Micrel, Inc.

MIC2199.

January 2010

13

M9999-011310

Decoupling Capacitor Selection

The 4.7µF decoupling capacitor is used to minimize noise on

proper operation of the IC. It must be placed right next to the

pins and routed with a wide trace. The capacitor should be a

good quality tantalum. An additional 1µF ceramic capacitor

maybenecessarywhendrivinglargeMOSFETswithhigh

capacitor will cause jitter or oscillations in the switching wave-

form and large variations in the overcurrent limit.

The capacitor should be placed near the IC and connected

PCB Layout and Checklist

PCBlayoutiscriticaltoachievereliable,stableandefficient

performance.AgroundplaneisrequiredtocontrolEMIand

minimize the inductance in power, signal and return paths.

The following guidelines should be followed to insure proper

operation of the circuit.

• Signalandpowergroundsshouldbekeptseparate

and connected at only one location. Large currents

orhighdi/dtsignalsthatoccurwhentheMOSFETs

turn on and off must be kept away from the small

signal connections.

• Theconnectionbetweenthecurrent-senseresis-

tor and the MIC2199 current-sense inputs (pin 4

and 5) should have separate traces, through a

10Ωresistoroneachpin.Thetracesshouldbe

routed as closely as possible to each other and

their length should be minimized. Avoid running

the traces under the inductor and other switching

components.The10Ωresistorshouldbeplaced

close as possible to pins 4 and 5 on the MIC2199

and a 1nF to 0.1µF capacitor placed between pins

4 and 5 will help attenuate switching noise on the

current sense traces. This capacitor should be

placed close to pins 4 and 5.

• Whenthehigh-sideMOSFETisswitchedon,the

criticalflowofcurrentisfromtheinputcapacitor

through the MOSFET, inductor, sense resistor,

output capacitor, and back to the input capacitor.

These paths must be made with short, wide pieces

of trace. It is good practice to locate the ground

terminals of the input and output capacitors close

to each.

• Whenthelow-sideMOSFETisswitchedon,cur-

rent flows through the inductor, sense resistor,

outputcapacitor,andMOSFET.Thesourceofthe

low-sideMOSFETshouldbelocatedclosetothe

output capacitor.

• Thefreewheelingdiode,D1inFigure2,conducts

currentduringthedeadtime,whenbothMOSFETs

are off. The anode of the diode should be located

close to the output capacitor ground terminal and

the cathode should be located close to the input

side of the inductor.

terminal (pin 7) must be located right at the IC. The

of this capacitor is very critical. Connections must

be made with wide trace. The capacitor may be

located on the bottom layer of the board and con-

nected to the IC with multiple vias.

to the IC and connected between pins 6 and 9.

Connections should be made with a ground and

power plane or with short, wide trace.