LTC3824

10

3824fc

APPLICATIONS INFORMATION

2. Power MOSFET switching and condution losses:

P

V

V

(I

DS(ON)

+ K(V

where D is the duty cycle

4. The inductor loss due to winding resistance:

5. Loss of the catch diode:

and inductor core losses, generally account for less

than 2% of total losses.

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 sufficient. 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.

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

internal current sense amplifier and should be connected

as close to the sense resistor pads as possible. A 100pF

noise filtering 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 specifications:

60V briefly during load dump condition, and operating

switching frequency = 400kHz.

Allow inductor ripple current to be 0.8A (40% of the

L

=

(18V – 5V)5V

(400kHz • 0.8A)18V

= 12μH

to satisfy the output voltage ripple requirement and the

bulk capacitance needed for loop stability. For this design

a 220μF tantalum capacitor is used.

1A ripple current (half of the maximum output current). A

47μF tantalum capacitor is adequate.

calculated by :

100mV

3.3A

= 0.03

and a 25mΩ resistor can be used.