LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 25

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Design Example
As a design example for one channel, assume V
12V(nominal), V
and f = 300kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR pin
to a resistive divider from the INTV
for 300kHz operation. The minimum inductance for 30%
ripple current is:
A 4.7 H inductor will produce 23% ripple current and a
3.3 H will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3 H value. Increasing the ripple current
will also help ensure that the minimum on-time of 100ns
is not violated. The minimum on-time occurs at maximum
V
The R
maximum current sense voltage specification with some
accommodation for tolerances:
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pin’s specified input
current.
IN
R
R
t
:
ON MIN
SENSE
1
I
L
(
SENSE
MAX
(
( )( )
V
)
f L
)
OUT
resistor value can be calculated by using the
5 84
60
24
24
V
.
IN MAX
mV
k
k
V
IN
– 1
(
OUT
A
2 4
2 4
= 22V(max), V
U
V
.
.
V
OUT
)
IN
0 01
V V
V
0 8
f
0 8
.
.
.
–
– .
U
V
V
1 8
22 300
OUT
V
V
1 8
(
.
OUT
CC
V
W
32
kHz
pin, generating 0.7V
= 1.8V, I
k
)
273
U
MAX
ns
= 5A,
IN
=
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Fairchild FDS6982S dual
MOSFET results in: R
215pF. At maximum input voltage with T(estimated) =
50 C:
A short-circuit to ground will result in a folded back current
of:
with a typical value of R
The resulting power dissipated in the bottom MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02 for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
IN
V
I
P
P
SC
0 035
is chosen for an RMS current rating of at least 3A at
MAIN
SYNC
ORIPPLE
5 2 3
.
– .
1
0 01
25
.
mV
1 8
100
22
22
= R
.
LTC3728L/LTC3728LX
2 3
V
V
V
1
.
ESR
22
mW
–
22
– .
5 1 0 005 50
2
1
V
1 8
V
( I
300
2
DS(ON)
2
120
V
DS(ON)
L
kHz
) = 0.02 (1.67A) = 33mV
5
3 3
2
2 1
ns
( .
A
.
.
(
= 0.035 /0.022 , C
A
22
and = (0.005/ C)(20) = 0.1.
H
4
2
332
V
)(
1 125 0 022
)
w w w . D a t a S h e e t 4 U . c
.
215
mW
C
2 1
pF
.
–
A
25
.
•
C
) •
MILLER
25
P–P
OUT
3728lxfa
is
=