MIC4423/4424/4425

Micrel, Inc.

MIC4423/4424/4425

8

July 2005

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

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

characteristicimpedanceofabout50Ω.Halfthatwidthsuffices

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

matching with a MIC4423/24/25 driver, on a 1.59mm (0.062")

board a land width of 42.75mm (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 tradeoff point between lands and wires comes

when lands narrower than 3.18mm (0.125") would be required

on a 1.59mm (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 trans-

formers 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.

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 noninductive 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.

Input Stage

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

MOSFET class A stage with an input capacitance of ≤38pF.

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 –2mA 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

~400mV 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 compat-

ible 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 addi-

tion to providing 2kV or more of ESD protection, also works to

prevent latchup 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 excur-

sions 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 30mA can be conducted through the driver from the

input terminal to its power supply terminal. This may overload

the output of whatever is driving the driver, and may cause

other devices that share the driver’s power supply, as well as

the driver, to operate when they are assumed to be off, but

it will not harm the driver itself. Excessive input voltage will

also slow the driver down, and result in much longer internal

propagation delays within the drivers. T

increase to several hundred nanoseconds. In general, while

the driver will accept this sort of misuse without damage,

proper termination of the line feeding the driver so that line

spiking and ringing are minimized, will always result in faster

and more reliable operation of the device, leave less EMI to

be filtered elsewhere, be less stressful to other components

in the circuit, and leave less chance of unintended modes of

operation.

Power Dissipation

CMOS circuits usually permit the user to ignore power dis-

sipation. Logic families such as 4000 series and 74Cxxx have

outputs which can only source or sink a few milliamps of cur-

rent, and even shorting the output of the device to ground or

V

hand, are intended to source or sink several Amps of current.

This is necessary in order to drive large capacitive loads at

frequencies into the megahertz range. Package power dis-

sipation of driver ICs can easily be exceeded when driving

large loads at high frequencies. Care must therefore be paid

to device dissipation when operating in this domain.

The Supply Current vs Frequency and Supply Current vs

Load characteristic curves furnished with this data sheet

aid in estimating power dissipation in the driver. Operating