Datasheet

LTC3834
22
3834fb
APPLICATIONS INFORMATION
sudden step change in output voltage if the load switch
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, 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:
ΔI
V
fL
V
V
L
OUT OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
()( )
–1
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
V
Vf
V
VkHz
n
ON MIN
OUT
IN MAX
()
()
.
()
== =
1 8
22 250
327 ss
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
≤≈Ω
80
5 84
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)
= 0.035Ω/0.022Ω, C
MILLER
= 215pF. At
maximum input voltage with T(estimated) = 50°C:
P
V
V
CC
MAIN
=
()
+°°
[]
1 8
22
5 1 0 005 50 25
00
2
.
(. )( – )•
.
335 22
5
2
4 215
1
523
1
2
2
Ω
()
+
()
⎛
⎝
⎜
⎞
⎠
⎟
Ω
()( )
+
V
A
pF •
–. .. 3
300 332
⎡
⎣
⎢
⎤
⎦
⎥
()
=kHz mW
A short-circuit to ground will result in a folded back cur-
rent of:
I
mV ns V
μH
A
SC
=
Ω
⎛
⎝
⎜
⎞
⎠
⎟
=
25
001
1
2
120 22
33
21
.
–
()
.
.
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
VV
V
A
SYNC
=
()( )
Ω
()
=
22 1 8
22
2 1 1 125 0 022
100
2
–.
...
mmW
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 0.02Ω 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
(ΔI
L
) = 0.02Ω(1.67A) = 33mV
P-P