July 2005

7

M9999-072205

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

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.

Figure 3. Self-Contained Voltage Doubler

MIC4429

1µF

50V

MKS2

UNITED CHEMCON SXE

0.1µF

WIMA

MKS2

1

8

6, 7

5

4

0.1µF

50V

5.6 kΩ

560 Ω

+15

220 µF 50V

BYV 10 (x 2)

35 µF 50V

(x2) 1N4448

2

+

+

+