MIC4420/4429
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 MIC4420/4429 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 disk capacitors
with short lead lengths (< 0.5 inch) should be used. A 1µF
low ESR film capacitor in parallel with two 0.1 µF low ESR
ceramic capacitors, (such as AVX RAM GUARD
®
), provides
adequate bypassing. Connect one ceramic capacitor di-
rectly between pins 1 and 4. Connect the second ceramic
capacitor directly between pins 8 and 5.
Grounding
The high current capability of the MIC4420/4429 demands
careful PC board layout for best performance Since the
MIC4429 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 MIC4429 input structure includes 300mV
of hysteresis to ensure clean transitions and freedom from
oscillation, but attention to layout is still recommended.
Figure 3 shows the feedback effect in detail. As the MIC4429
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 MIC4429 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 MIC4429 GND
pins will ensure full logic drive to the input and ensure fast
output switching. Both of the MIC4429 GND pins should,
however, still be connected to power ground.
+15
(x2) 1N4448
5.6 kΩ
560 Ω
0.1µF
50V
+
1
2
1µF
50V
MKS2
6, 7
+
BYV 10 (x 2)
8
0.1µF
WIMA
MKS2
MIC4429
4
5
220 µF 50V
+
35 µF 50V
UNIT E D CHE MCON S X E
Figure 3. Self-Contained Voltage Doubler
July 2005
7
M9999-072205
MIC [ MIC GROUP RECTIFIERS ]
