Datasheet

LTC3868-1
26
38681fd
APPLICATIONS INFORMATION
C
LOAD
to C
OUT
is greater than 1:50, the switch rise time
should be controlled so that the load rise time is limited
to approximately 25 • C
LOAD
. Thus a 10µF capacitor would
require a 250µs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example for one channel, assume V
IN
=
12V(nominal), V
IN
= 22V (max), V
OUT
= 3.3V, I
MAX
= 6A,
V
SENSE(MAX)
= 50mV and f = 350kHz.
The inductance value is chosen fi rst based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
ΔI
L(NOM)
=
V
OUT
f
()
L
()
1–
V
OUT
V
IN(NOM)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
A 3.9µH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 6.88A. Increasing the ripple
current will also help ensure that the minimum on-time
of 95ns is not violated. The minimum on-time occurs at
maximum V
IN
:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
f
()
=
3.3V
22V 350kHz
()
= 429ns
The equivalent R
SENSE
resistor value can be calculated by
using the minimum value for the maximum current sense
threshold (43mV):
R
SENSE
≤
43mV
6.88A
= 0.006Ω
Choosing 1% resistors: R
A
= 25k and R
B
= 78.1k yields
an output voltage of 3.299V.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
DS(ON)
= 0.035/0.022, C
MILLER
= 215pF. At
maximum input voltage with T(estimated) = 50°C:
P
MAIN
=
3.3V
22V
6A
()
2
1+ 0.005
()
50°C–25°C
()
⎡
⎣
⎤
⎦
0.035Ω
()
+ 22V
()
2
6A
2
2.5Ω
()
215pF
()
•
1
5V – 2.3V
+
1
2.3V
⎡
⎣
⎢
⎤
⎦
⎥
350kHz
()
= 433mW
A short-circuit to ground will result in a folded back cur-
rent of:
I
SC
=
25mV
0.006Ω
–
1
2
95ns 22V
()
3.9µH
⎛
⎝
⎜
⎞
⎠
⎟
= 3.9A
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
= 3.9A
()
2
1.125
()
0.022Ω
()
= 376mW
which is less than under full-load conditions.
C
IN
is chosen for an RMS current rating of at least 3A at
temperature assuming only this channel is on. C
OUT
is
chosen with an ESR of 0.02 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
) = 0.02(1.75A) = 35mV
P-P