15
LTC3830/LTC3830-1
3830fa
where L
O
is the inductor value in µH. With proper fre-
quency compensation, the combination of the inductor
and output capacitor values determine the transient recov-
ery time. In general, a smaller value inductor improves
transient response at the expense of ripple and inductor
core saturation rating. A 2µH inductor has a 0.81A/µs rise
time in this application, resulting in a 6.2µs delay in
responding to a 5A load current step. During this 6.2µs,
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 5V input LTC3830
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 efficiency
requirements. 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
VV V
fLV
RIPPLE
IN OUT OUT
OSC O IN
=
−()•()
••
f
OSC
= LTC3830 oscillator frequency = 200kHz
L
O
= Inductor value
Solving this equation with our typical 5V to 3.3V applica-
tion with a 2µH inductor, we get:
(–.)•.
••
.
53333
200 2 5
28
VVV
kHz H V
A
P
µ
=
-P
Peak inductor current at 10A load:
10A + (2.8A/2) = 11.4A
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
current 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 LTC3830 design places significant demands on
both the input and the output capacitors. During normal
steady load operation, a buck converter like the LTC3830
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 cur-
rent. Most of this current is supplied by the input bypass
capacitor. The resulting RMS current flow in the input
capacitor 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 cur-
rent 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 speci-
fication is recommended to extend the useful life of the
circuit. Lower operating temperature has the largest effect
on capacitor longevity.
APPLICATIO S I FOR ATIO
WUUU
