LTC1430
APPLICATIO S I FOR ATIO
inductor value, the input and output voltage and the
operating frequency. If the efficiency is high and can be
approximately equal to 1, the ripple current is approxi-
mately equal to:
∆
I
=
(
V
IN
−
V
OUT
)
•
DC
f
OSC
•
L
V
DC
=
OUT
V
IN
f
OSC
= LTC1430 oscillator frequency
L = inductor value
Solving this equation with our typical 5V to 3.3V applica-
tion, we get:
1.7
•
0.66
=
2.8A
P
−
P
200kHz
•
2
µ
H
Peak inductor current at 10A load:
10A
+
2.8A
=
11.4A
2
The inductor core must be adequate 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 the current may rise above
this maximum level in circuits under current limit or under
fault conditions in unlimited circuits; the inductor should
be sized to withstand this additional current.
Input and Output Capacitors
A typical LTC1430 design puts significant demands on
both the input and output capacitors. Under normal steady
load operation, a buck converter like the LTC1430 draws
square waves of current from the input supply at the
switching frequency, with the peak value equal to the
output current and the minimum value near zero. Most of
this current must come from the input bypass capacitor,
since few raw supplies can provide the current slew rate to
feed such a load directly. The resulting RMS current flow
in the input capacitor will heat it up, causing premature
capacitor failure in extreme cases. Maximum RMS current
occurs with 50% PWM duty cycle, giving an RMS current
U
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; further derating of the input capacitor
ripple current beyond the manufacturer’s specification is
recommended to extend the useful life of the circuit.
The output capacitor in a buck converter sees much less
ripple current under steady-state conditions than the input
capacitor. Peak-to-peak current is equal to that in the
inductor, usually a fraction of the total load current. 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 LTC1430 can adjust 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 0.05Ω ESR output capacitor will result in
a 250mV output voltage shift; this is a 7.6% output voltage
shift for a 3.3V supply! Because of the strong relationship
between output capacitor ESR and output load transient
response, the output capacitor is usually chosen for ESR,
not for capacitance value; a capacitor with suitable ESR
will usually have a larger capacitance value than is needed
to control steady-state output ripple.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1430 applications. OS-CON
electrolytic capacitors from Sanyo give excellent perfor-
mance and have a very high performance/size ratio for an
electrolytic capacitor. Surface mount applications can use
either electrolytic or dry tantalum capacitors. Tantalum
capacitors must be surge tested and specified for use in
switching power supplies; low cost, generic tantalums are
known to have very short lives followed by explosive
deaths in switching power supply applications. AVX TPS
series surface mount devices are popular tantalum capaci-
tors that work well in LTC1430 applications. A common
way to lower ESR and raise ripple current capability is to
parallel several capacitors. A typical LTC1430 application
might require an input capacitor with a 5A ripple current
capacity and 2% output shift with a 10A output load step,
which requires a 0.007Ω output capacitor ESR. Sanyo
W
U
U
9
LINER [ LINEAR TECHNOLOGY ]
