Micrel Inc.
MIC4451/4452
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 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 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 switching 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
2
φ
1 D R I V E S I G N A L
CONDUCTION ANGLE
C ONT R OL 0° TO 180°
CONDUCTION ANGLE
CONT ROL 1 80° T O 3 60 °
DRIVE
LOGIC
V DD
1µF
MIC4452
V DD
φ
1
M
φ
3
PHASE 1 OF 3 PHASE MOTOR
D R I VER U SI N G M I C 44 51 / 44 52
Figure 3. Direct Motor Drive
+15
(x2) 1N4448
5.6 kΩ
560
Ω
0.1µF
50V
+
1
2
0.1µF
WIMA
M KS2
8
MIC4451
5
4
1µF
50V
MKS2
6, 7
+
BYV 10 (x 2)
+
560µF 50V
100µF 50V
U NI TE D CH EM CO N S X E
Figure 4. Self Contained Voltage Doubler
January 2011
8
M9999-011811
MICREL [ MICREL SEMICONDUCTOR ]
