LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 21

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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Design Example
As a design example, assume V
22V(max), V
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:
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
The R
maximum current sense voltage specifi cation with some
accommodation for tolerances:
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
maximum input voltage with T(estimated) = 50°C:
APPLICATIONS INFORMATION
R
ΔI
t
ON MIN
SENSE
L
SENSE
(
=
( )( )
V
)
f L
≤
OUT
DS(ON)
=
resistor value can be calculated by using the
IN
OUT
5 84
80
:
V
.
IN MAX
⎛
⎝ ⎜
V
mV
= 1.8V, I
1–
(
OUT
A
= 0.035Ω/0.022Ω, C
≈
V
V
)
OUT
f
0 012
IN
.
=
MAX
⎞
⎠ ⎟
22 250
Ω
= 5A and f = 250kHz.
V
(
1 8
IN
.
= 12V(nominal), V
V
kHz
MILLER
)
=
327 s s
= 215pF . At
n
IN
=
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
The resulting power dissipated in the bottom MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only this channel is on. C
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:
IN
V
I
P
(
P
⎡
⎢
⎣
SC
0
is chosen for an RMS current rating of at least 3A at
SYNC
ORIPPLE
MAIN
5 2 3
. .
035
– .
=
1
0 01
25
=
=
=
Ω
.
mV
1 8
100
22
22
+
= R
)
.
Ω
+
2 3
V
V
V
(
1 1
ESR
.
22
m m W
–
22
– .
( )
5
⎤
⎥
⎦
2
1
V
1 8
V
(
(ΔI
⎛
⎝ ⎜
2
300
)
120
2
⎡
⎣
DS(ON)
V
L
1
⎛
⎝ ⎜
) = 0.02Ω(1.67A) = 33mV
(
kHz
+
3 3
5
2 1
ns
2
.
A
(
.
0 005 50
(
⎞
⎠ ⎟
μH
A
)
and δ = (0.005/°C)(20) = 0.1.
.
22
( )(
=
) (
4
2
V
Ω
332
)
1 125 0 022
)
⎞
⎠ ⎟
.
(
LTC3835-1
mW
215
=
2 1
°
C
)(
.
pF
–
A
)
.
25
•
°
C
Ω
)
21
⎤
⎦
)
P-P
OUT
•
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is