LM391 datasheet(9/12 Pages) NSC

9 / 12 page

Application Hints (Continued)

Since the free air thermal resistance of the MJE711

MJE721 is 100 CW no heat sink is required Using this

information and equation (6) we can find the minimum value

of RE required to prevent thermal runaway

RE t

100 (30) (0002)

30 a 1

019X

(6)

We must now use the SOA data on the TIP42 TIP41 tran-

sistors to set up the protection circuit Below is the SOA

curve with the 4X and 8X load lines Also shown are the

desired protection lines Note the value of VB is equal to the

supply voltage so we use the formulas in the table

DC SOA of TIP42 TIP41

Transistors

TLH7146 – 8

The data points from the curve are

VM e 60V VB e 23V IL e 3A IL e 7A

Using the dual slope protection formulas

RE e

065

022X

R2 e 1k

R1 e 1k

60b065

065

J 91k

R3 e 1k

7(022) b 065

J 24k

Note that an RE of 022X satisfies equation (6) The final

schematic of this amplifier is below If the output is shorted

the current will be 18A and VCE is 23V Since the input is

AC the average power is

short PD e

(18) (23)

21W

This power is greater than was used in the heat sink calcula-

tions so the transistors will overheat for long-duration

shorts unless a larger heat sink is used

Typical Applications (Continued)

20W-8X 30W-4X Amplifier with 1 Second Turn-ON Delay

TLH7146 – 9

Additional protection for LM391N Schottky diodes and R j 100X

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