ICL7667
2
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
7. Output stage I R power 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
dependent, with an I
input and 6mA maximum with a logic 1 input.
(from V to ground) during output transitions. This crossover
CC
of 0.1mA maximum with a logic 0
CC
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 1µs.
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
Application Notes
Although the ICL7667 is simply a dual level-shifting inverter,
there are several areas to which careful attention must be
paid.
be allowed to remain between V and V since this could
IL IH
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
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.
be seen in I
vs Frequency graph in the Typical
CC
Characteristics Graphs.
2
The output stage I R power dissipation is nothing more than
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
Bypas s ing
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 4.7µF
tantalum capacitor in parallel with a low inductance 0.1µF
capacitor is usually sufficient bypassing.
2
P
= CV
CC
f
AC
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
Output Damping
P
= Q V f
G CC
AC
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:
where Q = Charge required to switch the gate, in
G
Coulombs, f = Frequency.
Power MOS Driver Circuits
1. Reduce inductance by making printed circuit board traces
as short as possible.
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
2. Reduce inductance by using a ground plane or by closely
coupling the output lines to their return paths.
3. Use a 10Ω 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. Usegoodbypassingtechniquestopreventsupplyvoltage
ringing.
Power Dis s ipation
The power dissipation of the ICL7667 has three main
components:
capacitance and quickly turns the MOSFET fully on or off.
5. Input inverter current loss
6. Output stage crossover current loss
FN2853.5
5
April 4, 2006
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