13

A small heat sink VERSAWATT transistor is used as the

series pass element in the fold back current system, Figure

17, since dissipation levels will only approach 10W. In this

system, the D2201 diode is used for current sampling.

Foldback is provided by the 3k

Ω and 100kΩ divider network

connected to the base of the current sensing transistor.

Both regulators provide better than 0.02% load regulation.

Because there is constant loop gain at all voltage settings, the

regulation also remains constant. Line regulation is 0.1% per

volt. Hum and noise voltage is less than 200

µV as read with a

meter having a 10MHz bandwidth.

Figure 18A shows the turn ON and turn OFF characteristics

of both regulators. The slow turn on rise is due to the slow

rate of rise of the reference voltage. Figure 18B shows the

transient response of the regulator with the switching of a

20

Ω load at 20V output.

FIGURE 16. REGULATED POWER SUPPLY

FIGURE 17. REGULATED POWER SUPPLY WITH “FOLDBACK”

CURRENT LIMITING

5V/Div., 1s/Div.

FIGURE 18A. SUPPLY TURN-ON AND TURNOFF

CHARACTERISTICS

Top Trace: Output Voltage;

200mV/Div., 5

µs/Div.

Bottom Trace: Collector of load switching transistor, load = 1A;

5V/Div., 5

µs/Div.

FIGURE 18B. TRANSIENT RESPONSE

FIGURE 18. WAVEFORMS OF DYNAMIC CHARACTERISTICS OF POWER SUPPLY CURRENTS SHOWN IN FIGURES 16 AND 17

1

3

75

Ω

3k

Ω

100

Ω

2

1k

Ω 1kΩ

D2201

CURRENT

LIMITING

ADJUST

2N6385

POWER DARLINGTON

2

1k

Ω

1

3

8

2N2102

1k

Ω

+30V

INPUT

4

CA3140

7

1

6

5

100k

Ω

2

3

180k

Ω

56pF

1k

Ω

82k

Ω

250

µF

+

-

0.01

µF

100k

Ω

14

10

6

9

8

50k

Ω

13

5

µF

+

-

12

CA3086

2.2k

Ω

3

1

5

4

62k

Ω

VOLTAGE

ADJUST

10

µF

+

-

2.7k

Ω

1k

Ω

11

7

2

HUM AND NOISE OUTPUT <200

(MEASUREMENT BANDWIDTH

LINE REGULATION 0.1%/V

LOAD REGULATION

(NO LOAD TO FULL LOAD)

<0.02%

OUTPUT

0.1

⇒ 24V

AT 1A

1

2

1k

Ω 200Ω

D2201

“FOLDBACK” CURRENT

LIMITER

2N5294

3k

Ω

8

2N2102

1k

Ω

+30V

INPUT

4

CA3140

7

1

6

5

100k

Ω

2

3

180k

Ω

56pF

1k

Ω

82k

Ω

250

µF

+

-

0.01

µF

100k

Ω

14

10

6

9

8

50k

Ω

13

5

µF

+

-

12

CA3086

2.2k

Ω

3

1

5

4

62k

Ω

VOLTAGE

ADJUST

10

µF

+

-

2.7k

Ω

1k

Ω

11

7

2

HUM AND NOISE OUTPUT <200

(MEASUREMENT BANDWIDTH

LINE REGULATION 0.1%/V

LOAD REGULATION

(NO LOAD TO FULL LOAD)

<0.02%

OUTPUT

⇒ 0V TO 25V

25V AT 1A

3

100k

Ω

“FOLDS BACK”

TO 40mA

100k

Ω

CA3140, CA3140A