MIC4421/4422

Micrel

5-48

April 1998

Applications 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 MIC4421/4422 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.

Figure 3. Direct Motor Drive

Figure 4. Self Contained Voltage Doubler

30

29

28

27

26

25

0

50

100 150 200 250 300 350

mA

12

Ω LINE

OUTPUT VOLTAGE vs LOAD CURRENT

DRIVE

LOGIC

Ø1 DRIVE SIGNAL

CONDUCTION ANGLE

CONTROL 0

° TO 180°

CONDUCTION ANGLE

CONTROL 180

° TO 360°

MIC4451

VS

1µF

VS

MIC4452

VS

1µF

VS

Ø1

M

Ø3

Ø2

PHASE 1 of 3 PHASE MOTOR

DRIVER USING MIC4420/4429

MIC4421

1µF

50V

MKS 2

UNITED CHEMCON SXE

0.1µF

WIMA

MKS 2

1

8

6, 7

5

4

0.1µF

50V

5.6 k

Ω

560

Ω

+15

560µF 50V

BYV 10 (x 2)

100µF 50V

(x2) 1N4448

2

+

+

+

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 1

µF

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 MIC4421/4422 demands

careful PC board layout for best performance. Since the

MIC4421 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 MIC4421 input structure includes about

200mV of hysteresis to ensure clean transitions and free-

dom from oscillation, but attention to layout is still recom-

mended.

Figure 5 shows the feedback effect in detail. As the MIC4421

input begins to go positive, the output goes negative and

several amperes of current flow in the ground lead. As little

as 0.05

Ω of PC trace resistance can produce hundreds of

millivolts at the MIC4421 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 MIC4421 GND

pins will ensure full logic drive to the input and ensure fast

output switching. Both of the MIC4421 GND pins should,

however, still be connected to power ground.