LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 25

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250μs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example for one channel, assume V
12V(nominal), V
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 FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
A 4.7μH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 6.87A. 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
The R
maximum current sense voltage specifi cation with some
accommodation for tolerances:
Choosing 0.5% resistors: R
an output voltage of 3.296V.
APPLICATIONS INFORMATION
LOAD
SENSE(MAX)
R
ΔI
t
ON MIN
SENSE
L
SENSE
(
to C
=
( )( )
V
)
f L
OUT
≤
OUT
=
resistor value can be calculated by using the
IN
= 50mV and f = 350kHz.
6 87
50
:
V
.
V
is greater than 1:50, the switch rise time
IN
OUT
⎛
⎝ ⎜
mV
IN
1–
( )
A
f
= 22V (max), V
=
V
=
V
OUT
LOAD
0 007
IN
22 350
.
V
⎞
⎠ ⎟
A
. Thus a 10μF capacitor would
(
3 3
= 24.9k and R
Ω
.
V
kHz
OUT
)
= 3.3V, I
=
429
B
= 77.7k yields
ns
MAX
= 6A,
IN
=
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:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02Ω for low output ripple volt-
age. 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
is chosen for an RMS current rating of at least 3A at
MAIN
SYNC
ORIPPLE
=
30
=
=
=
(
0 007
⎡
⎢
⎣
%
76
DS(ON)
3 3
22
22
= R
0
.
5
.
. .
(
V
m
035
V
V
50
V
ESR
22
– .
W W
– .
( )
1
mV
Ω
6
2 3
= 0.035Ω/0.022Ω, C
V
Ω
3 3
(ΔI
A
)
)
V
+
2
V
L
–
) = 0.02Ω(1.73A) = 35mV
⎡
⎣
(
+
DS(ON)
(
1
22
2
1
1 9
+
2 3
.
⎛
⎜
⎝
.
V
(
1
90
A
0 005 50
)
V V
2
) (
.
and δ = (0.005/°C)(25°C)
ns
4 7
2
⎤
⎥
⎦
6
.
2
(
A
1 125 0 022
350
(
μH
22
.
LTC3858-1
( )(
)
2
(
V
Ω
kHz
MILLER
)
)(
°
⎞
⎟ =
⎠
C
215
)
–
=
.
1 1 9 . A
25
614
= 215pF . At
pF
°
Ω
)
C
25
P-P
mW
•
)
)
OUT
⎤
⎦
38581f
is