ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 23

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Design Example
As a design example for one channel, assume V
12V(nominal), V
and f = 250kHz.
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 PLLLPF pin
to GND, generating 250kHz operation. The minimum
inductance for 30% ripple current is:
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 180ns
is not violated. The minimum on-time occurs at maxi-
mum V
The R
maximum current sense voltage specification with some
accommodation for tolerances:
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
R
ΔI
t
ON MIN
SENSE
L
SENSE
(
IN
=
:
( )( )
V
f L
)
OUT
≤
resistor value can be calculated by using the
=
5 84
80
V
.
⎛
⎝ ⎜
IN MAX
IN
V
mV
1
(
OUT
–
= 22V(max), V
A
U
V
≈
V
OUT
)
IN
f
0 012
.
=
U
⎞
⎠ ⎟
22 250
Ω
V
(
1 8
.
OUT
W
V
kHz
= 1.8V, I
)
=
327 s s
U
MAX
n
= 5A,
IN
=
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 cur-
rent 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 0.02Ω 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:
IN
V
I
P
(
P
⎡
⎢
⎣
SC
0 0
is chosen for an RMS current rating of at least 3A at
MAIN
SYNC
ORIPPLE
5 2 3
. 3 3 5
– .
=
1
0 01
25
=
=
=
Ω
.
mV
= R
1 8
100
+
22
22
)
.
+
Ω
2
V
V
ESR
V
(
1
. . 3
22
m m W
–
22
– .
( )
⎤
⎥
⎦
5
2
1
V
1 8
(ΔI
V
(
⎛
⎝ ⎜
DS(ON)
300
2
)
120
DS(ON)
2
[
L
V
1
) = 0.02Ω(1.67A) = 33mV
⎛
⎝ ⎜
+
(
kHz
3 3
5
2 1
2
ns
.
( .
A
.
= 0.035Ω/0.022Ω, C
0 005 50
⎞
⎠ ⎟
μ
and δ = (0.005/°C)(20) = 0.1.
(
)
A
22
( )(
H
=
) (
4
2
Ω
V
332
)(
)
1 125 0 022
⎞
⎠ ⎟
LTC3827-1
.
215
mW
=
°
2 1
C
pF •
.
)
–
(
A
)
25
.
°
C
MILLER
23
) •
Ω
P–P
]
OUT
38271fd
)
is
=