output. In this role it
determines the output ripple, so low impedance (at the
switching frequency) is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT3971A’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and pro-
vide 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 F. 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. 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.
APPLICATIONS INFORMATION
