MIC28512 datasheet(26/34 Pages) MICROCHIP

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MIC28512

DS20005524A-page 26

 2016 Microchip Technology Inc.

6.0

PCB LAYOUT GUIDELINES

PCB Layout is critical to achieve reliable, stable and

efficient performance. A ground plane is required to

control EMI and minimize the inductance in power,

signal and return paths.

Figure 6-1 is optimized from a small form-factor point of

view and shows the top and bottom layers of a

four-layer PCB. It is recommended to use Mid-Layer 1

as a continuous ground plane.

FIGURE 6-1:

Top and Bottom Layers of a

Four-Layer Board.

The following guidelines should be followed to ensure

proper operation of the MIC28512 converter.

6.1

• The analog ground pin (AGND) must be

connected directly to the ground planes. Do not

route the AGND pin to the PGND pin on the top

layer.

• Place the IC close to the point-of-load (POL).

• Use copper planes to route the input and output

power lines.

• Analog and power grounds should be kept

separate and connected at only one location.

6.2

Input Capacitor

• Place the input capacitors on the same side of the

board and as close to the PVIN and PGND pins as

possible.

• Place several vias to the ground plane close to

the input capacitor ground terminal.

• Use either X7R or X5R dielectric input capacitors.

Do not use Y5V or Z5U type capacitors.

• Do not replace the ceramic input capacitor with

any other type of capacitor. Any type of capacitor

can be placed in parallel with the input capacitor.

• If a Tantalum input capacitor is placed in parallel

with the input capacitor, it must be recommended

for switching regulator applications and the

operating voltage must be derated by 50%.

• In “Hot-Plug” applications, a Tantalum or

Electrolytic bypass capacitor must be used to limit

the overvoltage spike seen on the input supply

with power is suddenly applied.

6.3

SW Node

• Do not route any digital lines underneath or close

to the SW node.

• Keep the switch node (SW) away from the

feedback (FB) pin.

6.4

Output Capacitor

• Use a copper island to connect the output

capacitor ground terminal to the input capacitor

ground terminal.

• Phase margin will change as the output capacitor

value and ESR changes. Contact the factory if the

output capacitor is different from what is shown in

the BOM.

• The feedback trace should be separate from the

power trace and connected as close as possible

to the output capacitor. Sensing a long

high-current load trace can degrade the DC load

regulation.

6.5

Thermal Measurements

Measuring the IC’s case temperature is recommended

to ensure it is within its operating limits. Although this

might seem like a very elementary task, it is easy to get

erroneous results. The most common mistake is to use

the standard thermal couple that comes with a thermal

meter. This thermal couple wire gauge is large, typically

22 gauge, and behaves like a heatsink, resulting in a

lower case measurement.

Two methods of temperature measurement are using a

smaller

thermal

couple

wire

infrared

thermometer. If a thermal couple wire is used, it must

be constructed of 36 gauge wire or higher then (smaller

wire size) to minimize the wire heat-sinking effect. In

addition, the thermal couple tip must be covered in

either thermal grease or thermal glue to make sure that

the thermal couple junction is making good contact with

the case of the IC. Omega brand thermal couple

(5SC-TT-K-36-36) is adequate for most applications.

Wherever possible, an infrared thermometer is

recommended. The measurement spot size of most

infrared thermometers is too large for an accurate

reading on a small form factor ICs. However, a IR

thermometer from Optris has a 1 mm spot size, which

makes it a good choice for measuring the hottest point

on the case. An optional stand makes it easy to hold the

beam on the IC for long periods of time.

For more information about the Evaluation board layout, please contact Microchip sales.

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