DATA SHEET

KH561

12

REV. 1A January 2004

Figure 10: Thermal Model

delta V’s. For bipolar swings, the two powers for each

output polarity are developed as shown above then

ratioed by the duty cycle. Having the total internal power,

as well as its component parts, the maximum junction

temperature may be computed as follows.

θ

output transistor junction temperature

hottest internal junction temperature

The Limiting Factor for Output Power is Maximum

Junction Temperature

Reducing

θ

ca

through

either

heatsinking

and/or

airflow can greatly reduce the junction temperatures.

One effective means of heatsinking the KH561 is to use

a thermally conductive pad under the part from the pack-

age bottom to a top surface ground plane on the compo-

nent side. Tests have shown a

θ

using a “Sil Pad” available from Bergquist (800-347-

4572).

As an example of calculating the maximum internal junc-

tion temperatures, consider the circuit of Figure 1 driving

±2.5V, 50% duty cycle, square wave into a 50

Ω load.

here since the 50% duty cycle output splits the power

evenly between the two halves of the circuit whereas the

total powers were used to get case temperature.

Even with the output current internally limited to 250mA,

the KH561’s short circuiting capability is principally a

thermal issue. Generally, the KH561 can survive short

duration shorts to ground without any special effort. For

protection against shorts to the ±15 volt supply voltages,

it is very useful to reduce some of the voltage across the

output stage transistors by using some external output

Evaluation Board

An evaluation board (part number 730019) for the KH561

is available.

R50

410

5

51

45.6

I

2.5V / 45.6

54.9mA

I

54.9mA

54.9mA

.06

68.1mA

P

68.1mA 15

2.5

0.7 15.3

68.1mA

733mW

total power in both sides of the output stage

P

2 68.1mA 15 1.4 17.3

68.1mA

169mW

total power in both sides of hottest

eq

o

T

1

2

22

T

q

=

⋅

−









=

=

=+









=

=−

−

−

⋅

=⋅

−

−

⋅

Ω

Ω

Ω

Ω

Ω

Ω

0.

junctions

junctions

prior to output stage

P

1.3 15

2 68.1mA

54.9mA 19.2mA

733mW 169mW

1.058W

power in the remainder of circuit

With these powers and T

25 C and

35 C / W

T

25 C

.733 .169 1.058 35

94 C

case temperature

From this, the hottest internal junctions may be found as

T t

94 C

.733 20

101

circuit

Aca

c

j

1

2

=⋅

−

+

[]

−−

=

=°

=

°

=° +

+

+

°

θ

C

C output stage

T q

94 C

.169 200

111 C

hottest internal junction

j

1

2

°

Ambient

Temperature

200

°C/W

20

°C/W

Case Temperature

Case to Ambient

Termal Impedance

I

V / R

total output current

with R

R

RA

A1

total load

I

I

I

.06

total internal output stage current

P

I

V

1.4 17.3

I output stage power

P

.2 I

V

V

0.7 15.3

I

power in hottest internal junction

prior

oo

eq

eq

L

f

L

L

t

1

2

oo

2

2

tt

CC

t

qt

CC

ot

=

=

−













=+

+

()









=⋅

−

−

⋅

()

=⋅ ⋅

−

−

−

⋅

()

Ω

Ω

0

to

to output stage

P

1.3 V

2 I

I

19.2mA

P

P

power in remainder of circuit [note V

| V

|]

circuit

CC

to

t

q

CC

CC

=⋅

⋅

⋅ −

+

=−