MIC4451/4452
Micrel
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 film capacitor in parallel with two 0.1µF low ESR ceramic
capacitors, (such as AVX RAM GUARD
®
), provides ad-
equate 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 MIC4451/4452 demands
careful PC board layout for best performance. Since the
MIC4451 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. The MIC4451 input structure includes 200mV of
hysteresis to ensure clean transitions and freedom from
oscillation, but attention to layout is still recommended.
Figure 5 shows the feedback effect in detail. As the MIC4451
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 MIC4451 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 MIC4451 GND
pins will ensure full logic drive to the input and ensure fast
output switching. Both of the MIC4451 GND pins should,
however, still be connected to power ground.
Applications Information
Supply Bypassing
Charging and discharging large capacitive loads quickly
requires large currents. For example, changing a 10,000pF
load to 18V in 50ns requires 3.6A.
The MIC4451/4452 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 DD
1µF
MIC4451
V DD
φ
φ
1 DRIVE SIGNAL
CONDUCTION ANGLE
CONTROL 0° TO 180°
CONDUCTION ANGLE
CONTROL 180° TO 360°
DRIVE
LOGIC
V DD
1µF
φ
1
2
M
φ
3
V DD
MIC4452
PHASE 1 OF 3 PHASE MOTOR
DRIVER USING MIC4451/4452
Figure 3. Direct Motor Drive
+15
(x2) 1N4448
5.6 kΩ
OUTPUT VOLTAGE vs LOAD CURRENT
560
Ω
0.1µF
50V
1µF
50V
MKS 2
6, 7
+
30
29
VOLTS
+
28
12
Ω
LINE
27
26
25
0
50
100 150 200 250 300 350
mA
1
8
2
0.1µF
WIMA
MKS 2
MIC4451
5
4
BYV 10 (x 2)
+
560µF 50V
100µF 50V
UNITED CHEMCON SXE
Figure 4. Self Contained Voltage Doubler
5-76
April 1998
MICREL [ MICREL SEMICONDUCTOR ]
