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Application Note 2

AN2-1

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August 1984

Performance Enhancement Techniques forThree-Terminal RegulatorsJim Williams

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

Three terminal regulators provide a simple, effective solu-tion 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 modi cation involves extend-ing 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 speci ed regulators to achieve a simple paralleled con 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 resistors 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 LT1005 multifunction regulator to 12A, while retaining the LT1005s 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 0.05 shunt. When the shunt voltage is large enough, Q2 comes on, biasing Q3 and shutting down the regulator via the LT1005s enable pin. The shunts value can be selected for the desired current limit. The 100C thermo-switch limits dissipation in Q1 during prolonged short circuits by disabling the LT1005. It should be mounted on Q1s heat sink.

+

LT1083ADJ

IN OUTVIN 6.5V

LT1083ADJ

IN

121 200F

UPDATEThe LT3080 and LT3083are better for parallel operation

AN02 F01

5V15A

0.01

0.01

365 NOTE: THIS CIRCUIT WILL NOT WORK WITH LM-TYPE DEVICES

OUT

+100F

Figure 1

Application Note 2

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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 Q1s common emitter con guration has voltage gain, transients approaching the input voltage are possible when the load drops out. Here, the 100F capacitor damps Q1s tendency to overshoot, while the 20 value provides turn-off bias. The 250F unit maintains Q1s 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 (regula-tor output current is Trace C) is placed across the output terminals. The regulator output voltage recovers quickly, with minimal aberration.

While the 100F output capacitor aids stability, it prevents the regulator output from dropping quickly when the enable command is given. Because Q1 cannot sink current, the 100F units 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, cut-ting off the LT1005 and forcing Q1 off. Simultaneously, Q4 comes on, pulling down the regulator output (Trace B), and sinks the 100F capacitors discharge current (Trace C). If fast turn-off is not needed, Q4 may be omitted.

LT1005 GNDIN OUT

10k

Q32N2222

10k

AUXILIARY ENABLE

100C N.0.THERMO-SWITCH

ON HEAT SINK

20

0.05*

250F

Q12N4398

(HEAT SINK)

1kQ2

2N2907

8.5 MININPUT

ENABLELO

1k

10k

0.05 100F

*SELECT FOR I LIMIT = 12A

AN02 F02

OUTPUT5V12A

Q42N6387

1k

+

+

Figure 2

A = 10V/DIV

B = 0.5V/DIVAC-COUPLED

C = 5A/DIV

HORIZONTAL = 10s/DIV AN02 F03

Figure 3

A = 10V/DIV

B = 2V/DIV

C = 2A/DIV


Figure 4

Application Note 2

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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 inef 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 con-trol loop maintains the voltage across it at a minimal value, regardless of line or load changes. Although this approach is not quite as ef 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

of the circuit. The LT350A functions in the conventional fashion, supplying a regulated output at 3A capacity. The remaining components form the switched-mode dissipa-tion limiting control. This loop forces the potential across the LT350A to equal the 3.7V value of VZ. When the input 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 ow (Trace C) from the circuit input into the 4500F capacitor, raising the regulators input voltage. When the regulator input rises far enough, the comparator goes high, Q1 cuts off and the capacitor ceases charging.

LT350AADJ

IN

LT10041.2

LT10042.5

15k

240* 15k

OUTPUT

10k

AN02 F05

2.0k

28V

1k

10k

28VINPUT

Q12N6667

1M

2.2k

68pF

1N4003

*1% FILM RESISTOR 1MHY = DALE TD-5 TYPE

1MHY

4500

10k

OUT

VZ

VZ

+LT1018

+

UPDATEThe LT3083 allows adjustmentto zero. Various single chipswitching regulators can be used

Figure 5

A = 100mV/DIVAC-COUPLED ON15.7V DC LEVEL

B = 50V/DIV

C = 4A/DIV

D = 20V/DIV


Figure 6

Application Note 2

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The 1N4003 damps the yback spike of the current-limit-ing inductor. The 4.7k unit ensures circuit start-up and the 68pF-1M combination sets loop hysteresis at about 80mVP-P . This free-running oscillation control mode substantially reduces dissipation in the regulator, while preserving its performance. Despite changes in the input voltage, different regulated outputs or load shifts, the loop always ensures the minimum possible dissipation in the regulator.

Figure 7 shows the dissipation limiting technique applied in a more sophisticated circuit. The AC-powered version provides 0V-35V, 10A regulation under high line-low line (90VAC-140VAC) conditions with good ef ciency. In this version, two SCRs and a center-tapped transformer source power to the inductor-capacitor combination. The trans-former output is also diode recti ed (Trace A, Figure 8), divided down, and used to reset the 0.1F unit (Trace B)

+LT1038 OR

LT1083

LT10041.2

LT10042.5

16k*

750* 100F+

+

10,000F1N400343

1N4003

1N4148

82k

*1% FILM RESISTOR T1 = SPRAGUE 11Z-2003SCRs = G.E. C-220B 1MHY = DALE TD-5 TYPE

15V

1F

1N4003

20

110AC

STANCORP-8675

10k

20

1k

tt 2

T11

1MHY 0V-35V0A-10A(7.5A FOR LT1083)

20k

2.7k15V

LT10041.2V

11k*200k

15k

1F

16k*

11k*

AN02 F07

15V

2N3904100pF

15V

15V

10k

0.11

4

7

2

3

15V15V

15V

15V15k

VZ

8

8

3

7

2

15V

+

+

+

4

15V

1

18

C1LT1011

C2LT1011

A1LM301A

VZ

UPDATEParalled LT3083s allowadjustment to zerowithout the LT1004

Figure 7

Application Note 2

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via C1. The resulting AC line synchronous ramp at C1s output is compared to A1s offset output by C2. A1s output represents the deviation from the VZ value that the loop is trying to force across the LT1038. When the ramp output exceeds C2s + input value, C2 pulls low, dumping current through T1s primary (Trace C). This res the appropriate SCR and a path from the main transformer to the LC pair occurs (Trace D). The resultant current ow (Trace E) is limited by the inductor and charges the capacitor. When the AC line cycle drops low enough, the SCR commutates and charging ceases. On the next half cycle the process repeats, except that the alternate SCR does the work. In this fashion, the loop controls the phase angle at which the SCRs re to keep the voltage across the LT1038 at VZ

(3.7V). As a result, the circuit functions over all line, load and output voltage conditions with good ef ciency. The 1.2V LT1004 at the LT1038 allows the output voltage to be set down to 0.00 and the 2N3904 clamp at A1 prevents loop hang-up. Figure 7A shows a way to trigger the SCRs without using a transformer.

Although A1s output is an analog voltage, the AC-driven nature of the circuit makes it approximate a smoothed, sample loop response. Conversely, the regulator consti-tutes a true linear system. Because these two feedback systems are interlocked, frequency compensation can be dif cult.

+

C2

TO 10k-15k JUNCTIONFROM A1 OUTPUT

TOC1 OUTPUT

3

2

10k

2010k

1N4148 1MHY

20

0.68

AN02 F07A

10,000F

15V 15V

TOSCR

GATES

2N2219

1N4148

Figure 7A

A = 50V/DIV

B = 10V/DIV

C = 100mA/DIV

D = 50V/DIV

E = 10A/DIV

HORIZONTAL = 2ms/DIV AN02 F08

Figure 8

Application Note 2

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In practice, A1s 1F capacitor keeps dissipation loop gain at a low enough frequency for stable characteristics, without in uencing the LT1038s transient response char-acteristic. Trace A, Figure 9 shows the output noise while the circuit is operating at 35V into a 10A load (350W). Note the absence of fast switching transients and harmonics. The output noise is made up of residual 120Hz ripple and regulator noise. Re ected noise into the AC power line is also negligible (Trace B) because the inductor limits cur-rent rise time to about 1ms, much slower than the normal switching supplies. Figure 10 shows a plot of ef ciency versus output voltage for a 10A load. At low output volt-ages, where the static losses across the regulator and SCRs are signi cant, ef ciency suffers, but 85% is attained at the upper extreme.

High voltage output is another area for regulator enhance-ment. In theory, because the regulator does not have a ground pin, it can regulate high voltages. In normal opera-tion the regulator oats at the supplys upper level, and as long as the VINVOUT maximum differential is not exceeded there are no problems. However, if the output is shorted, the VINVOUT maximum is exceeded and device destruc-tion will occur. The circuit of Figure 11 shows a complete high voltage regulator that delivers 100V at 100mA and withstands shorts to ground. Even at 100V output the LT317A functions in the normal mode, maintaining 1.2V between its output and adjustment pin.

10mV/DIVAC-COUPLED

ON 35V OUTPUT

200V/DIV

HORIZONTAL = 2ms/DIV AN02 F09

Figure 9

OUTPUT VOLTAGE0

0

EFFI

CIEN

CY (%

)

20

40

60

5 10 15 20

AN02 F10

25

80

100

10

30

50

70

90

30

P = 10W

P = 50W

P = 100W

P = 200WP = 300W

LOAD CURRENT = 10AFOR ALL CONDITIONS

Figure 10

+ LT317AT

1N4004

120V

500F

1N4004

115AC

TRIADN-48X

ADJIN

1N4148 332

10

100VOUTPUT

25.5k

AN02 F11

500pF

2k5W

1N303130V

0.02F

1kOUTPUTADJ

Q12N6533

OUT

UPDATENewer regulators such asthe LT3080 and LT3081allow adjustment to zero

Figure 11

Application Note 2

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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Under these conditions the 30V Zener is off and Q1 con-ducts. When an output short occurs, the Zener conducts, forcing Q1s base to 30V. This causes Q1s emitter to clamp 2 VBEs below VZ, well within the VINVOUT rating of the regulator. Under these conditions, Q1, a high voltage device, sustains 90V VCE at whatever current the transformer and the regulators current limit will support. The transformer speci ed saturates at 130mA, keeping Q1 well within its safe area as it dissipates 12W. If Q1 and the LT317A are thermally coupled, the regulator will soon go into thermal shutdown and oscillation will commence. This action will continue, protecting the load and the regulator as long as the output remains shorted. the 500pF capacitor and the 10-0.02F damper aid transient response and the diodes provide safe discharge paths for the capacitors.

This approach to high voltage regulation is primarily lim-ited by the power dissipation capability of the device in series with the regulator. Figure 11A uses a vacuum tube (remember them?) to achieve very high short-circuit dis-sipation capability. The tube allows high voltage operation and is extremely tolerant of overloads. This circuit allows the LT317A to control 600W at 2000V (V1s plate limit is 300mA) with full short-circuit protection.

Power is not the only area in which regulator performance can be augmented. Figure 12 shows a way to increase the stability of a regulators output over time and temperature. This is particularly useful in powering strain gauge-based transducers. In this circuit the output voltage is divided down and compared to the 2.5V reference by A1, a precision ampli er. A1s output is used to force the LT317As adjust-ment pin to whatever voltage is required to maintain the 10V output. A1 contributes negligible error. The resistors speci ed will track within 5ppm/C and the reference con-tributes about 20ppm/C. The regulators internal circuitry protects against short circuits and thermal overload.

Figure 13s circuit allows a regulator to remotely sense the feedback voltage, eliminating the effects of voltage drop in the supply lines. This is a concern where high currents must be transmitted over relatively long supply rails or PC traces. Figure 13s circuit uses A1 to sense the voltage at the point of load. A1s output, summed with the regulators output, modi es the adjustment pin voltage to compensate for the voltage lost across RDROP . The feedback divider is returned through a separate lead from the load, complet-ing the remote sensing scheme. The 5F capacitor lters noise and the 1k value limits bypass capacitor discharge when power is turned off.

LT317ATADJ

IN

V1

1.2k

1.8M2W

OUTPUT2000V

180k50W

AN02 F11A

1N3031

75-THEIMAC FILIMENT

2500V

500kOUTPUTTRIM

OUT

UPDATEThe LT3085 will allowVOUT to go to zero

LT317AHADJ

2k

INVIN

2k 15k*

4.99k**RESISTORS = TRW MAR-6

OUTPUT10V

AN02 F12

OUT

LT10092.5V

+A1

LT1001

Figure 11A Figure 12

Application Note 2

AN2-8

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Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LINEAR TECHNOLOGY CORPORATION 1986

GP/IM 286 5K PRINTED IN USA

A nal circuit allows voltage regulator-powered circuity to run from 110VAC or 220VAC without having to switch trans-former windings. Regulator dissipation does not increase for 220VAC inputs. In Figure 14, when T1 is driven from 110VAC, the LT1011 output goes high, allowing the SCR to receive gate bias through the 1.2k resistor. The 1N4002 is off. T1s output is recti ed by the SCR and the regulator sees about 8.5V at its input. If T1 is plugged into a 220VAC source, the negative input at the LT1011 is driven beyond 2.5V and the devices output clamps low. This steers the SCRs gate bias to ground through the LT1011s output transistor. The diodes in the LT1011 output line prevent

reverse voltages from reaching the SCR or the LT1011 output. Now, the SCR goes off and the 1N4002 sources current to the regulator from T1s center tap. Although T1s input voltage has doubled, its output potential has halved and the regulator power dissipation remains the same. Figure 15 shows the AC line input versus regulator input voltage transfer function. The switch to center tap drive occurs midway between 110VAC and 220VAC. The hysteresis, a desirable characteristic, occurs because T1s output voltage shifts with the step change in loading.

LT350AADJ

22

VIN

5V AT 3A

18

4

INVIN OUT

+

1k

RLOAD

AN02 F13

RDROP(MAX DROP = 300mV)

365 100pF 25

5F

HIGH CURRENTRETURN

TO GROUND

+

A1LM301A

121

Figure 13

LT1086ADJ

IN

240* 10F

VOUT5V

720*

OUT +

1.6k1k

6.2k

AN02 F14

= 1N4148 UNLESS MARKED

*1% FILM RESISTOR T1 = STACO #SP05A012

1M

5000F

1k

83

21F

LT1009C2.5V

7

1

4

C-106 (G.E.)

1N4002

T1

110-220AC

+

1.2k

+LT1011

UPDATEThe LT3080 regulatorallows VOUT to go to zero

Figure 14

AC LINE VOLTAGERMS0

0

REGU

LATO

R IN

PUT

VOLT

AGE

2

6

8

10

160

18

AN02 F15

4

8040 200 240120 280

12

14

16

Figure 15

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