July 2005

7

MIC4423/4424/4425

MIC4423/4424/4425

Micrel, Inc.

Application Information

Although the MIC4423/24/25 drivers have been specifically

constructed to operate reliably under any practical circum-

stances, there are nonetheless details of usage which will

provide better operation of the device.

Supply Bypassing

Charging and discharging large capacitive loads quickly

requires large currents. For example, charging 2000pF from

0 to 15 volts in 20ns 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. PLEASE NOTE

that 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 (in-house

we use WIMA™ film capacitors andAVX Ramguard™ ceram-

ics; several other manufacturers of equivalent devices also

exist). 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 2.5cm or less.

Bypasscapacitance,anditsclosemountingtothedriverserves

two purposes. Not only does it allow optimum performance

from the driver, it minimizes the amount of lead length radiat-

ing at high frequency during switching, (due to the large Δ I)

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 switch-

ing, 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 to proper

operation of high speed driver ICs.

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-laidoutgroundplanecanbeworsethannone.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 2cm long land, 1.59mm (0.062") wide on a

PCB with no ground plane is approximately 45nH. Assum-

ing a dl/dt of 0.3A/ns (which will allow a current of 3A to flow

after 10ns, and is thus slightly slow for our purposes) a volt-

age of 13.5 Volts will develop along this land in response to

our postulated ∆Ι. For a 1cm land, (approximately 15nH) 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 1.59mm (0.062") land of

2oz. Copper carrying 3A will be about 4mV/cm (10mV/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 com-

mon 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.

Output Lead Inductance

The same descriptions just given for PCB land inductance

apply equally well for the output leads from a driver to its load,

except that commonly the load is located much further away

from the driver than the driver’s ground bus.

Generally, the best way to treat the output lead inductance

problem, when distances greater than 4cm (2") are involved,

requires treating the output leads as a transmission line. Un-

fortunately, as both the output impedance of the driver and the

input impedance of the MOSFET gate are at least an order of

magnitude lower than the impedance of common coax, using

coax is seldom a cost-effective solution. A twisted pair works

about as well, is generally lower in cost, and allows use of a

wider variety of connectors. The second wire of the twisted

pair should carry common from as close as possible to the