LTC3738 Linear Technology, LTC3738 Datasheet - Page 25

• Manufacturer Part Number: LTC3738
• Description: 3-Phase Buck Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If C
than 2% of C
so that the load rise time is limited to approximately
1000 • R
R
the charging current to about 1A.
Design Example (Using Three Phases)
As a design example, assume V
20V(max), V
AVP slope is 1mV/A. The inductance value is chosen first
based upon a 30% ripple current assumption. The highest
value of ripple current in each output stage occurs at the
maximum input voltage.
Using L = 0.6µH, a commonly available value results in
34% ripple current. The worst-case output ripple for the
three stages operating in parallel will be less than 11% of
the peak output current.
R
a conservative maximum sense current threshold of 65mV
and taking into account half of the ripple current:
Use a commonly available 0.003Ω sense resistor.
Take R
SENSE
SENSE1,
R
R
L
PREAVP
SENSE
=
=
≥
AVP
0 68
resistor would require a 500µs rise time, limiting
(
V
f I
SENSE
400
( )
.
R
OUT
∆
SENSE2
as recommended value 100Ω, the R
=
µ
OUT
=
OUT
kHz
15 1
⎛
⎜
⎝
H
• C
0 003
1
.
A
, the switch rise time should be controlled
−
1 3
)(
LOAD
= 1.3V, I
.
and R
⎛
⎜
⎝
65
30
V
U
V
OUT
V
+
mV
Ω
IN
%
. Thus a 250µF capacitor and a 2mΩ
34
1
)(
100
SENSE3
⎞
⎟
⎠
mV A
2
15
%
U
MAX
Ω
A
⎞
⎟
⎠
/
)
=
= 45A, f = 400kHz and the
⎛
⎜
⎝
can be calculated by using
=
1
0 0037
IN
300
.
−
= 12V(nominal), V
W
1 3
20
.
Ω
V
V
Ω
⎞
⎟
⎠
LOAD
U
PREAVP
is greater
IN
is:
=
Next verify the minimum on-time is not violated. The
minimum on-time occurs at maximum V
The output voltage will be set by the VID code according
to Table 1.
The power dissipation on the topside MOSFET can be
estimated. Using a Siliconix Si7390DP for example, R
= 13.5mΩ, C
input voltage with T(estimated) = 50°C:
using a Siliconix Si7356DP as bottom side MOSFET.
The worst-case power dissipation by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is:
A short circuit to ground will result in a folded back current
of:
with a typical value of R
0.25. The resulting power dissipated in the bottom MOSFET
is:
which is less than one third of the normal, full load
conditions. Incidentally, since the load no longer dissi-
pates any power, total system power is decreased by over
90%. Therefore, the system actually cools significantly
during a shorted condition!
P
t
P
I
P
ON MIN
SYNC
SC
MAIN
SYNC
(
≈
=
(
≈
= (7.5A)
2 3
)
0 0135
20
25
=
⎛
⎜
⎝
+
1 3
20
.
5
.
MILLER
mV
V
V
V
V
20
)
IN MAX
V
m
−
– .
(
V
( )
1
V
Ω
2
1 3
OUT
15 1
1 8
Ω
(1.25)(0.004Ω) ≈ 0.28W
.
= 2.1nC/15V = 140pF. At maximum
+
+
V
)
2
V
( )
[
2
1
( )
DS(ON)
(
f
+
20
15
⎛
⎜
⎝
+
=
150
1 8
(
A
.
2
0 005 50
1
20 400
) (
⎛
⎜
⎝
0 6
.
2
V
ns
and d = (0.005/°C)(50°C) =
( )( )
.
V
45
2 3
⎞
⎟
⎠
1 25 0 004
(
µ
(
(
. 1 3
.
20
400
)
H
A
(
V
V
)
⎞
⎟ Ω
⎠
(
kHz
kHz
)
( )(
° −
⎞
⎟ =
⎠
2
C
.
CC
LTC3738
)
)
7 5
=
25
=
:
140
Ω
.
162
0 51
°
)
A
.
C
=
pF
)
ns
25
]
W
1 05
DS(ON)
)
.
3738f
W