LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 18

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC3878
V
voltage on the bottom MOSFET switch. As a result, the
average DC current limit includes signifi cant temperature
and component variability. Design to guarantee that the
average DC current limit will always exceed the rated oper-
ating output current by assuming worst-case component
tolerance and temperature.
The worst-case minimum INTV
MOSFET worst-case R
temperature is 80°C above a 70°C ambient with ρ
1.5. Set T
low and the inductor 15% high.
By setting I
voltage which corresponds to a V
Verify that the calculated nominal T
assumed worst-case T
Because the top MOSFET is on for a short time, an
RJK0305DPB (R
(C
θ
tion at current limit with = ρ
The junction temperatures will be signifi cantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary.
18
RNG
JA
MILLER
P
P
V
V
T
T
= 40°C/W) is suffi cient. Checking its power dissipa-
TOP
J
TOP
J
DS
RNG
sets current limit by fi xing the maximum peak V
=
=
=
70
70
=
=
=
=
⎛
⎝ ⎜
= Q
ON
(
0
28
° ° +
1 2
° +
15
7 5
28
2 5
.
LIMIT
C
C
.
. •
1 1 8
.
equal to the minimum specifi cation of 15%
GD
A
V
V
V
28
Ω
W
1 25
0 76
– • .
– .
V
/10V = 150pF , V
(
.
)
.
equal to 15A we get 79mV for peak V
V
15
DS
1 2
2
1
+
⎛
⎝ ⎜
DS(ON)
150
0 58
A
W
W
V
5 1
.
)
2
(
•
•
DS(ON)
J
pF
15
A
40
• . •
40
W
in the bottom MOSFET:
1 4 13
•
⎛
⎝ ⎜
A
°
°
= 10mΩ (nominal) 13mΩ
=
0 85
1 15
2 5
C W
C W
)
.
2
.
2
.
0 65
/
100°C
/
is 3.9mΩ and the junction
.
• . • .
⎞
⎠ ⎟
1 5 3 9
BOOST
⎞
⎠ ⎟
CC
=
m
=
+
W
RNG
3 9
5 15
120
Ω
100
= 1.4:
1 2
5 3
.
is 5.15V. The bottom
.
3
.
.
+
m
equal to 592mV:
J
⎞
⎠ ⎟
V V
°
= 5V), V
°
(
Ω
V
C
C
m
28
400
is less than the
• .
Ω
V
1 5
kHz
=
)
2
1 25
⎛
⎝ ⎜
GS
.
15
2
150°C
= 4.5V,
W
A
⎞
⎟ ⎟
⎠
DS
DS
=
Select C
at 85°C. The output capacitor C
ESR of 4.5mΩ to minimize output voltage changes due to
inductor ripple current and load steps. The output voltage
ripple is given as:
However, a 0A to 10A load step will cause an output
change of up to:
Optional 2 × 47μF ceramic output capacitors are included
to minimize the effect of ESR and ESL in the output ripple
and to improve load step response.
PC Board Layout Checklist
The LTC3878 PC board layout can be designed with or
without a ground plane. A ground plane is generally pre-
ferred based on performance and noise concerns.
When using a ground plane, use a dedicated ground plane
layer. In addition, for high current it is recommended to
use a multilayer board to help with heat sinking power
components.
l
l
l
l
l
The ground plane layer should have no traces and be
as close as possible to the routing layer connecting the
power MOSFET’s.
Place LTC3878 Pins 9 to 16 facing the power compo-
nents. Keep components connected to Pin 1 close to
LTC3878 (noise sensitive components).
Place C
compact area. It may help to have some components
on the bottom side of the board.
Use an immediate via to connect components to the
ground plane SGND and PGND of LTC3878. Use several
larger vias for power components.
Use compact switch node (SW) plane to improve cool-
ing of the MOSFETs and to keep EMI down.
Δ
Δ
V
V
OUT RIPPLE
OUT STEP
IN
(
(
IN
to give an RMS current rating greater than 4A
, C
OUT
)
=
=
)
=
=
10
Δ
, MOSFETs, D
5 1 4 5
Δ
I
LOAD
A
. • .
I
L MAX
• .
(
4 5
(
ESR
m
m
)
(
Ω
Ω
ESR
)
=
OUT1
=
B
45
23
and inductor all in one
)
mV
mV
is chosen for a low
3878f