ISL6529ACB datasheet(14/20 Pages) RENESAS

14 / 20 page

ISL6529, ISL6529A

FN9070 Rev 5.00

Page 14 of 20

Apr 12, 2005

The final test for a system is transient load current

performance. Ringing or oscillation indicates that the

compensation network must be adjusted to assure stable

operation with component and environmental variations.

Figures 14 and 15 are scope shots that show the regulator

with only with a 1500

F, 100m capacitor with high

frequency ringing with no compensation. A 27pF capacitor,

C16 was added in Figure 15. The output step is about

260mV for the 3A load current for an ESR in the order of

90m

. After the load current is removed, the output network

parasitics ring for about 5

s.

Application Guidelines

Layout Considerations

Layout is very important in high frequency switching

converter design. With power devices switching efficiently at

600kHz, 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 over-voltage stress. Careful component

layout and printed circuit board design minimizes the voltage

spikes in the converters.

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

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 lower MOSFET and

parasitic 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 minimizes

the magnitude of voltage spikes.

There are two sets of critical components in a DC-DC converter

using the ISL6529, ISL6529A. 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

16 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

through 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 terminal to the output inductor short. The power

plane should support the input and output power nodes. Use

copper filled polygons on the top and bottom circuit layers for

the phase node. Use the remaining printed circuit layers for

small signal wiring. The wiring traces from the UGATE pin to

the MOSFET gate should be kept short and wide enough to

easily handle the 1A of drive current.

The switching components should be placed close to the

ISL6529, ISL6529A 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. Position the output inductor and

output capacitors between the upper MOSFET and lower

diode and the load.

FIGURE 14. 3A TRANSIENT LOAD APPLIED TO THE

REGULATOR NO COMPENSATION

FIGURE 15. 3A TRANSIENT LOAD APPLIED TO THE

REGULATOR 27pF COMPENSATION

OUTPUT VOLTAGE

200mV/DIV

LOAD CURRENT, 1A/DIV

OUTPUT VOLTAGE

200mV/DIV

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