MM74C908 查看數據表（PDF） - Fairchild Semiconductor

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MM74C908

Dual CMOS 30-Volt Relay Driver

Fairchild Semiconductor 

Power Considerations

Calculating Output “ON” Resistance (RL > 18Ω)

The output “ON” resistance, RON, is a function of the junc-

tion temperature, TJ, and is given by:

RON = 9 (TJ − 25) (0.008) + 9: (1)

and TJ is given by:

TJ = TA + PDAV θJA,: (2)

where TA = ambient temperature, θJA = thermal resistance,

and PDAV is the average power dissipated within the

device. PDAV consists of normal CMOS power terms (due

to leakage currents, internal capacitance, switching, etc.)

which are insignificant when compared to the power dissi-

pated in the outputs. Thus, the output power term defines

the allowable limits of operation and includes both outputs,

A and B. PD is given by:

PD = IOA2RON + IOB2 RON, (3)

where IO is the output current, given by:

(4)

VL is the load voltage.

The average power dissipation, PDAV, is a function of the

duty cycle:

PDAV = IOA2RON (Duty CycleA) + (5)

IOB2 RON(Duty CycleB)

where the duty cycle is the % time in the current source

state. Substituting equations (1) and (5) into (2) yields:

TJ = TA + θJA [9 (TJ − 25) (0.008) + 9]: (6a)

[IOA2 (Duty CycleA) + IOB2 (Duty CycleB)]

simplifying:

Equations (1), (4), and (6b) can be used in an iterative

method to determine the output current, output resistance

and junction temperature.

For example, let VCC = 15V, RLA = 100Ω, RLB = 100Ω,

VL = 0V, TA = 25°C, θJA = 110°C/W, Duty CycleA = 50%,

Duty CycleB = 75%.

Assuming RON = 11Ω, then:

and

Applications

(See AN-177 for applications)

and RON = 9 (TJ − 25) (0.008) + 9

= 9(52.6 − 25) (0.008) + 9 = 11Ω

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