LTC3824EMSE-PBF LINER [Linear Technology], LTC3824EMSE-PBF Datasheet - Page 10

• Manufacturer Part Number: LTC3824EMSE-PBF
• Description: High Voltage Step-Down Controller With 40?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3824
2. Power MOSFET switching and condution losses:
3. The I
4. The inductor loss due to winding resistance:
5. Loss of the catch diode:
6. Other losses, including C
PCB Layout Considerations
To achieve best performance from a LTC3824 circuit, the PC
board layout must be carefully designed. For lower power
applications, a 2-layer PC board is suffi cient. However, at
higher power levels, a multiple layer PC board is recom-
mended. Using a solid ground plane under the circuit is
the easiest way to ensure that switching noise does not
affect the operation.
In order to help dissipate the power from the MOSFET and
diode, keep the ground plane on the layers closest to the
layers where power components are mounted. Use power
planes for the MOSFET and diode in order to improve the
spreading of heat from these components into the PCB.
For best electrical performance the LTC3824 circuit should
be laid out as following:
Place all power components in a tight area. This will
minimize the size of high current loops. Orient the input
and output capacitors and current sense resistor in a way
that minimizes the distance between the pads connected
to ground plane.
10
where D is the duty cycle
and inductor core losses, generally account for less
than 2% of total losses.
P
MOSFET
P
P
P
(SENSE R)
(WINDING)
(DIODE)
2
R losses of the current sense resistor:
+ K(V
=
V
= I
V
OUT
IN
= (I
= (I
OUT
IN
+ V
)
+ V
OUT
2
OUT
• V
(I
D
OUT
D
)
)
D
2
(I
2
• R • D
• (1–D)
OUT
• R
)(C
IN
W
RSS
)
and C
2
(1+ δ)R
)(f)
OUT
DS(ON)
ESR dissipation
Place the LTC3824 and associated components tightly to-
gether and next to the section with power components.
Use a local via to ground plane for all pads that connect to
ground. Use multiple vias for power components.
Connect the current sense input directly to the current
sense resistor pad. V
internal current sense amplifi er and should be connected
as close to the sense resistor pads as possible. A 100pF
capacitor is required across the V
noise fi ltering and should be placed as close to the pins
as possible.
Design Example
As an example, the LTC3824 is designed for an automotive
5V power supply with the following specifi cations:
Maximum I
60V briefl y during load dump condition, and operating
switching frequency = 400kHz.
For f = 400kHz, R
Allow inductor ripple current to be 0.8A (40% of the
maximum output current) at V
C
to satisfy the output voltage ripple requirement and the
bulk capacitance needed for loop stability. For this design
a 220μF tantalum capacitor is used.
For worse-case conditions C
1A ripple current (half of the maximum output current). A
47μF tantalum capacitor is adequate.
A current limit of 3.3A is selected and R
calculated by :
and a 25mΩ resistor can be used.
OUT
L =
R
SENSE
will be selected based on the ESR that is required
(400kHz • 0.8A)18V
(18V – 5V)5V
=
OUT
100mV
3.3A
= 2A, typical V
SET
= 0.03
CC
is chosen to be 180k.
and SENSE are the inputs of the
= 12μH
IN
IN
should be rated for at least
= 6V to 18V and can reach
IN
= 18V,
CC
and sense pins for
SENSE
can be
3824fc