MIC4225 datasheet(12/20 Pages) MIC

12 / 20 page

Micrel, Inc.

MIC4223/MIC4224/MIC4225

June 2009

M9999-061109-A

(408) 944-0800

Bypass Capacitor Selection

Bypass capacitors are required for proper operation by

supplying the charge necessary to drive the external

MOSFETs as well as minimize the voltage ripple on the

supply pins.

Ceramic capacitors are recommended because of their

low impedance and small size. Z5U type ceramic capacitor

dielectrics are not recommended due to the large change

capacitance

over

temperature

and

voltage.

Manufacturer specifications should be checked to insure

voltage and temperature do not reduce the capacitance

below the value needed. A minimum value of 1µF is

required regardless of the MOSFETs being driven. Larger

MOSFETs, with their higher input capacitance may require

larger decoupling capacitance values for proper operation.

The voltage rating of the capacitors depends on the supply

voltage, ambient temperature and the voltage derating

used for reliability.

Placement of the decoupling capacitors is critical. The

bypass capacitor for VDD should be placed as close as

possible between the VDD and GND pins. The etch

connections must be short, wide and direct. The use of a

ground plane to minimize connection impedance is

recommended. Multiple vias insure a low inductance path

and help with power dissipation. Refer to the section on

layout and component placement for more information.

Grounding, Component Placement and Circuit Layout

Nanosecond switching speeds and ampere peak currents

in and around the MOSFET driver necessitate proper

placement and trace routing of all components. Improper

placement may cause degraded noise immunity, false

switching and excessive ringing.

Figure 6 shows the critical current paths when the driver

outputs go high and turn on the external MOSFETs. It also

helps demonstrate the need for a low impedance ground

plane. Charge needed to turn-on the MOSFET gates

comes from the decoupling capacitors CVDD. Current in the

gate driver flows from CVDD through the internal driver, into

the MOSFET gate and out the Source. The return

connection back to the decoupling capacitor is made

through the ground plane. Any inductance or resistance in

the ground return path causes a voltage spike or ringing to

appear on the source of the MOSFET. This voltage works

against the gate drive voltage and can either slow down or

turn off the MOSFET during the period where it should be

turned on.

Figure 6. Driver Turn-On Current Path

Figure 7 shows the critical current paths when the driver

outputs go low and turn off the external MOSFETs. Short,

low impedance connections are important during turn-off

for the same reasons given in the turn-on explanation.

Current from the VDD supply replenishes charge in the

decoupling capacitor, CVDD.

Figure 7. Driver Turn-Off Current Path

The following circuit guidelines should be adhered to for

optimum circuit performance:

The VDD bypass capacitor must be placed close to the

VDD and ground pins. It is critical that the etch length

between the decoupling capacitor and the VDD and GND

pins be minimized to reduce pin inductance. Multiple vias

in parallel help minimize inductance in the ground and VDD

paths.

A ground plane is recommended to minimize parasitic

inductance and impedance of the return paths. The

MIC4223 family of drivers is capable of high peak currents

and very fast transition times.

Any impedance between

the driver, the decoupling capacitors and the external

MOSFET will degrade the performance of the circuit.

Trace out the high di/dt and dv/dt paths, as shown in

Figures 6 and 7 and minimize etch length and loop area

for

these connections. Minimizing

these

parameters

decreases the parasitic inductance and the radiated EMI

generated by fast rise and fall times.

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