ML4819

13

CONSTRUCTION AND LAYOUT TIPS

High frequency power circuits require special care during

breadboard construction and layout. Double sided printed

circuit boards with ground plane on one side are highly

recommended. All critical switching leads (power FET,

output diode, IC output and ground leads, bypass

capacitors) should be kept as small as possible. This is to

minimize both the transmission and pickup of switching

noise.

There are two kinds of noise coupling; inductive and

capacitive. As the name implies inductive coupling is

due to fast changing (high di/dt) circulating switching

currents. The main source is the loop formed by Q1, D6,

and

C3–C4. Therefore this loop should be as small as possible,

and the above capacitors should be good, high frequency

types.

The second form of noise coupling is due to fast changing

voltages (high dv/dt). The main source in this case is the

drain of the power FET. The radiated noise in this case can

be minimized by insulating the drain of the FET from the

heatsink and then tying the heatsink to the source of the

FET with a high frequency capacitor.

The IC has two ground pins named PWR GND and Signal

GND. These two pins should be connected together with a

very short lead at the printed circuit board exit point. In

general grounding is very important and ground loops

should be avoided. Star grounding schemes are preferred.

FROM

R23, C14

OSC

SLOPE

COMP.

C6

DUTY CYCLE

2

PWM B

1V

FROM U2, R15

0.7V

R13

R14

+

–

+

–

+

–

7

20

11

9

8

OSC

–

+

Figure 15. Voltage Mode Configuration