LTC1430
APPLICATIO S I FOR ATIO
Once the threshold voltage has been selected, R
ON
should
be chosen based on input and output voltage, allowable
power dissipation and maximum required output current.
In a typical LTC1430 buck converter circuit operating in
continuous mode, the average inductor current is equal to
the output load current. This current is always flowing
through either M1 or M2 with the power dissipation split
up according to the duty cycle:
V
OUT
V
IN
V
DC (M2)
=
1
−
OUT
V
IN
(
V
IN
−
V
OUT
)
=
V
IN
DC (M1)
=
The R
ON
required for a given conduction loss can now be
calculated by rearranging the relation P = I
2
R:
R
ON
(M1)
=
=
P
MAX
(M1)
DC (M1)
•
I
MAX2
V
IN
•
P
MAX
(M1)
V
OUT
•
I
MAX2
R
ON
(M2)
=
=
P
MAX
(M2)
DC (M2)
•
I
MAX2
V
IN
•
P
MAX
(M2)
(
V
IN
−
V
OUT
)
•
I
MAX2
P
MAX
should be calculated based primarily on required
efficiency. A typical high efficiency circuit designed for 5V
in, 3.3V at 10A out might require no more than 3%
efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
P
MAX
value of (3.3V • 10A/0.9) • 0.03 = 1.1W per FET and
a required R
ON
of:
R
ON
(M1)
=
5V
•
1.1W
=
0.017
Ω
2
3.3V
•
10A
5V
•
1.1W
R
ON
(M2)
=
=
0.032
Ω
(
5V
−
3.3V
)
•
10A
2
8
U
Note that the required R
ON
for M2 is roughly twice that of
M1 in this example. This application might specify a single
0.03Ω device for M2 and parallel two more of the same
devices to form M1. Note also that while the required R
ON
values suggest large MOSFETs, the dissipation numbers
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 LTC1430 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 LTC1430
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 LTC1430. In a typical 5V to
3.3V application, the maximum rise time will be:
90%
•
W
U
U
(
V
IN
−
V
OUT
)
L
AMPS
1.53A I
=
SECOND
µ
s 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 3.xV LTC1430 circuits.
Different combinations of input and output voltages 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
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