ISL85003AEVAL2Z datasheet(21/24 Pages) RENESAS

21 / 24 page

ISL85003, ISL85003A

FN7968 Rev.3.02

Page 21 of 23

Apr 15, 2024

Layout Considerations

The layout is very important in high frequency switching

converter design. With power devices switching efficiently at

500kHz, the resulting current transitions from one device to

another cause voltage spikes across the interconnecting

impedances and parasitic circuit elements. These voltage spikes

can degrade efficiency, radiate noise into the circuit, and lead to

device overvoltage stress. Careful component layout and printed

circuit board design minimizes these voltage spikes.

As an example, consider the turn-off transition of the upper

MOSFET. Prior to turn-off, the MOSFET is carrying the full load

current. During turn-off, current stops flowing in the MOSFET and

is picked up by the internal body diode. Any parasitic inductance

in the switched current path generates a large voltage spike

during the switching interval. Careful component selection, tight

layout of the critical components and short, wide traces minimize

the magnitude of voltage spikes.

There are two sets of critical components in the regulator

switching converter. The switching components are the most

critical because they switch large amounts of energy and

therefore tend to generate large amounts of noise. Next are the

small signal components, which connect to sensitive nodes or

supply critical bypass current and signal coupling.

A multi-layer printed circuit board is recommended. Figure 50

shows the connections of the critical components in the

converter. Note that capacitors CIN and COUT could each

represent numerous physical capacitors. Dedicate one solid

layer, usually a middle layer of the PC board, for a ground plane

and make all critical component ground connections with vias to

this layer. Dedicate another solid layer as a power plane and

break this plane into smaller islands of common voltage levels.

Keep the metal runs from the PHASE terminals to the output

inductor short. The power plane should support the input power

and output power nodes. Use copper filled polygons on the top

and bottom circuit layers for the phase nodes. Use the remaining

printed circuit layers for small signal wiring.

In order to dissipate heat generated by the internal LDO and

MOSFETs, the ground pad should be connected to the internal

ground plane through at least five vias. This allows the heat to

move away from the IC and also ties the pad to the ground plane

through a low impedance path.

The switching components should be placed close to the

regulator first. Minimize the length of the connections between

the input capacitors, CIN, and the power switches by placing

them nearby. Position both the ceramic and bulk input capacitors

as close to the upper MOSFET drain as possible.

The critical small signal components include any bypass

capacitors, feedback components and compensation

components. Place the compensation components close to the

FB and COMP pins. The feedback resistors should be located as

close as possible to the FB pin with vias tied straight to the

ground plane.

FIGURE 50. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS

VIN

ISL85003

PHASE

PGND

COMP

PGND PAD

COUT1

VOUT1

CIN

VIN

ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER

VIA CONNECTION TO GROUND PLANE

KEY

ISL85003A

ISL85003AEVAL2Z Datasheet(PDF) 21 Page - Renesas Technology Corp

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