26
LTC1628/LTC1628-PG
1628fb
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 first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQSET pin
to the INTV
CC
pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
∆I
V
fL
V
V
L
OUT OUT
IN
=
()()
–1
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 200ns
is not violated. The minimum on-time occurs at maximum
V
IN
:
t
V
Vf
V
V kHz
ns
ON MIN
OUT
IN MAX
()
()
.
()
== =
18
22 300
273
The R
SENSE
resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances:
R
mV
A
SENSE
≤≈Ω
50
584
001
.
.
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 pins specified input
current.
Rk
V
VV
K
V
VV
k
MAX
OUT
124
08
24
24
08
24 18
32
()
.
.–
.
.–.
=
=
=
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 Siliconix Si4412DY results
in; R
DS(ON)
= 0.042Ω, C
RSS
= 100pF. At maximum input
voltage with T(estimated) = 50°C:
P
V
V
CC
V A pF kHz
mW
MAIN
=
()
+°°
[]
Ω
()
+
()()( )( )
=
18
22
5 1 0 005 50 25
0 042 1 7 22 5 100 300
220
2
2
.
(. )( – )
..
A short-circuit to ground will result in a folded back current
of:
I
mV ns V
H
A
SC
=
Ω
+
µ
=
25
001
1
2
200 22
33
32
.
()
.
.
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
VV
V
A
mW
SYNC
=
()()
Ω
()
=
22 1 8
22
32 11 0042
434
2
–.
...
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 0.02Ω 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(∆IL)
= 0.02Ω(1.67A) = 33mV
P–P
APPLICATIO S I FOR ATIO
WUUU
