Noty an2f Linear Technology
Application Note 2
Performance Enhancement Techniques for
Three-Terminal Regulators
Jim Williams
August 1984
Three terminal regulators provide a simple, effective solution to voltage regulation requirements. In many situations
the regulator can be used with no special considerations.
Some applications, however, require special techniques
to enhance the performance of the device.
Probably the most common modifi cation involves extending the output current of regulators. Conceptually, the
simplest way to do this is by paralleling devices. In practice,
the voltage output tolerance of the regulators can cause
problems. Figure 1 shows a way to use two regulators to
achieve an output current equal to their sum. This circuit
capitalizes on the 1% output tolerance of the specifi ed
regulators to achieve a simple paralleled confi guration.
Both regulators sense from the same divider string and
the small value resistors provide ballast to account for the
slightly differing output voltages. This added impedance
degrades total circuit regulation to about 1%.
Figure 2 shows another way to extend current capability
in a regulator. Although this circuit is more complex than
Figure 1, it eliminates the ballasting resistor’s effects
and has a fast-acting logic-controlled shutdown feature.
Additionally, the current limit may be set to any desired
®
value. This circuit extends the 1A capacity of the LT
1005
multifunction regulator to 12A, while retaining the LT1005’s
enable feature and auxiliary 5V output. Q1, a booster
transistor, is servo-controlled by the LT1005, while Q2
senses the current dependent voltage across the 0.05Ω
shunt. When the shunt voltage is large enough, Q2 comes
on, biasing Q3 and shutting down the regulator via the
LT1005’s enable pin. The shunt’s value can be selected
for the desired current limit. The 100°C thermo-switch
limits dissipation in Q1 during prolonged short circuits
by disabling the LT1005. It should be mounted on Q1’s
heat sink.
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LT1083
IN OUTVIN ≥ 6.5V
ADJ
LT1083
IN
+
100μF
OUT
ADJ
Figure 1
0.01Ω
0.01Ω
121Ω
365Ω
NOTE: THIS CIRCUIT
WILL NOT WORK WITH
LM-TYPE DEVICES
5V
15A
UPDATE
The LT3080 and LT3083
are better for parallel operation
+
200μF
AN02 F01
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AN2-1
Application Note 2
Boosted regulator schemes of this type are often poorly
dynamically damped. Such improper loop compensation
results in large output transients for shifts in the load. In
particular, because Q1’s common emitter confi guration
has voltage gain, transients approaching the input voltage
are possible when the load drops out. Here, the 100μF
capacitor damps Q1’s tendency to overshoot, while the
20Ω value provides turn-off bias. The 250μF unit maintains
Q1’s emitter at DC. Figure 3 shows that this “brute force”
compensation works quite well. Normally the regulator
sees no load. When Trace A goes high, a 12A load (regulator output current is Trace C) is placed across the output
terminals. The regulator output voltage recovers quickly,
with minimal aberration.
250μF
+
Q1
2N4398
8.5 MIN
INPUT
ENABLE
“LO”
0.05Ω*
Q2
2N2907
1k
(HEAT SINK)
20Ω
1k
AUXILIARY ENABLE
10k
10k
1k
IN OUT
LT1005 GND
10k
Q3
2N2222
100°C N.0.
THERMO-SWITCH
ON HEAT SINK
While the 100μF output capacitor aids stability, it prevents
the regulator output from dropping quickly when the enable
command is given. Because Q1 cannot sink current, the
100μF unit’s discharge time is load limited. Q4 corrects
this problem, even when there is no load. When the enable
command is given (Trace A, Figure 4) Q3 comes on, cutting off the LT1005 and forcing Q1 off. Simultaneously, Q4
comes on, pulling down the regulator output (Trace B), and
sinks the 100μF capacitor’s discharge current (Trace C). If
fast turn-off is not needed, Q4 may be omitted.
OUTPUT
+
0.05Ω 100μF
Q4
2N6387
*SELECT FOR I LIMIT = 12A
AN02 F02
5V
12A
A = 10V/DIV
B = 0.5V/DIV
AC-COUPLED
C = 5A/DIV
AN2-2
HORIZONTAL = 10μs/DIV
Figure 3
AN02 F03
Figure 2
A = 10V/DIV
B = 2V/DIV
C = 2A/DIV
HORIZONTAL = 100μs/DIV
Figure 4
AN02 F04
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Application Note 2
Power dissipation control is another area where regulators
can be helped by additional circuitry. Increasing heat sink
area can be used to offset dissipation problems, but is a
wasteful and ineffi cient approach. Instead, the regulator can
be placed within a switched-mode loop that servo-controls
the voltage across the regulator. In this arrangement the
regulator functions normally while the switched-mode control loop maintains the voltage across it at a minimal value,
regardless of line or load changes. Although this approach
is not quite as effi cient as a classical switching regulator,
it offers lower noise and the fast transient response of
the linear regulator. Figure 5 details a DC driven version
2.2k
Q1
28V
2N6667
INPUT
10k
1k
1N4003
68pF
1MHY
28V
LT1018
1M
+
4500
V
Z
IN
LT1004
1.2
LT1004
2.5
15k
+
10k
–
of the circuit. The LT350A functions in the conventional
fashion, supplying a regulated output at 3A capacity. The
remaining components form the switched-mode dissipation limiting control. This loop forces the potential across
the LT350A to equal the 3.7V value of V
. When the input
Z
of the regulator (Trace A, Figure 6) decays far enough, the
LT1018 output (Trace B) switches low, turning on Q1 (Q1
collector is Trace D). This allows current fl ow (Trace C)
from the circuit input into the 4500μF capacitor, raising
the regulator’s input voltage. When the regulator input
rises far enough, the comparator goes high, Q1 cuts off
and the capacitor ceases charging.
V
Z
LT350A
*1% FILM RESISTOR
1MHY = DALE TD-5 TYPE
OUT
ADJ
2.0k
240Ω* 15k
OUTPUT
10k
UPDATE
The LT3083 allows adjustment
to zero. Various single chip
switching regulators can be used
AN02 F05
A = 100mV/DIV
AC-COUPLED ON
15.7V DC LEVEL
B = 50V/DIV
C = 4A/DIV
D = 20V/DIV
Figure 5
HORIZONTAL = 100μs/DIV
Figure 6
AN02 F06
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AN2-3
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