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Date: 3/6/06

SP6656, 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output

© Copyright 2006 Sipex Corporation

APPLICATION INFORMATION

Output Voltage Ripple Frequency

An important consideration in a power sup-

ply application is the frequency value of the

output ripple. Given the control technique of

the SP6656 (as described in the operations

section), the frequency of the output ripple

will vary when in light to moderate load in

the discontinuous or PFM mode. For mod-

erate to heavy loads greater than about

100mA inductor current ripple, (for the typi-

cal 10µH inductor application on 100mA is

half the 200mA inductor current ripple), the

output ripple frequency will be fairly con-

stant. From the operations section, this maxi-

mum loop frequency in continuous conduc-

tion mode is

1

*

*

Data for loop frequency, as measured from

output voltage ripple frequency, can be found

in the typical performance curves.

Layout Considerations

Proper layout of the power and control circuits

is necessary in a switching power supply to

obtain good output regulation with stability

and a minimum of output noise. The SP6656

application circuit can be made very small and

reside close to the IC for best performance

and solution size, as long as some layout

techniques are taken into consideration. To

avoid excessive interference between the

SP6656 high frequency converter and the

other active components on the board, some

rules should be followed. Refer to the typical

application schematic on page 1 and the

sample PCB layout shown in the following

figures to illustrate how to layout a SP6656

power supply.

Avoid injecting noise into the sensitive part of

circuit via the ground plane. Input and output

capacitors conduct high frequency current

through the ground plane. Separate the con-

trol and power grounds and connect them

together at a single point. Power ground

plane is shown in the figure titled PCB top

sample layout and connects the ground of the

The control ground plane connects from pin 9

These two separate control and power ground

planes come together in the figure titled PCB

top sample layout where SP6656 pin 9 GND

is connected to pin 10 PGND.

Power loops on the input and output of the

converter should be laid out with the shortest

and widest traces possible. The longer and

narrower the trace, the higher the resistance

and inductance it will have. The length of

traces in series with the capacitors increases

its ESR and ESL and reduces their effective-

ness at high frequencies. Therefore, put the

GND pins of the converter as possible, the

output capacitor as close to the inductor as

possible. The external voltage feedback net-

should be placed very close to the FB pin. Any

noise traces like the LX pin should be kept

away from the voltage feedback network and

separated from it by using power ground

copper to minimize EMI.