MIC4425 datasheet(12/26 Pages) MICROCHIP

12 / 26 page

MIC4423/4/5

DS20006638A-page 12



2022 Microchip Technology Inc. and its subsidiaries

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. TD2, for example,

may 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.

4.6

Power Dissipation

CMOS circuits usually permit the user to ignore power

dissipation. Logic families such as 4000 series and

74Cxxx have outputs which can only source or sink a

few milliamps of current, and even shorting the output

of the device to ground or VCC may not damage the

device. CMOS drivers, on the other driver 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 dissipation 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 in the

Section 2.0 “Typical Performance

Curves” furnished with this data sheet aid in

estimating power dissipation in the driver. Operating

frequency, power supply voltage, and load all affect

power dissipation.

Given the power dissipation in the device, and the

thermal resistance of the package, junction operating

temperature for any ambient is easy to calculate. For

example, the thermal resistance of the 8-pin plastic DIP

package, from the data sheet, is 150°C/W. In a 25°C

ambient, then, using a maximum junction temperature

of 150°C, this package will dissipate 960 mW.

Accurate power dissipation numbers can be obtained

by summing the three sources of power dissipation in

the device:

• Load power dissipation (PL)

• Quiescent power dissipation (PQ)

• Transition power dissipation (PT)

Calculation of load power dissipation differs depending

on whether the load is capacitive, resistive or inductive.

4.7

Resistive Load Power Dissipation

Dissipation caused by a resistive load can be

calculated in the following Equation 4-1:

EQUATION 4-1:

4.8

Capacitive Load Power

Dissipation

Dissipation caused by a capacitive load is simply the

energy placed in, or removed from, the load

capacitance by the driver. The energy stored in a

capacitor is described in the following Equation 4-2:

EQUATION 4-2:

As this energy is lost in the driver each time the load is

charged or discharged, for power dissipation

calculations the 1/2 is removed. This equation also

shows that it is good practice not to place more voltage

in the capacitor than is necessary, as dissipation

increases as the square of the voltage applied to the

capacitor. For a driver with a capacitive load.

EQUATION 4-3:

PL I

2 R D



Where:

I =

The current drawn by the load

RO = The output resistance of the driver when the

output is high, at the power supply voltage

used (See

Section 2.0 “Typical Perfor-

mance Curves”)

D =

Fraction of time the load is conducting (duty

cycle)

1 2



C V2



PL f C VS

 2



Where:

f =

Operating frequency

C =

Load capacitance

VS = Driver supply voltage

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