ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 24

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LTC3851-1
A 4.7μH inductor will produce 28% ripple current and
a 3.3μH will result in 40%. The peak inductor current
will be the maximum DC value plus one-half the ripple
current, or 6A, for the 3.3μH value. Increasing the ripple
current will also help ensure that the minimum on-time
of 90ns is not violated. The minimum on-time occurs at
maximum V
The R
maximum current sense voltage specifi cation 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 topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
maximum input voltage with T (estimated) = 50°C:
APPLICATIONS INFORMATION
24
R
t
P
ON MIN
SENSE
MAIN
SENSE
(
=
)
≤
DS(ON)
=
(
resistor value can be calculated by using the
⎡
⎢
⎣
IN
1 8
0
22
5 2 3
50
.
:
. .
V
035
−
6
V
IN MAX
V
mV
1
A
V
(
( )
.
OUT
= 0.035Ω/0.022Ω, C
5
Ω
=
2
)
+
+
)
⎡ ⎣
0 0083
( )
1 0 005 50
2 3
.
(
f
1 1
+
.
22
=
(
⎤
⎥
⎦
V
(
22 250
.
250
)
2
Ω
⎛
⎝ ⎜
V
5
kHz
1 8
(
)
2
A
(
.
⎞
⎠ ⎟
)
V
( )(
° −
=
kHz
2
C
MILLER
Ω
185
)
25
215
=
mW
°
327 7 ns
= 215pF . At
C
pF
)
⎤ ⎦
)
•
•
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than under full-load conditions.
C
temperature. C
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
SC
is chosen for an RMS current rating of at least 3A at
ORIPPLE
SYNC
=
0 0125
=
.
29
22
22
= R
mV
V
V
OUT
ESR
Ω
(
2 02
.
–
is chosen with an ESR of 0.02Ω for
(ΔI
2
1
A
L
⎛
⎝ ⎜
) (
) = 0.02Ω (2A) = 40mV
DS(ON)
90
2
1 125 0 022
3 3
ns
.
.
(
μH
22
and δ = (0.005/°C)(25°C)
)(
V
)
⎞
⎠ ⎟
.
=
2 02
Ω
.
)
=
A A
101 0
P-P
.
mW
38511f