Micrel, Inc.
MIC4421A/4422A
To guarantee low supply impedance over a wide
frequency range, a parallel capacitor combination is
recommended for supply bypassing. Low inductance
ceramic disk capacitor swith short lead lengths (< 0.5
inch) should be used. A 1µF low ESR film 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 MIC4421A/4422A
demands careful PC board layout for best performance.
Since the MIC4421A 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 MIC4421A
input structure includes about 600mV of hysteresis to
ensure clean transitions and freedom from oscillation,
but attention to layout is still recommended.
Figure 7 shows the feedback effect in detail. As the
MIC4421A 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 MIC4421A
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.
Connecting the logic ground directly to the MIC4421A
GND pins will ensure full logic drive to the input and
ensure fast output switching. Both of the MIC4421A
GND pins should, however, still be connected to power
ground.
Application 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 MIC4421A/4422A 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.
V
S
1µF
MIC4421A
Ø2
V
S
Ø1 Drive Signal
Conduction Angle
Control 0°C to 180°C
Conduction Angle
Control 180°C to 360°C
Drive
Logic
Ø1
Ø3
V
S
MIC4422A
1µF
V
S
Phase 1 of 3 Phase Motor
Driver Using MIC4421A/22A
Figure 5. Direct Motor Drive
1N4448
(x2)
30
Output Voltage
vs. Load Current
V
IN
+15V
BYV 10
(x2)
500µF
50V
United Chemcon SXE
VOLTS
29
0.1µF
50V
MIC4422A
1µF
WIMA
MKS2
28
27
26
25
0.1µF
WIMA
MKS2
100µF
50V
0
50 100 150 200 250 300 350
mA
Figure 6. Self Contained Voltage Doubler
June 2007
8
M9999-062707
MIC [ MIC GROUP RECTIFIERS ]
