Datasheet

LTC3851
24
3851fb
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
ON(MIN)
=
V
OUT
V
IN(MAX)
f
()
=
1.8V
22V 250kHz
()
= 327ns
The R
SENSE
resistor value can be calculated by connect-
ing I
LIM
to INTV
CC
and using the maximum current sense
voltage specifi cation with some accommodation for toler-
ances. Tie I
LIM
to INTV
CC
.
R
SENSE
≤
75mV
6A
= 0.0125Ω, so 0.01Ω is selected
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
MAIN
=
1.8V
22V
5
()
2
1+ 0.005
()
50°C − 25°C
()
⎡
⎣
⎤
⎦
•
0.035Ω
()
+ 22V
()
2
5A
2
⎛
⎝
⎜
⎞
⎠
⎟
2Ω
()
215pF
()
•
1
5 − 2.3
+
1
2.3
⎡
⎣
⎢
⎤
⎦
⎥
250kHz
()
=185mW
APPLICATIONS INFORMATION
A short-circuit to ground will result in a folded back cur-
rent of:
I
SC
=
29mV
0.0125Ω
–
1
2
90ns 22V
()
3.3µH
⎛
⎝
⎜
⎞
⎠
⎟
= 2.02A
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
SYNC
=
22V
22V
2.02A
()
2
1.125
()
0.022Ω
()
=101.0mW
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