LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 23

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Third, design the inductors for about 40% ripple current
at the maximum V
A standard 1μH inductor will result in 45% of ripple current
(4.5A) at worst case.
L2 can also use 1μH to save some BOM (Bill of Material)
cost; the resulting ripple current is 3.4A.
The selection of MOSFETs is simplifi ed by the fact that both
channels have the same maximum output current. Select
the top and bottom MOSFETs for one channel and the
same MOSFETs can be used for the other. Take channel 1
for calculation and begin with the bottom synchronous
MOSFET. As stated previously in the Power MOSFET Se-
lection section, the major criterion in selecting the bottom
MOSFET is low R
R
The nominal sense voltage is:
Tying V
range for a nominal value of 110mV with the current
limit occurring at 146mV. To check if the current limit is
acceptable, assume a junction temperature of about 80°C
above a 70°C ambient with ρ
and double check the assumed T
Because the top MOSFET is on for only a short time,
an Si4884 will be suffi cient: R
DS(ON)
V
T
L
L
I
P
LIMIT
J
SNS(NOM)
BOT
1
2
= 70°C + (1.90W)(40°C/W) = 146°
=
=
RNG1
(
(
= 0.0083Ω (nom) 0.010Ω (max), θ
=
500
≥
500
28
( )
1 5 0 010
kHz
kHz
to 1.1V will set the current sense voltage
V
= (10A)(1.3)(0.0083) = 108mV
.
28
146
– .
2 5
1 8
(
)( )(
V
.
)( )(
DS(ON)
2 5
.
IN
0 4 10
.
0 4 10
mV
V
V
.
:
.
V
(
Ω
. Choose an Si4874 for example:
11 8
)
A
.
A
+
)
)
A
150°C
⎛
⎜
⎝
2
1
⎛
⎜
⎝
) ( )
1
1
(
2
DS(ON)
–
4 1
–
J
1 5 0 010
.
2 5
28
1 8
.
28
in the MOSFET:
= 1.5:
A
.
.
)
V
(
V
V
V
=
= 0.0165Ω (max),
⎞
⎟ =
⎠
⎞
⎟ =
⎠
.
11 8
.
1 1
0 8
JA
Ω
.
A
.
μ
)
= 40°C/W.
μ
=
H
H
1 9
.
W
C
power dissipation at current limit with ρ
The junction temperatures for both top and bottom
MOSFETs will be signifi cantly less at nominal current, but
the above analysis shows that careful attention to PCB
layout and heat sinking will be necessary in this circuit.
The same MOSFETs (Si4874 and Si4884) can be used
for channel 2.
Finally, an input capacitor is chosen for an RMS current
rating of about 5A at 85°C and the output capacitors are
chosen for a low ESR of 0.013Ω to minimize output volt-
age changes due to inductor ripple current and load steps.
The ripple voltage will be only:
However, a 0A to 10A load step will cause an output
change of up to:
An optional 22μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 13.
Δ
Δ
P
T
RSS
V
V
TOP
J
ΔV
OUT RIPPLE
OUT RIPPLE
= 70°C + (1.43W)(42°C/W) = 130°
= 190pF , V
OUT(STEP)
=
=
1
2
(
(
0 33
(
2 5
28
11 8
.
.
V
.
V
W
)
(
A
)
=
=
=
=
11 8
= ΔI
= Δ
)(
+
= Δ
GS(TH)
4 5
60
3 4
46
190
.
1 10
.
.
.
I
LOAD(ESR)
mV
mV
I
L
A
L
A
A
1
2
) ( )
pF
2
•
•
•
W
= 1V, θ
•
⎛
⎜
⎝
⎛
⎜
⎝
⎛
⎜
⎝
)(
1 6 0 0165
⎛
⎜
⎝
ESR
0 013
0 013
=
ESR
.
500
.
.
1 43
(
.
= (10A)(0.013Ω) = 130mV
+
kHz
JA
+
Ω +
Ω +
.
8
W
8
= 42°C/W. Checking its
• •
)
• •
( )
f C
8 500
2
8 500
f C
Ω
Ω
1
•
•
1
)
OUT
LTC3708
OUT
⎛
⎜
⎝
+
130°C
5
( )(
kHz
kHz
V
⎞
⎟
⎠
0 5 28
⎞
⎟
⎠
.
1
–
1
1
1
= 1.6:
•
•
V
470
470
23
+
V
1
μ
μ
3708fb
)
1
V
2
F
F
⎞
⎟
⎠
⎞
⎟
⎠
⎞
⎟
⎠