MIC2199

Micrel

MIC2199

14

November 2004

Decoupling Capacitor Selection

The 4.7µF decoupling capacitor is used to minimize noise on

the V

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

may be necessary when driving large MOSFETs with high

gate capacitance. Incorrect placement of the V

capacitor will cause jitter or oscillations in the switching

waveform and large variations in the overcurrent limit.

The capacitor should be placed near the IC and connected

directly to between pin 6 (V

PCB Layout and Checklist

PCB layout is critical to achieve reliable, stable and efficient

performance. A ground plane is required to control EMI and

minimize the inductance in power, signal and return paths.

The following guidelines should be followed to insure proper

operation of the circuit.

• Signal and power grounds should be kept separate

and connected at only one location. Large currents

or high di/dt signals that occur when the MOSFETs

turn on and off must be kept away from the small

signal connections.

• The connection between the current-sense resis-

tor and the MIC2199 current-sense inputs (pin 4

and 5) should have separate traces, through a 10Ω

resistor on each pin. The traces should be 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. The 10Ω resistor should be placed

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.

• When the high-side MOSFET is switched on, the

critical flow of current is from the input capacitor

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.

• When the low-side MOSFET is switched on, cur-

rent flows through the inductor, sense resistor,

output capacitor, and MOSFET. The source of the

low-side MOSFET should be located close to the

output capacitor.

• The freewheeling diode, D1 in Figure 2, conducts

current during the dead time, when both MOSFETs

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

V

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

connected to the IC with multiple vias.

• The V

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.