Datasheet

14
FN957.10
July 11, 2005
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 3kΩ 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 200µV 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
20Ω 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., 5µs/Div.
Bottom Trace: Collector of load switching transistor, load = 1A;
5V/Div., 5µs/Div.
FIGURE 18B. TRANSIENT RESPONSE
FIGURE 18. WAVEFORMS OF DYNAMIC CHARACTERISTICS OF POWER SUPPLY CURRENTS SHOWN IN FIGURES 16 AND 17
1
3
75Ω
3kΩ
100Ω
2
1kΩ 1kΩ
D2201
CURRENT
LIMITING
ADJUST
2N6385
POWER DARLINGTON
2
1kΩ
1
3
8
2N2102
1kΩ
+30V
INPUT
4
CA3140
7
1
6
5
100kΩ
2
3
180kΩ56pF
1kΩ
82kΩ
250µF
+
-
0.01µF
100kΩ
14
10
6
9
8
50kΩ
13
5µF
+
-
12
CA3086
2.2kΩ
3
1
5
4
62kΩ
VO LTAGE
ADJUST
10µF
+
-
2.7kΩ
1kΩ
11
7
2
HUM AND NOISE OUTPUT <200µV
RMS
(MEASUREMENT BANDWIDTH ~10MHz)
LINE REGULATION 0.1%/V
LOAD REGULATION
(NO LOAD TO FULL LOAD)
<0.02%
OUTPUT
0.1 ⇒ 24V
AT 1A
1
2
1kΩ 200Ω
D2201
“FOLDBACK” CURRENT
LIMITER
2N5294
3kΩ
8
2N2102
1kΩ
+30V
INPUT
4
CA3140
7
1
6
5
100kΩ
2
3
180kΩ56pF
1kΩ
82kΩ
250µF
+
-
0.01µF
100kΩ
14
10
6
9
8
50kΩ
13
5µF
+
-
12
CA3086
2.2kΩ
3
1
5
4
62kΩ
VO LTAGE
ADJUST
10µF
+
-
2.7kΩ
1kΩ
11
7
2
HUM AND NOISE OUTPUT <200µV
RMS
(MEASUREMENT BANDWIDTH ~10MHz)
LINE REGULATION 0.1%/V
LOAD REGULATION
(NO LOAD TO FULL LOAD)
<0.02%
OUTPUT ⇒ 0V TO 25V
25V AT 1A
3
100kΩ
“FOLDS BACK”
TO 40mA
100kΩ
CA3140, CA3140A