LTC3831
15
3831fb
APPLICATIONS INFORMATION
capacitor values determine the transient recovery time.
In general, a smaller value inductor improves transient
response at the expense of ripple and inductor core satura-
tion rating. A 2µH inductor has a 0.57A/µs rise time in this
application, resulting in a 8.8µs delay in responding to a 5A
load current step. During this 8.8µs, the difference between
the inductor current and the output current is made up
by the output capacitor. This action causes a temporary
voltage droop at the output. To minimize this effect, the
inductor value should usually be in the 1µH to 5µH range for
most LTC3831 circuits. To optimize performance, different
combinations of input and output voltages and expected
loads may require different inductor values.
Once the required value is known, the inductor core type
can be chosen based on peak current and effi ciency re-
quirements. Peak current in the inductor will be equal to
the maximum output load current plus half of the peak-
to-peak inductor ripple current. Ripple current is set by
the inductor value, the input and output voltage and the
operating frequency. The ripple current is approximately
equal to:
I
RIPPLE
=
(V
IN
− V
OUT
)•(V
OUT
)
f
OSC
•L
O
•V
IN
f
OSC
= LTC3831 oscillator frequency = 200kHz
L
O
= Inductor value
Solving this equation with our typical 2.5V to 1.25V ap-
plication with 2µH inductor, we get:
(2.5V – 1.25V) • 1.25V
200kHz • 2μH • 2.5V
= 1.56A
P-P
Peak inductor current at 5A load:
5A + (1.56A/2) = 5.78A
The ripple current should generally be between 10% and
40% of the output current. The inductor must be able to
withstand this peak current without saturating, and the
copper resistance in the winding should be kept as low
as possible to minimize resistive power loss. Note that in
circuits not employing the current limit function, the cur-
rent in the inductor may rise above this maximum under
short circuit or fault conditions; the inductor should be
sized accordingly to withstand this additional current.
Inductors with gradual saturation characteristics are often
the best choice.
Input and Output Capacitors
A typical LTC3831 design places signifi cant demands on
both the input and the output capacitors. During normal
steady load operation, a buck converter like the LTC3831
draws square waves of current from the input supply at
the switching frequency. The peak current value is equal
to the output load current plus 1/2 the peak-to-peak ripple
current. Most of this current is supplied by the input bypass
capacitor. The resulting RMS current fl ow in the input ca-
pacitor heats it and causes premature capacitor failure in
extreme cases. Maximum RMS current occurs with 50%
PWM duty cycle, giving an RMS current value equal to
I
OUT
/2. A low ESR input capacitor with an adequate ripple
current rating must be used to ensure reliable operation.
Note that capacitor manufacturers’ ripple current ratings
are often based on only 2000 hours (3 months) lifetime at
rated temperature. Further derating of the input capacitor
ripple current beyond the manufacturer’s specifi cation
is recommended to extend the useful life of the circuit.
Lower operating temperature has the largest effect on
capacitor longevity.
The output capacitor in a buck converter under steady-
state conditions sees much less ripple current than the
input capacitor. Peak-to-peak current is equal to inductor
ripple current, usually 10% to 40% of the total load cur-
rent. Output capacitor duty places a premium not on power
dissipation but on ESR. During an output load transient,
the output capacitor must supply all of the additional load
current demanded by the load until the LTC3831 adjusts
the inductor current to the new value. ESR in the output
capacitor results in a step in the output voltage equal to
the ESR value multiplied by the change in load current. A
5A load step with a 0.05 ESR output capacitor results
in a 250mV output voltage shift; this is 20% of the output
voltage for a 1.25V supply! Because of the strong rela-
tionship between output capacitor ESR and output load
transient response, choose the output capacitor for ESR,
