MIC4425 datasheet(10/26 Pages) MICROCHIP

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MIC4423/4/5

DS20006638A-page 10

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2022 Microchip Technology Inc. and its subsidiaries

4.0

APPLICATION INFORMATION

Although the MIC4423/4/5 drivers have been

speciﬁcally constructed to operate reliably under any

practical circumstances. There are, nonetheless,

details of usage that will provide better operation of the

device.

4.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, charging

2000 pF from 0 to 15 volts in 20 ns requires a constant

current of 1.5A. In practice, the charging current is not

constant, and will usually peak at around 3A. In order

to charge the capacitor, the driver must be capable of

drawing this much current, this quickly, from the system

power supply. In turn, this means that as far as the

driver is concerned, the system power supply, as seen

by the driver, must have a very low impedance.

As a practical matter, this means that the power supply

bus must be capacitively bypassed at the driver with at

least 100X the load capacitance in order to achieve

optimum driving speed. It also implies that the

bypassing capacitor must have very low internal

inductance and resistance at all frequencies of interest.

Generally, this means using two capacitors, one a

high-performance low ESR ﬁlm, the other a low internal

resistance ceramic, as together the valleys in their two

impedance curves allow adequate performance over a

broad enough band to get the job done. Many ﬁlm

capacitors can be sufﬁciently inductive as to be useless

for this service. Likewise, many multilayer ceramic

capacitors have unacceptably high internal resistance.

Use capacitors intended for high pulse current service.

The high pulse current demands of capacitive drivers

also mean that the bypass capacitors must be mounted

very close to the driver in order to prevent the effects of

lead inductance or PCB land inductance from nullifying

what you are trying to accomplish. For optimum results

the sum of the lengths of the leads and the lands from

the capacitor body to the driver body should total

2.5 cm or less.

Bypass capacitance, and its close mounting to the

driver serves two purposes. Not only does it allow

optimum performance from the driver, it minimizes the

amount of lead length radiating at high frequency

during switching, (due to the large ΔI) thus minimizing

the amount of EMI later available for system disruption

and subsequent cleanup. It should also be noted that

the actual frequency of the EMI produced by a driver is

not the clock frequency at which it is driven, but is

related to the highest rate of change of current

produced during switching, a frequency generally one

or two orders of magnitude higher, and thus more

difﬁcult to ﬁlter if you let it permeate your system.

Good

bypassing practice is essential for proper

operation of high speed driver ICs.

4.2

Grounding

Both proper bypassing and proper grounding are

necessary for optimum driver operation. Bypassing

capacitance only allows a driver to turn the load ON.

Eventually (except in rare circumstances) it is also

necessary to turn the load OFF. This requires attention

to the ground path. Two things other than the driver

affect the rate at which it is possible to turn a load off:

The adequacy of the grounding available for the driver,

and the inductance of the leads from the driver to the

load. The latter will be discussed in a separate section.

Best practice for a ground path is obviously a well laid

out ground plane. However, this is not always practical,

and a poorly-laid out ground plane can be worse than

none.Attention to the paths taken by return currents

even in a ground plane is essential. In general, the

leads from the driver to its load, the driver to the power

supply, and the driver to whatever is driving it should all

be as low in resistance and inductance as possible. Of

the three paths, the ground lead from the driver to the

logic driving it is most sensitive to resistance or

inductance, and ground current from the load are what

is most likely to cause disruption. Thus, these ground

paths should be arranged so that they never share a

land, or do so for as short a distance as is practical.

To illustrate what can happen, consider the following:

The inductance of a 2 cm long land, 1.59 mm (0.062")

wide on a PCB with no ground plane is approximately

45 nH. Assuming a dl/dt of 0.3 A/ns (which will allow a

current of 3A to ﬂow after 10 ns, and is thus slightly

slow for our purposes) a voltage of 13.5 volts will

develop along this land in response to our postulated

ΔI. For a 1 cm land, (approximately 15 nH) 4.5 volts is

developed. Either way, anyone using TTL level input

signals to the driver will ﬁnd that the response of their

driver has been seriously degraded by a common

ground path for input to and output from the driver of

the given dimensions. Note that this is before

accounting for any resistive drops in the circuit. The

resistive drop in a 1.59 mm (0.062") land of 2 oz.

Copper carrying 3A will be about 4 mV/cm (10 mV/in)

at DC, and the resistance will increase with frequency

as skin effect comes into play.

The problem is most obvious in inverting drivers where

the input and output currents are in phase so that any

attempt to raise the driver’s input voltage (in order to

turn the driver’s load off) is countered by the voltage

developed on the common ground path as the driver

attempts to do what it was supposed to. It takes very

little common ground path, under these circumstances,

to alter circuit operation drastically.

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