LTC1430 LINER [Linear Technology], LTC1430 Datasheet - Page 8

• Manufacturer Part Number: LTC1430
• Description: High Power Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1430
Once the threshold voltage has been selected, R
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:
The R
calculated by rearranging the relation P = I
P
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
a required R
8
MAX
MAX
DC M
R
R
R
R
DC M
ON
ON
ON
ON
ON
value of (3.3V • 10A/0.9) • 0.03 = 1.1W per FET and
should be calculated based primarily on required
(
( )
( )
(
( )
(
M
M
M
M
required for a given conduction loss can now be
2
1
1
2
1
2
)
)
)
ON
1
V
DC M
V
3 3
V
DC M
V
OUT
V
of:
IN
5
V
.
5
IN
IN
OUT
IN
V
V
V
P
P
V
( )
V
V
(
OUT
V
MAX
IN
MAX
U
IN
P
5
IN
V
1
1 1
V
MAX
2
3 3
10
V
OUT
.
)
I
.
OUT
P
( )
MAX
(
W
M
I
A
M
MAX
1 1
V
I
MAX
( )
MAX
2
1
U
.
M
2
)
2
W
1
(
10
2
M
2
I
0 017
MAX
2
.
A
)
2
2
W
0 032
.
2
R:
ON
U
should
Note that the required R
M1 in this example. This application might specify a single
0.03 device for M2 and parallel two more of the same
devices to form M1. Note also that while the required R
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
of gate drive; both work well in LTC1430 circuits with up
to 10A output current. A higher P
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:
where L is the inductor value in H. A 2 H inductor would
have a 0.76A/ s rise time in this application, resulting in a
6.5 s delay in responding to a 5A load current step. During
this 6.5 s, 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 1 H
to 5 H 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
90
%
V
IN
L
V
OUT
ON
SECOND
ON
values of 0.03 or below with 5V
AMPS
for M2 is roughly twice that of
MAX
1 53
.
value will generally
s
A
L
I
ON