TC429

DS21416C-page 8

 2003 Microchip Technology Inc.

4.0

APPLICATIONS INFORMATION

4.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, charging

a 2500 pF load to 18V in 25 nsec requires a 1.8A

current from the device's power supply.

To ensure low supply impedance over a wide frequency

range, a parallel capacitor combination is recom-

mended for supply bypassing. Low-inductance ceramic

disk capacitors with short lead lengths (< 0.5 in.)

should be used. A 1 µF film capacitor in parallel with

one or two 0.1 µF ceramic disk capacitors normally

provides adequate bypassing.

4.2

Grounding

The high-current capability of the TC429 demands

careful PC board layout for best performance. Since

the TC429 is an inverting driver, any ground lead

impedance will appear as negative feedback that can

degrade switching speed. The feedback is especially

noticeable with slow rise-time inputs, such as those

produced by an open-collector output with resistor pull-

up. The TC429 input structure includes about 300 mV

of hysteresis to ensure clean transitions and freedom

from oscillation, but attention to layout is still

recommended.

Figure 4-3 shows the feedback effect in detail. As the

TC429 input begins to go positive, the output goes

negative and several amperes of current flow in the

ground lead. A PC trace resistance of as little as 0.05

Ω

can produce hundreds of millivolts at the TC429 ground

pins. If the driving logic is referenced to power ground,

the effective logic input level is reduced and oscillations

may result.

To ensure optimum device performance, separate

ground traces should be provided for the logic and

power connections. Connecting logic ground directly to

the TC429 GND pins ensures full logic drive to the input

and fast output switching. Both GND pins should be

connected to power ground.

FIGURE 4-1:

Inverting Driver Switching

Time Test Circuit.

FIGURE 4-2:

Switching Speed.

0.1 µF

1µF

Input

Output

Input: 100 kHz,

square wave,

4, 5

2

6, 7

1, 8

TC429

Output

Input

+5V

10%

90%

10%

90%

10%

90%

18V

0V

0V

TIME (100ns/DIV)

CL = 2500pF

5V

INPUT

OUTPUT

100ns

TIME (100ns/DIV)

CL = 2500pF

5V

INPUT

OUTPUT

100ns