MIC4120/4129
Micrel, Inc.
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.
M9999-081105
6
August 2005
MICREL [ MICREL SEMICONDUCTOR ]
