Datasheet

LTC3851-1
24
38511fa
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
IN
:
t
V
Vf
V
VkHz
ON MIN
OUT
IN MAX
()
()
.
=
()
=
()
=
18
22 250
3277ns
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
≤ =
50
6
0 0083. Ω
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
DS(ON)
= 0.035Ω/0.022Ω, C
MILLER
= 215pF. At
maximum input voltage with T (estimated) = 50°C:
P
V
V
CC
MAIN
=
()
+
()
° − °
()
⎡
⎣
⎤
⎦
18
22
5 1 0 005 50 25
0
2
.
.•
.. •
.
035 22
5
2
2 215
1
523
2
ΩΩ
()
+
()
⎛
⎝
⎜
⎞
⎠
⎟
()( )
−
+
V
A
pF
11
23
250 185
.
⎡
⎣
⎢
⎤
⎦
⎥
()
=kHz mW
APPLICATIONS INFORMATION
A short-circuit to ground will result in a folded back cur-
rent of:
I
mV
ns V
μH
SC
=
()
⎛
⎝
⎜
⎞
⎠
⎟
=
29
0 0125
1
2
90 22
33
202
.
–
.
.
Ω
AA
with a typical value of R
DS(ON)
and δ = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P
V
V
AmW
SYNC
=
()()( )
=
22
22
2 02 1 125 0 022 101 0
2
... .Ω
which is less than under full-load conditions.
C
IN
is chosen for an RMS current rating of at least 3A at
temperature. 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
(∆I
L
) = 0.02Ω (2A) = 40mV
P-P