LTC3851A-1
25
3851a1fa
Design Example
As a design example, assume V
IN
= 12V (nominal), V
IN
=
22V (maximum), V
OUT
= 1.8V, I
MAX
= 5A, and f = 250kHz
(refer to Figure 12).
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. Connect a
160k resistor between the FREQ/PLLFLTR and GND pins,
generating 250kHz op eration. The minimum inductance
for 30% ripple current is:
∆I
L
=
1
f
( )
L
( )
V
OUT
1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
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
ON(MIN)
=
OUT
V
IN(MAX)
f
( )
=
22V 250kHz
( )
= 327ns
The R
SENSE
resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances.
R
SENSE
≤
50mV
= 0.0083Ω
Choosing 1% resistors: R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
applicaTions inForMaTion
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
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
=
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
