
MIC4421/4422
Applications Information
Supply Bypassing
Charging and discharging large capacitive loads quickly
requires large currents. For example, charging a 10,000pF
load to 18V in 50ns requires 3.6A.
The MIC4421/4422 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.
VS
Micrel, Inc.
To guarantee low supply impedance over a wide frequency
range, a parallel capacitor combination is recommended for
supply bypassing. Low inductance ceramic disk capacitors
with short lead lengths (< 0.5 inch) should be used. A 1μF low
ESR ?lm capacitor in parallel with two 0.1μF low ESR ceramic
capacitors, (such as AVX RAM Guard ? ), provides 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 MIC4421/4422 demands
careful PC board layout for best performance. Since the
MIC4421 is an inverting driver, any ground lead impedance
will appear as negative feedback which can degrade switching
speed. Feedback is especially noticeable with slow-rise time
inputs. The MIC4421 input structure includes about 200mV
of hysteresis to ensure clean transitions and freedom from
oscillation, but attention to layout is still recommended.
?1 DRIV E S IGNA L
CONDUCTION ANGLE
CONT ROL 0° TO 180°
CONDUCTION ANGLE
CONTROL 180° TO 360°
DRIVE
L OGIC
MIC4451
VS
MIC4452
1μF
VS
?2
?1 M ?3
1μF VS
PHASE 1 of 3 PHASE MOTOR
DRIVER USING MIC4420/4429
Figure 5 shows the feedback effect in detail. As the MIC4421
input begins to go positive, the output goes negative and
several amperes of current ?ow in the ground lead. As little
as 0.05Ω of PC trace resistance can produce hundreds of
millivolts at the MIC4421 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 MIC4421 GND pins
will ensure full logic drive to the input and ensure fast output
switching. Both of the MIC4421 GND pins should, however,
still be connected to power ground.
Figure 3. Direct Motor Drive
+15
(x2) 1N4448
0.1μF
50V
5.6 k?
560 ?
30
29
OUTPUT VOLTAGE vs LOAD CURRENT
1
+
1μF
50V
MKS2
BYV 10 (x 2)
28
27
12 ? LINE
2
0.1μF
WIMA
MKS2
8
MIC4421
5
4
6, 7 +
560μF 50V
100μF 50V
UNIT E D CHE MCON S X E
+
26
25
0
50
100 150 200 250 300 350
mA
Figure 4. Self Contained Voltage Doubler
August 2005
7
M9999-081005