LTC3709 Linear Technology, LTC3709 Datasheet - Page 20

• Manufacturer Part Number: LTC3709
• Description: No RSENSE Synchronous DC/DC Controller
• Manufacturer: Linear Technology

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LTC3709
APPLICATIO S I FOR ATIO
Selecting a standard value of 1.8µH results in a maximum
ripple current of:
Next, choose the synchronous MOSFET switch. Choosing
a Si4874 (R
q
Tying V
for a nominal value of 110mV with 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 such a short time, an
Si4884 R
40°C/W will be sufficient. Checking its power dissipation
at current limit with ρ
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary in this circuit.
C
at 85°C. The output capacitors are chosen for a low ESR
of 0.013Ω to minimize output voltage changes due to
20
JA
IN
V
T
T
I
P
P
∆ =
LIMIT
= 40°C/W) yields a nominal sense voltage of:
J
J
SNS(NOM)
BOT
TOP
is chosen for an RMS current rating of about 10A
I
L
= 70°C + (1.97W)(40°C/W) = 149°C
= 70°C + (0.7W)(40°C/W) = 98°C
RNG
=
=
( )(
=
≥
(
1 7 28
DS(ON)(MAX)
250
2 5
.
0 30
28
28
⎛
⎜
⎝
to 1.1V will set the current sense voltage range
.
.
( )
DS(ON)
1 5 0 010
= (10A)(1.3)(0.0083Ω) = 108mV
V
V
V
kHz
28
.
W
– .
2 5
⎛
⎜
⎝
146
.
V
2 5
V
(
24
)
+
) (
(
U
V
2
.
2
1 8
= 0.0083Ω (NOM) 0.010Ω (MAX),
mV
0 40
A
150°C
V
.
.
100°C
12
= 0.0165Ω, C
⎞
⎟
⎠
⎛
⎜
⎝
µ
2
24
Ω
A
( )
H
W
U
1 4 0 0165
2
)(
= 1.5:
)
)
.
A
= 1.4:
⎛
⎜
⎝
100
=
+
1
⎞
⎟
⎠
(
0 7
2
–
2
1
.
( )
pF
.
(
1 5 0 010
2 5
28
J
5 1
W
.
.
.
in the MOSFET:
)(
W
V
V
250
A
RSS
(
⎞
⎟ =
⎠
)
Ω
⎞
⎟
⎠
.
)
kHz
•
= 100pF, θ
+
5 1
2 24
.
=
Ω
)
A
)
U
=
A
1 97
.
JA
W
=
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 9.
PC Board Layout Checklist
When laying out a PC board follow one of the two sug-
gested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents,
it is recommended to use a multilayer board to help with
heat sinking power components.
• The ground plane layer should not have any traces and
• Place C
• Use an immediate via to connect the components to
• Use a compact plane for switch node (SW) to keep EMI
• Use planes for V
• Flood all unused areas on all layers with copper. Flood-
When laying out a printed circuit board, without a ground
plane, use the following checklist to ensure proper opera-
tion of the controller. These items are also illustrated in
Figure 9.
∆V
∆V
it should be as close as possible to the layer with power
MOSFETs.
one compact area. It may help to have some compo-
nents on the bottom side of the board.
ground plane including SGND and PGND of LTC3709.
Use several larger vias for power components.
down.
filtering and to keep power losses low.
ing with copper will reduce the temperature rise of
power component. You can connect the copper areas to
any DC net (V
your system).
OUT(RIPPLE)
OUT(STEP)
IN
, C
OUT
= ∆I
IN
= ∆I
= (5.1A) (0.013Ω) = 66mV
, MOSFETs, D1, D2 and inductors all in
, V
LOAD
IN
OUT
L(MAX)
and V
(ESR) = (10A) (0.013Ω) = 130mV
, GND or to any other DC rail in
OUT
(ESR)
to maintain good voltage
3709f