LM340.pdf

LM340 Series Three

Terminal Positive

Regulators

National Semiconductor

Application Note 103

George Cleveland

August 1980

INTRODUCTION

The LM340-XX are three terminal 1.0A positive voltage reg-

ulators, with preset output voltages of 5.0V or 15V. The

LM340 regulators are complete 3-terminal regulators requir-

ing no external components for normal operation. However,

by adding a few parts, one may improve the transient re-

sponse, provide for a variable output voltage, or increase

the output current. Included on the chip are all of the func-

tional blocks required of a high stability voltage regulator;

these appear in Figure1.

TL/H/7413±2

FIGURE 2. Simplified Volt Reference

Figure2 illustrates a simplified reference using the predict-

able temperature, voltage, and current relationship of emit-

ter-base junctions.

Assuming J Q1 l J Q2 ,I CQ2 n I BQ2 e I BQ3 ,

Area (emitter Q1) e Area (emitter Q2), and

V BEQ1 e V BEQ3 ,

(1-1)

then

TL/H/7413±1

FIGURE 1. Functional Block of the LM340

The error amplifier is internally compensated; the voltage

reference is especially designed for low noise and high pre-

dictability; and, as the pass element is included, the regula-

tor contains fixed current limiting and thermal protection.

The LM340 is available in either metal can TO-3 or plastic

TO-220 package.

1. CIRCUIT DESIGN

Voltage Reference

Usually IC voltage regulators use temperature-compensat-

ed zeners as references. Such zeners exhibit BV l 6.0V

which sets the minimum supply voltage somewhat above

6.0V. Additionally they tend to be noisy, thus a large bypass

capacitor is required.

V REF j

q ln R2

R3 a V BEQ3

(1-2)

Simplified LM340

In Figure3 the voltage reference includes R1±R3 and Q1±

Q5. Q3 also acts as an error amplifier and Q6 as a buffer

between Q3 and the current source. If the output drops, this

drop is fed back, through R4, R5, Q4, Q5, to the base of Q3.

Q7 then conducts more current re-establishing the output

given by:

V OUT e V REF

R4 a R5

C 1995 National Semiconductor Corporation

TL/H/7413

RRD-B30M115/Printed in U. S. A.

Complete Circuit of the LM340 (Figure4)

Here (J Q2 ,J Q3 ) l (J Q4 ,J Q5 ) and a positive TC D V BE ap-

pears across R6. This is amplified by 17, (R6/R6 e 17) and

is temperature compensated by the V BE of Q6, Q7, Q8 to

develop the reference voltage. R17 is changed to get the

various fixed output voltages.

Short Circuit Protection

A) V IN ±V OUT k 6.0V: There is no current through D2 and

the maximum output current will be given by:

I OUT MAX e

V BEQ14

R16

j 2.2A (T j e 25 § C)

(1-4)

B) V IN ±V OUT l 6.0V: To keep Q16 operating within its

maximum power rating the output current limit must de-

crease as V IN ±V OUT increases. Here D2 conducts and

the drop across R16 is less than V BE to turn on Q14. In

this case I OUT maximum is:

I OUT MAX e

R16

R14

e 0.077 [37.2 b (V IN b V OUT )] (A)

(1-5)

at T j e 25 § C

TL/H/7413±3

FIGURE 3. LM340 Simplified

*Series pass element

Starting up resistor

TL/H/7413±4

FIGURE 4. Complete Circuit of the LM340

V BEQ14 b

[(V IN b V OUT ) b V ZD2 b V BEQ14 ]

R13

Thermal Shut Down

InFigure4the V BE of Q13 is clamped to 0.4V. When the die

temperature reaches approximately a 175 § C the V BE to turn

on Q13 is 0.4V. When Q13 turns on it removes all base

drive from Q15 which turns off the regulator thus preventing

a further increase in die temperature.

Power Dissipation

The maximum power dissipation of the LM340 is given by:

P D MAX e (V IN MAX b V OUT )I OUT MAX a V IN MAX I Q (W)

(1-6)

The maximum junction temperature (assuming that there is

no thermal protection) is given by:

T jM e 36 b 13 I OUT MAX b (V IN b V OUT )

0.0855

load impedance of 0 to 85 X . Using the following definitions

and the notation shown on Figure5,Z OUT and I OUT are:

Q CC /V e Quiescent current change per volt of input/out-

put (pin 1 to pin 2) voltage change of the LM340

L r /V e Line regulation per volt: the change in the LM340

output voltage per volt of input/output voltage

change at a given I OUT .

D I OUT e (Q CC /V) D V OUT a L r /V

R1 D V OUT

(2-1)

Z OUT e D V OUT

D I OUT

(2-2)

Z OUT e

D V OUT

(Q CC /V) D V OUT a (L r /V)

(2-3)

a 25 § C (1-7)

D V OUT

Example:

V IN MAX e 23V, I OUT MAX e 1.0A, LM340T-15.

Equation (1-7) yields: T jM e 200 § C. So the T j max of 150 § C

specified in the data sheet should be the limiting tempera-

ture.

From (1-6) P D j 8.1W. The thermal resistance of the heat

sink can be estimated from:

i s-a e T jMAX b T A

P D

Z OUT e

(2-4)

(L r /V)

(Q CC /V) a

b ( i j-c a i c-s )( § C/W)

(1-8)

The LM340-5.0 data sheet lists maximum quiescent current

change of 1.0 mA for a 7.0V to 25V change in input voltage;

and a line regulation (interpolated for I OUT e 200 mA) of

35 mV maximum for a 7.0V to 25V change in input voltage:

Q CC /V e 1.0 mA

15V

e 55 m A/V

(2-5)

The thermal resistance i j-c (junction to case) of the TO-220

package is 6 § C/W, and assuming a i c-s (case to heat sink)

of 0.4, equation (1-8) yields:

i s-a e 8.4 § C/W

2. CURRENT SOURCE

The circuit shown on Figure5 provides a constant output

current (equal to V OUT /R1 or 200 mA) for a variable

L r /V e 35 mV

18V j 2 mV/V

(2-6)

The worst case change in the 200 mA output current for a

1.0V change in output or input voltage using equation 2-1 is:

D I OUT

1.0V e 55 m A a

2mV

25 X e 135 m A

(2-7)

and the output impedance fora0to85 X change in Z L using

equation 2-4 is:

Z OUT e

e 7.4 k X

(2-8)

2mV

25 X

55 m A a

TL/H/7413±5

Typical measured values of Z OUT varied from 10±12.3 k X ,

or 81±100 m A/V change input or output (approximately

0.05%/V).

3. HIGH CURRENT REGULATOR WITH SHORT CIRCUIT

CURRENT LIMIT

The 15V regulator circuit of Figure 6 includes an external

boost transistor to increase output current capability to

5.0A. Unlike the normal boosting methods, it maintains the

LM340's ability to provide short circuit current limiting and

thermal shut-down without use of additional active compo-

nents. The extension of these safety features to the exter-

nal pass transistor Q1 is based on a current sharing scheme

*Required if regulator is located far from power supply filter

FIGURE 5. Current Source

TL/H/7413±6

*Solid tantalum

Note 1: Current sharing between the LM340 and Q1 allows the extension of short

circuit current limit, safe operating area protection, and (assuming Q1's heat sink

has four or more times the capacity of the LM340 head sink) thermal shutdown

protection.

Note 2: I SHORT CIRCUIT is approximately 5.5 amp.

Note 3: I OUT MAX at V OUT e 15V is approximately 9.5 amp.

FIGURE 6. 15V 5.0A Regulator with Short Circuit Current Limit

using R1, R2, and D1. Assuming the base-to-emitter voltage

of Q1 and the voltage drop across D1 are equal, the voltage

drops across R1 and R2 are equal. The currents through R1

and R2 will then be inversely proportional to their resistanc-

es. For the example shown on Figure 6, resistor R1 will

have four times the current flow of R2. For reasonable val-

ues of Q1 beta, the current through R1 is approximately

equal to the collector current of Q1; and the current through

R2 is equal to the current flowing through the LM340.

Therefore, under overload or short circuit conditions the

protection circuitry of the LM340 will limit its own output

current and, because of the R1/R2 current sharing scheme,

the output current of Q1 as well. Thermal overload protec-

tion also extends Q1 when its heat sink has four or more

times the capacity of the LM340 heat sink. This follows from

the fact that both devices have approximately the same in-

put/output voltage and share the load current in a ratio of

four to one.

The circuit shown on Figure 6 normally operates at up to

5.0A of output current. This means up to 1.0A of current

flows through the LM340 and up to 4.0A flows through Q1.

For short term overload conditions the curve of Figure 7

shows the maximum instantaneous output current versus

temperature for the boosted regulator. This curve reflects

the approximately 2.0A current limit of the LM340 causing

an 8.0A current limit in the pass transistor, or 10A, total.

Under continuous short circuit conditions the LM340 will

heat up and limit to a steady total state short circuit current

of 4.0A to 6.0A as shown in Figure8. This curve was taken

using a Wakefield 680-75 heat sink (approximately

7.5 § C/W) at a 25 § C ambient temperature.

TL/H/7413±8

FIGURE 8. Continuous Short Circuit

Current vs Input Voltage

For optimum current sharing over temperature between the

LM340 and Q1, the diode D1 should be physically located

close to the pass transistor on the heat sink in such a man-

ner as to keep it at the same temperature as that of Q1. If

the LM340 and Q1 are mounted on the same heat sink the

LM340 should be electrically isolated from the heat sink

since its case (pin 3) is at ground potential and the case of

Q1 (its collector) is at the output potential of the regulator.

Capacitors C1 and C2 are required to prevent oscillations

and improve the output impedance respectively. Resistor

R3 provides a path to unload excessive base charge from

the base of Q1 when the regulator goes suddenly from full

load to no load. The single point ground system shown on

Figure6 allows the sense pins (2 and 3) of the LM340 to

monitor the voltage directly at the load rather than at some

point along a (possibly) resistive ground return line carrying

up to 5.0A of load current. Figure9 shows the typical varia-

tion of load regulation versus load current for the boosted

regulator. The insertion of the external pass transistor in-

creases the input/output differential voltage from 2.0V to

TL/H/7413±7

FIGURE 7. Maximum Instantaneous

Current vs Junction Temperature

approximately 4.5V. For an output current less than 5.0A,

the R2/R1 ratio can be set lower than 4:1. Therefore, a less

expensive PNP transistor may be used.

Example:

I OVERLOAD e 5.0A

I LED e 40 mA (light intensity of 16 mcd)

V LED e 1.75, R5 j

V IN b 2.65

I LED

(4-1)

5. ADJUSTABLE OUTPUT VOLTAGE REGULATOR FOR

INTERMEDIATE OUTPUT VOLTAGES

The addition of two resistors to an LM340 circuit allows a

non-standard output voltage while maintaining the limiting

features built into IC. The example shown in Figure11 pro-

vides a 10V output using an LM340K-5.0 by raising the ref-

erence (pin number 3) of the regulator by 5.0V.

TL/H/7413±9

FIGURE 9. Load Regulation

4. 5.0V, 5.0A VOLTAGE REGULATOR FOR TTL

The high current 5.0V regulator for TTL shown in Figure10

uses a relatively inexpensive NPN pass transistor with a

lower power PNP device to replace the single, higher cost,

power PNP shown in Figure6. This circuit provides a 5.0V

output at up to 5.0A of load current with a typical load regu-

lation of 1.8% from no load to full load. The peak instanta-

neous output current observed was 10.4A at a 25 § C junction

temperature (pulsed load with a 1.0 ms ON and a 200 ms

OFF period) and 8.4A for a continuous short circuit. The

typical line regulation is 0.02% of input voltage change

(I OUT e 0).

One can easily add an overload indicator using the Nation-

al's new NSL5027 LED. This is shown with dotted lines in

Figure10. With this configuration R2 is not only a current

sharing resistor but also an overload sensor. R5 will deter-

mine the current through the LED; the diode D2 has been

added to match the drop across D1. Once the load current

exceeds 5.0A (1.0A through the LM340 assuming perfect

current sharing and V D1 e V D2 ) Q3 turns ON and the over-

load indicator lights up.

TL/H/7413±11

FIGURE 11. 10V Regulator

The 5.0V pedestal results from the sum of regulator quies-

cent current I Q and a current equal to V REG /R1, flowing

through potenteniometer R2 to ground. R2 is made adjusta-

ble to compensate for differences in I Q and V REG output.

The circuit is practical because the change in I Q due to line

voltage and load current changes is quite small.

The line regulation for the boosted regulator is the sum of

the LM340 line regulation, its effects on the current through

TL/H/7413±10

*Solid tantalum

FIGURE 10. 5.0V, 5.0A Regulator for TTL (with short circuit, thermal shutdown protection, and overload indicator)

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