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CA3282

Octal Low Side Power Driver with Serial Bus Control

Intersil 

CA3282

For the CA3282, the maximum positive output current rating

is 1A when one output is ON. When ALL outputs are ON, the

rating is reduced to 0.625A because the total maximum cur-

rent is limited to 5A. For any given application, all output driv-

ers on a chip may or may not have a different level of

loading. The discussion here is intended to provide relatively

simple methods to determine the maximum dissipation and

current ratings as a general solution and, as a special solu-

tion, when all switched ON outputs have the same current

loading.

General Solution

A general equation for dissipation should specify that the

total power dissipation in a package is the sum of all signifi-

cant elements of dissipation on the chip. However, in Power

BiMOS Circuits very little dissipation is needed to control the

logic and predriver circuits on the chip. The over-all chip dis-

sipation is primarily the sum of the I2R dissipation losses in

each channel where the current, I is the output current and

the resistance, R is the NMOS channel resistance, rDS(ON)

of each output driver. As such, the total dissipation, PD for n

output drivers is:

n

PD = ∑ Pk

k=1

(EQ. 1)

This expression sums the dissipation, PK of each output

driver without regard to uniformity of dissipation in each

MOS channel. The dissipation loss in an NMOS channel is:

Pk = I2 × rDS(ON)

(EQ. 2)

where the current, I is determined by the output load when

the channel is turned ON. The channel resistance, rDS(ON)

is a function of the circuit design, level of gate voltage and

the chip temperature. Refer to the Electrical Specifications

values for worse case channel resistance.

The temperature rise in the package due to the dissipation is

the product of the on-chip dissipation, PD and the package

Junction-to-Case thermal resistance, θJC. To determine the

junction temperature, TJ, given the case (heat sink tab)

temperature, TC, the linear heat flow solution is:

TJ = TC + PD × θJC

(EQ. 3)

or

TC = TJ – PD × θJC

(EQ. 3A)

Since this solution relates only to the package, further

consideration must be given to a practical heat sink. The

equation of linear heat flow assumes that the Junction-to-

Ambient thermal resistance, θJA, is the sum of the thermal

resistance from Junction-to-Case and the thermal resistance

from Case (heat sink)-to-Ambient, θCA. The Junction-to-

Ambient thermal resistance, θJA is the sum of all thermal

paths from the chip junction to the ambient temperature (TA)

environment and can be expressed as:

Equation 3 and Equation 3A may be expressed as:

TJ = TA + PD × θJA

(EQ. 5)

or

TA = TJ – PD × θJA

(EQ. 5A)

Not all Integrated Circuit packages have a directly definable

case temperature because the heat is spread thru the lead

frame to a PC Board which is the effective heat sink.

Calculation Example 1

For the CA3282, θJC = 3oC/W and the worst case junction

temperature, as an application design solution, should not

exceed 150oC. For any given application, Equation 1 deter-

mines the dissipation, PD.

Assume the package is mounted to a heat sink having a

thermal resistance of 6oC/W and, for a given application, the

dissipation, PD

perature, TA =

= 3W.

100oC.

Assume the operating ambient tem-

The calculated Junction-to-Ambient

thermal resistance is:

θJA = θJC + θCA = 9oC/W

The solution for junction temperature by Equation 5 is :

TJ = 100oC + 3W x 9oC/W = 127oC

Calculation Example 2

Using the CA3282 maximum Junction-to-Ambient Thermal

Resistance, θJA value of

the worst case Junction

45oC/W (no external heat sink) and

Temperature, TC of 150oC we have

an application design solution for the maximum ambient

temperature or dissipation. For example; Using Equation 1

and assuming a device dissipation, PD of 1W, the maximum

allowable Ambient Temperature, TA from Equation 5A is

calculated as follows:

TA = 150oC - 1.0W x 45oC/W = 105oC

Equal Current Loading Solution

Where a given application has equal current loading in the

output drivers, equal rDS(ON) and temperature conditions

may be assumed. As such, a convenient method to show

rating boundaries is to substitute the dissipation Equation 2

into the junction temperature Equation 3. For m outputs that

are ON with equal currents, where I = I1 = I2..... = Im, we

have the following solution for dissipation:

PD = m × Pk = m × I2 × rDS(ON)

(EQ. 6)

I = m-------×-----θ---J--T--C--J---×-–----r-T--D--C--S-----(--O-----N-----)

(EQ. 7)

θJA = θJC + θCA

(EQ. 4)

7

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