LTC3707-SYNC
25
3707sfa
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 the INTV
CC
pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
ΔI
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
VkHz
n
ON MIN
OUT
IN MAX
()
()
.
()
== =
18
22 300
273 ss
The R
SENSE
resistor value can be calculated by using the
maximum current sense voltage specifi cation with some
accommodation for tolerances:
R
mV
A
SENSE
≤≈Ω
60
584
001
.
.
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.
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.
APPLICATIONS INFORMATION
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Fairchild FOS6982S results
in; R
DS(ON)
= 0.035Ω/0.022Ω, calculated C
MILLER
is
3nC/15V = 200pF. At maximum input voltage with
T(estimated) = 50°C:
P
V
V
CC
MAIN
=
()
+°°
[]
18
22
5 1 0 005 50 25 0 03
2
.
(. )( – ) .
55
1
2
22 5 4 200
1
52
1
2
3
2
Ω
()
+
()()
Ω
()( )
+
⎛
⎝
⎜
⎞
⎠
⎟
VA pF
–
000
323
kHz
mW
()
=
A short-circuit to ground will result in a folded back
current of:
I
mV ns V
µH
A
SC
=
Ω
+
⎛
⎝
⎜
⎞
⎠
⎟
=
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 0022
227
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
(ΔI
L
) = 0.02Ω(1.67A) = 33mV
P–P
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the IC. These items are also illustrated graphically in the
layout diagram of Figure 10. The Figure 11 illustrates the
current waveforms present in the various branches of the
2-phase synchronous regulators operating in the continu-
ous mode. Check the following in your layout:
