MIC4120/4129

Micrel, Inc.

M9999-072010

6

July 2010

Applications Information

Supply Bypassing

Charging and discharging large capacitive loads quickly

requires large currents. For example, charging a 2500pF

load to 18V in 25ns requires a 1.8 A current from the device

power supply.

The MIC4120/4129 has double bonding on the supply pins,

the ground pins and output pins This reduces parasitic lead

inductance. Low inductance enables large currents to be

switched rapidly. It also reduces internal ringing that can

cause voltage breakdown when the driver is operated at or

near the maximum rated voltage.

Internal ringing can also cause output oscillation due to

feedback. This feedback is added to the input signal since it

is referenced to the same ground.

To guarantee low supply impedance over a wide frequency

range, a parallel capacitor combination is recommended

for supply bypassing. Low inductance ceramic capacitors

should be used. A 1µF low ESR film capacitor in parallel with

two 0.1 µF low ESR ceramic capacitors provide adequate

bypassing. Connect one ceramic capacitor directly between

pins 1 and 4. Connect the second ceramic capacitor directly

between pins 8 and 5.

Grounding

The high current capability of the MIC4120/4129 demands

careful PC board layout for best performance. Since the

MIC4129 is an inverting driver, any ground lead impedance

will appear as negative feedback which can degrade switch-

ing speed. Feedback is especially noticeable with slow-rise

time inputs.

Figure 3 shows the feedback effect in detail. As the MIC4129

input begins to go positive, the output goes negative and

several amperes of current flow in the ground lead. As little

as 0.05Ω of PC trace resistance can produce hundreds of

millivolts at the MIC4129 ground pins. If the driving logic is

referenced to power ground, the effective logic input level is

reduced and oscillation may result.

To insure optimum performance, separate ground traces

should be provided for the logic and power connections. Con-

necting the logic ground directly to the MIC4129 GND pins

will ensure full logic drive to the input and ensure fast output

switching. Both of the MIC4129 GND pins should, however,

still be connected to power ground.

The E-Pad and MLF packages have an exposed pad under

the package. It's important for good thermal performance that

this pad is connected to a ground plane.