 2002 Microchip Technology Inc.

DS21416B-page 5

TC429

3.0

APPLICATIONS INFORMATION

3.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, charging

a 2500pF load to 18V in 25nsec 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.

3.2

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 which 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 300mV of

hysteresis to ensure clean transitions and freedom

from

oscillation, but attention to layout is

still

recommended.

Figure 3-3 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. As little as 0.05

Ω of PC trace resistance

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 3-1:

INVERTING DRIVER

SWITCHING TIME

TEST CIRCUIT

FIGURE 3-2:

SWITCHING SPEED

18

45

26

7

0.1

µF

+5V

18V

0V

0V

90%

10%

10%

10%

90%

90%

Input

Input

Output

1

µF

Input: 100kHz,

square wave,

Output

TC429

TIME (100ns/DIV)

CL = 2500pF

5V

INPUT

OUTPUT

100ns

TIME (100ns/DIV)

CL = 2500pF

5V

INPUT

OUTPUT

100ns