Datasheet

LTC3827-1
23
38271fe
APPLICATIONS INFORMATION
resistance is low and it is driven quickly. If the ratio of
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
= 1.8V, I
MAX
= 5A,
and f = 250kHz.
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 PLLLPF
pin to GND, generating 250kHz operation. The minimum
inductance for 30% ripple current is:
I
L
=
V
OUT
(f)(L)
1–
V
OUT
V
IN






A 4.7µH inductor will produce 23% ripple current and a
3.3µH will result in 33%. The peak inductor current will
be the maximum DC value plus one half the ripple cur-
rent, or 5.84A, for the 3.3µH value. Increasing the ripple
current will also help ensure that the minimum on-time
of 180ns 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 using the
maximum current sense voltage specifi cation with some
accommodation for tolerances:
R
SENSE

80mV
5.84A
 0.012
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)
= 0.035/0.022, 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)
[]
•
0.035
()
+ 22V
()
2
5A
2






4
()
215pF
()
•
1
5–
2.3
+
1
2.3






300kHz
()
= 332mW
A short-circuit to ground will result in a folded back cur-
rent of:
I
SC
=
25mV
0.01
–
1
2
120ns(22V)
3.3μH






= 2.1A
with a typical value of R
DS(ON)
and δ = (0.005/°C)(20) =
0.1. The resulting power dissipated in the bottom MOSFET
is:
P
SYNC
=
22V – 1.8V
22V
2.1A
()
2
1.125
()
0.022
()
= 100mW
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
(ΔI
L
) = 0.02(1.67A) = 33mV
P–P