LTC3728L-1
25
3728l1fc
APPLICATIONS INFORMATION
Design Example
As a design example for one channel, assume V
IN
=
12V(nominal), V
IN
= 22V(max), V
OUT
= 1.8V, I
MAX
= 5A,
and f = 300kHz.
The inductance value is chosen fi rst 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
CC
pin, generating
0.7V for 300kHz operation. The minimum inductance for
30% ripple current is:
L
V
IN
–V
OUT
(f)(I
RIPPLE
)
•
V
OUT
V
IN
or 3.7µH. Using standard inductor values:
I
L
=
V
OUT
(f)(L)
1–
V
OUT
V
IN
A 4.7µH inductor will produce 23% ripple current and a
3.3µH 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 3.3µH 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
IN
:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
f
=
1.8V
22V(300kHz)
= 273ns
The R
SENSE
resistor value can be calculated by using the
maximum current sense voltage specifi cation with some
accommodation for tolerances:
R
SENSE
60mV
5.84
A
0.01
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 specifi ed
input current.
R1
(MAX)
= 24k
0.8V
2.4V – V
OUT
= 24k
0.8V
2.4V – 1.8
V
= 32k
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
DS(ON)
= 0.035/0.022, C
MILLER
= 215pF. At
maximum input voltage with T(estimated) = 50°C:
P
MAIN
=
1.8V
22V
5
()
2
1+ (0.005)(50°C–25°C)
[]
•
0.035
()
+ 22V
()
2
5A
2
4
()
215pF
()
•
1
5–
2.3
+
1
2.3
300kHz
()
= 332mW
A short-circuit to ground will result in a folded back
current of:
I
SC
=
25mV
0.01
–
1
2
120ns(22V)
3.3μH
= 2.1A
with a typical value of R
DS(ON)
and δ = (0.005/°C)(20) = 0.1.
The resulting power dissipated in the bottom MOSFET is:
P
SYNC
=
22V – 1.8V
22V
2.1A
()
2
1.125
()
0.022
()
= 100mW
which is less than under full-load conditions.
C
IN
is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. C
OUT
is
chosen with an ESR of 0.02 for low output ripple. The
output ripple in continuous mode will be highest at the
