11
LTC1430A
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
are only 1.1W per device or less— large TO-220 packages
and heat sinks are not necessarily required in high effi-
ciency applications. Siliconix Si4410DY (in SO-8) and
Motorola MTD20N03HL (in DPAK) are two small, surface
mount devices with R
ON
values of 0.03Ω or below with 5V
of gate drive; both work well in LTC1430A circuits with up
to 10A output current. A higher P
MAX
value will generally
decrease MOSFET cost and circuit efficiency and increase
MOSFET heat sink requirements.
Inductor
The inductor is often the largest component in an LTC1430A
design and should be chosen carefully. Inductor value and
type should be chosen based on output slew rate require-
ments and expected peak current. Inductor value is prima-
rily controlled by the required current slew rate. The
maximum rate of rise of the current in the inductor is set
by its value, the input-to-output voltage differential and the
maximum duty cycle of the LTC1430A. In a typical 5V to
3.3V application, the maximum rise time will be:
90% =
(V
IN
– V
OUT
)
L
AMPS
SECOND
1.53A
µs
I
L
where L is the inductor value in µH. A 2µH inductor would
have a 0.76A/µs rise time in this application, resulting in a
6.5µs delay in responding to a 5A load current step. During
this 6.5µs, the difference between the inductor current and
the output current must be made up by the output capaci-
tor, causing a temporary droop at the output. To minimize
this effect, the inductor value should usually be in the 1µH
to 5µH range for most typical 5V to 2.xV-3.xV LTC1430A
circuits. Different combinations of input and output volt-
ages and expected loads may require different 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 added to half the peak-
to- peak inductor ripple current. Ripple current is set by the
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 = DC
(V
IN
–
V
OUT
)
(f
OSC
)(L)
DC =
V
OUT
V
IN
f
OSC
= LTC1430A oscillator frequency
L = inductor value
Solving this equation with our typical 5V to 3.3V applica-
tion, we get:
= 2.8A
P–P
(1.7)(0.66)
(200kHz)(2µH)
Peak inductor current at 10A load:
= 11.4A10A +
2.8A
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 LTC1430A design puts significant demands on
both the input and output capacitors. Under normal steady
load operation, a buck converter like the LTC1430A 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
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.
