MIC4425 datasheet(11/26 Pages) MICROCHIP

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DS20006638A-page 11

MIC4423/4/5

4.3

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 4 cm

(2") are involved, requires treating the output leads as

a transmission line. Unfortunately, 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 ground pin of the driver directly to the

ground terminal of the load. Do not use a twisted pair

where the second wire in the pair is the output of the

other driver, as this will not provide a complete current

path for either driver. Likewise, do not use a twisted

triad with two outputs and a common return unless both

of the loads to be driver are mounted extremely close

to each other, and you can guarantee that they will

never be switching at the same time.

For output leads on a printed circuit, the general rule is

to make them as short and as wide as possible. The

lands should also be treated as transmission lines: i.e.

minimize sharp bends, or narrowings in the land, as

these will cause ringing. For a rough estimate, on a

1.59 mm (0.062") thick G-10 PCB a pair of opposing

lands each 2.36 mm (0.093") wide translates to a

characteristic impedance of about 50Ω. Half that width

sufﬁces on a 0.787 mm (0.031") thick board. For

accurate impedance matching with a MIC4423/24/25

driver, on a 1.59 mm (0.062") board a land width of

42.75 mm (1.683") would be required, due to the low

impedance of the driver and (usually) its load. This is

obviously impractical under most circumstances.

Generally the trade-off point between lands and wires

comes when lands narrower than 3.18 mm (0.125")

would be required on a 1.59 mm (0.062") board.

To obtain minimum delay between the driver and the

load, it is considered best to locate the driver as close

as possible to the load (using adequate bypassing).

Using matching transformers at both ends of a piece of

coax, or several matched lengths of coax between the

driver and the load, works in theory, but is not optimum.

4.4

Driving at Controlled Rates

Occasionally there are situations where a controlled

rise or fall time (which may be considerably longer than

the normal rise or fall time of the driver’s output) is

desired for a load. In such cases it is still prudent to

employ best possible practice in terms of bypassing,

grounding and PCB layout, and then reduce the

switching speed of the load (not the driver) by adding a

non-inductive series resistor of appropriate value

between the output of the driver and the load. For

situations where only rise or only fall should be slowed,

the resistor can be paralleled with a fast diode so that

switching in the other direction remains fast. Due to the

Schmitt trigger action of the driver’s input it is not

possible to slow the rate of rise (or fall) of the driver’s

input signal to achieve slowing of the output.

4.5

Input Stage

The input stage of the MIC4423/24/25 consists of a

single-MOSFET class A stage with an input

capacitance of ≤38 pF.

This capacitance represents the maximum load from

the driver that will be seen by its controlling logic. The

drain load on the input MOSFET is a –2 mA current

source. Thus, the quiescent current drawn by the driver

varies, depending on the logic state of the input.

Following the input stage is a buffer stage which

provides ~400 mV of hysteresis for the input, to prevent

oscillations when slowly-changing input signals are

used or when noise is present on the input. Input

voltage switching threshold is approximately 1.5V

which makes the driver directly compatible with TTL

signals, or with CMOS powered from any supply

voltage between 3V and 15V.

The MIC4423/24/25 drivers can also be driven directly

by the SG1524/25/26/27, TL494/95, TL594/95,

NE5560/61/62/68, TSC170, MIC38C42, and similar

switch mode power supply ICs. By relocating the main

switch drive function into the driver rather than using

the somewhat limited drive capabilities of a PWM IC.

The PWM IC runs cooler, which generally improves its

performance and longevity, and the main switches

switch faster, which reduces switching losses and

increase system efﬁciency.

The input protection circuitry of the MIC4423/24/25, in

addition to providing 2 kV or more of ESD protection,

also works to prevent latch-up or logic upset due to

ringing or voltage spiking on the logic input terminal. In

most CMOS devices when the logic input rises above

the power supply terminal, or descends below the

ground terminal, the device can be destroyed or

rendered inoperable until the power supply is cycled

OFF and ON. The MIC4423/24/25 drivers have been

designed to prevent this. Input voltages excursions as

great as 5V below ground will not alter the operation of

the device. Input excursions above the power supply

voltage will result in the excess voltage being

conducted to the power supply terminal of the IC.

Because the excess voltage is simply conducted to the

power terminal, if the input to the driver is left in a high

state when the power supply to the driver is turned off,

currents as high as 30 mA can be conducted through

the driver from the input terminal to its power supply

terminal. This may overload the output of whatever is

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