3-77

peak current capability of the ICL7667 enables it to drive a

1000pF load with a rise time of only 40ns. Because the

output stage impedance is very low, up to 300mA will flow

through the series N-Channel and P-channel output devices

current is responsible for a significant portion of the internal

power dissipation of the ICL7667 at high frequencies. It can be

minimized by keeping the rise and fall times of the input to the

ICL7667 below 1

µs.

Application Notes

Although the ICL7667 is simply a dual level-shifting inverter,

there are several areas to which careful attention must be

paid.

Grounding

Since the input and the high current output current paths

both include the ground pin, it is very important to minimize

and common impedance in the ground return. Since the

ICL7667 is an inverter, any common impedance will

generate negative feedback, and will degrade the delay, rise

and fall times. Use a ground plane if possible, or use

separate ground returns for the input and output circuits. To

minimize any common inductance in the ground return,

separate the input and output circuit ground returns as close

to the ICL7667 as is possible.

Bypassing

The rapid charging and discharging of the load capacitance

requires very high current spikes from the power supplies. A

parallel combination of capacitors that has a low impedance

over a wide frequency range should be used. A 4.7

µF

tantalum capacitor in parallel with a low inductance 0.1

µF

capacitor is usually sufficient bypassing.

Output Damping

Ringing is a common problem in any circuit with very fast

rise or fall times. Such ringing will be aggravated by long

inductive lines with capacitive loads. Techniques to reduce

ringing include:

1. Reduce inductance by making printed circuit board traces

as short as possible.

2. Reduce inductance by using a ground plane or by closely

coupling the output lines to their return paths.

3. Use a 10

Ω to 30Ω resistor in series with the output of the

ICL7667. Although this reduces ringing, it will also slightly

increase the rise and fall times.

4. Use good bypassing techniques to prevent supply voltage

ringing.

Power Dissipation

The power dissipation of the ICL7667 has three main

components:

5. Input inverter current loss

6. Output stage crossover current loss

The sum of the above must stay within the specified limits for

reliable operation.

As noted above, the input inverter current is input voltage

input and 6mA maximum with a logic 1 input.

The output stage crowbar current is the current that flows

through the series N-Channel and P-channel devices that

form the output. This current, about 300mA, occurs only

during output transitions. Caution: The inputs should never

leave the output stage in a high current mode, rapidly

leading to destruction of the device. If only one of the drivers

is being used, be sure to tie the unused input to a ground.

NEVER leave an input floating. The average supply current

drawn by the output stage is frequency dependent, as can

Characteristics Graphs.

the product of the output current times the voltage drop

across the output device. In addition to the current drawn by

any resistive load, there will be an output current due to the

charging and discharging of the load capacitance. In most

high frequency circuits the current used to charge and

discharge capacitance dominates, and the power dissipation

is approximately

where C = Load Capacitance, f = Frequency

In cases where the load is a power MOSFET and the gate

drive requirement are described in terms of gate charge, the

ICL7667 power dissipation will be

Coulombs, f = Frequency.

Power MOS Driver Circuits

Power MOS Driver Requirements

Because it has a very high peak current output, the ICL7667

the at driving the gate of power MOS devices. The high

current output is important since it minimizes the time the

power MOS device is in the linear region. Figure 9 is a

typical curve of charge vs gate voltage for a power MOSFET.

The flat region is caused by the Miller capacitance, where

the drain-to-gate capacitance is multiplied by the voltage

gain of the FET. This increase in capacitance occurs while

the power MOSFET is in the linear region and is dissipating

significant amounts of power. The very high current output of

the ICL7667 is able to rapidly overcome this high

capacitance and quickly turns the MOSFET fully on or off.

ICL7667