Datasheet

LTC3728
28
3728fg
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 using the INTV
CC
pin generating
1V for 300kHz operation. The minimum inductance for
30% ripple current is:
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 cur-
rent, 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
5.84A
 0.01
Since the output voltage is below 2.4V the output resis-
tive divider will need to be sized to not only set the output
voltage but also to absorb the SENSE pins 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 topside MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in R
DS(ON)
= 0.042, C
RSS
= 100pF. At maximum input
voltage with T(estimated) = 50°C:
P
MAIN
=
1.8V
22V
5
()
2
1+ (0.005)(50°C–25°C)
[]
0.042
()
+ 1.7 22V
()
2
5A
()
100pF
()
300kHz
(
= 220mW
A short-circuit to ground will result in a folded back cur-
rent of:
I
SC
=
25mV
0.01
+
1
2
200ns(22V)
3.3μH






= 3.2A
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
3.2A
()
2
1.1
()
0.042
()
= 434mW
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
maximum input voltage. The output voltage ripple due to
ESR is approximately:
V
ORIPPLE
= R
ESR
(∆I
L
) = 0.02(1.67A) = 33mV
P–P