UPC16312 - NEC Electronics - Datasheet.Directory

Document No. G12702EJ8V0UM00 (8th edition)

Date Published May 2000 N CP(K)

Printed in Japan

User’s Manual

Usage of Three-Terminal Regulators

User’s Manual G12702EJ8V0UM00

[MEMO]

User’s Manual G12702EJ8V0UM00 3

The application circuits and the circuit constants in this document are only examples, and not intended for

use in the actual design of application systems for mass-production.

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M7D 98. 12

User’s Manual G12702EJ8V0UM00

CONTENTS

1. INTRODUCTION................................................................................................................................ 5

2. BASIC STRUCTURE OF A POWER SUPPLY IC....................................................................... 5

2.1 Structure of a Bipolar IC........................................................................................................................... 5

2.2 About Power Supply IC Equivalent Circuits.............................................................................................. 6

3. BASIC CIRCUITS OF A POWER SUPPLY IC............................................................................ 7

3.1 Basic Circuits ........................................................................................................................................... 7

3.2 Operating Principles of Adjustable Output Types..................................................................................... 11

3.3 Operating Principles of Low Saturation Types ......................................................................................... 12

4. POWER SUPPLY IC APPLICATION CIRCUITS .......................................................................... 13

4.1 Typical Circuit Connection........................................................................................................................ 13

4.2 Application Circuit Set.............................................................................................................................. 17

5. PRECAUTIONS ON APPLICATION ............................................................................................... 22

5.1 Shorting Input Pins and Ground Pins......................................................................................... .............. 22

5.2 Floating Ground Pins ............................................................................................................................... 22

5.3 Applying Transient Voltage to Input Pins.................................................................................................. 23

5.4 Reverse Bias Between Output Pin and GND Pin..................................................................................... 23

5.5 Precautions Related to Low Saturation Types ......................................................................................... 24

5.6 Thinking on Various Protection Circuits.................................................................................................... 24

6. POWER SUPPLY IC DATA SHEET APPEARANCE AND DESIGN METHODS..................... 24

6.1 Absolute Maximum Ratings...................................................................................................................... 24

6.2 Recommended Operating Conditions ...................................................................................................... 24

6.3 Electrical Specifications................................................................................................... ......................... 25

6.4 Design Methods....................................................................................................................................... 28

User’s Manual G12702EJ8V0UM00 5

1. INTRODUCTION

NEC produces a variety of ICs for power supplies that differ in their on-chip functions and usage. Within these,

large quantities of three-terminal regulators have come to be used to configure stabilized power supplies easily using

few external components.

However, the occurrence of unexpected irregularities when designing power supply circuits also has increased.

Therefore, this manual starts with the basic structure of the main bipolar process that is used in ICs for power

supplies and gives precautions pertaining to actual applications.

2. BASIC STRUCTURE OF A POWER SUPPLY IC

As mentioned in chapter 1, a power supply IC mainly uses a bipolar process. Understanding the structure of an

IC that uses a bipolar process also is useful for applications.

2.1 Structure of a Bipolar IC

The following elements can be made into an IC in a general bipolar process.

NPN transistor

PNP transistor

Resistor

Capacitor

Figures 2-1 through 2-3 show the structure of each.

Figure 2-1. Structure of NPN Transistor and PNP Transistor

Separation region Separation region Separation region

Collector CollectorBase Base

P-type substrate

Emitter Emitter

NPN transistor PNP transistor

pp p

ppp

User’s Manual G12702EJ8V0UM00

Figure 2-2. Structure of Resistor

Separation region Base diffused resistor Base pinch resistor

Pinch

region

Separation region

Separation

region

Epitaxial layer

electrode

Resistor

electrode Resistor

electrode

Resistor

electrode

P-type diffusion

layer P-type diffusion

layer

P-type substrate

pp p

n+ diffusion layer

Figure 2-3. Structure of Capacitor

Separation region Separation region

Junction capacitor MOS capacitor

AI electrode

P-type substrate P-type substrate

+−

Oxide

layer Separation region

There is a point to heed in applying power supply ICs. It is that a method known as "junction separation" is used

as the method of electrically separating each of the elements above. By connecting a separation region so that it is

formed by a P-type semiconductor and is the same lowest potential as the substrate, the element region and the

separation region are electrically separated and insulated by being in (PN junction) reverse bias states. If for some

reason the potential of this separation region becomes a higher potential than the element region (for example the

NPN transistor collector region in Figure 2-1), normal operation cannot be expected since the PN junction enters a

forward bias state and the separation state between the elements cannot be maintained. For example, when using a

positive output three-terminal regulator, the GND pin always must be made a lower potential than the potential of

other pins.

2.2 About Power Supply IC Equivalent Circuits

Equivalent circuits that are shown in data sheets are so designated assuming the premise of the preceding

section (that separation regions and substrate are made the lowest potential). Be careful not to reference these

when this premise is violated.

User’s Manual G12702EJ8V0UM00 7

3. BASIC CIRCUITS OF A POWER SUPPLY IC

3.1 Basic Circuits

Although the basic circuits that make up a power supply IC differ according to the product type, the following

elements are necessary.

<1> Reference voltage circuit

<2> Error amplifier

<3> Active load (constant current circuit)

<4> Output stage power transistor

<5> Startup circuit

The following protection circuits also are on-chip.

<6> Overcurrent protection circuit

<7> Limiting circuit for securing safe operating area (SOA)

<8> Overheat protection circuit

Figure 3-1 shows a block diagram of a power supply IC.

Figure 3-1. Power Supply IC Block Diagram

INPUT

OUTPUT

Series bus

transistor

Protection

circuit

Current

source

Startup

circuit Reference

voltage

Error amplification

circuit Split resistor

GND

−

User’s Manual G12702EJ8V0UM00

The operation of each block is explained in simple terms below.

<1> Reference voltage circuit

The reference voltage circuit, which determines the output voltage of the power supply IC, is an extremely

important part within the circuit. The method for configuring this circuit is as follows.

•Band gap reference method: Use the forward characteristic between the base and emitter of the transistor.

The possibility of making the reference voltage 2 V or less is a feature of this method.

Figure 3-2 shows the principles of the band gap reference method. Figure 3-3 is a simple circuit diagram

of the band gap reference reference voltage used in the

PC7800A Series.

Figure 3-2. Band Gap Reference Circuit

∆V

REF

= V

BE3

+ ( ln )

KT R

GND

V+

The reference voltage is as follows.

VREF = VBE3 + (IC2 + IB3) • R2

= VBE3 + (

∆

VBE) + IB3 • R2

= VBE3 + ln ................................................................................................................ (3 - 3)

The temperature coefficient is as follows.

= + • ln .............................................................................................................. (3 - 4)

By optimally choosing the ratio of • , a temperature compensated reference voltage is known to be

obtained.

. .

qR2

∂

VREF

∂

VBE3

∂

qR2

User’s Manual G12702EJ8V0UM00 9

Figure 3-3. (Simplified) Band Gap Reference Circuit of

µµ

PC7800A Series

REF

GND

<2> Error amplifier

This circuit controls the output voltage by detecting and comparing the reference voltage created by the

reference voltage circuit and the resistor split output voltage. If VOUT is the output voltage and VREF is the

reference voltage (refer to Figure 3-1), the following relationship holds.

VOUT = VREF ........................................................................................................................ (3 - 1)

Here, A is the open loop gain of the error amplifier and

= RA / (RA + RB).

<3> Active load (constant current circuit)

Expression (3 - 1) becomes the following if the open loop gain A of the error amplifier is sufficiently large

compared to 1.

VOUT = VREF/

A small bias current and high resistance are realized by using a constant current circuit in the error amplifier

load to make A 60 to 80 dB.

<4> Output stage power transistor

The output stage power transistor supplies current to the load. Although normally a Darlington form NPN, the

low saturation type of power supply IC uses a PNP single transistor.

. .

(1 + A)

User’s Manual G12702EJ8V0UM00

<5> Startup circuit

A power supply IC has an on-chip constant current circuit for use as an error amplifier load or for biasing the

reference voltage circuit. A constant current circuit, which consists of paired transistors, does not begin to

operate as long as the diode connected transistors are not in a steady bias state. A startup circuit therefore

is set up and it biases the active load at power-on to cause normal operation to begin whether the

temperature of the transistors is low or high.

<6> Overcurrent protection circuit

This is a protection circuit for preventing the load current from exceeding the current capacity of the output

stage power transistor. It restricts the base current of the output stage power transistor by biasing the current

restriction transistor more deeply in accordance with the voltage drop in the current detection resistor inserted

in the load current route.

<7> Limiting circuit for securing safe operating area (SOA)

The limiting circuit for securing SOA operates to cut down the output current if the voltage between input and

output (voltage between the collector and emitter of the output stage power transistor) becomes large so that

the safe operating area of the output stage power transistor is not exceeded.

If the voltage difference between input and output exceeds the breakdown voltage (7 to 8 V) of a Zener diode

connected between input and output, it limits the base current of the output stage power transistor by biasing

the current limiting transistor more deeply using the breakdown current. Since the larger the voltage

difference between input and output the more the base current of the output stage power transistor is limited,

the load characteristic is a "foldback" type drooping characteristic as a result.

Figure 3-4 shows the parts of a general overcurrent protection circuit and limiting circuit for securing SOA.

Figure 3-4. Example of Overcurrent Protection Circuit and Limiting Circuit for Securing SOA

(

µµ

PC7800A Series)

: Current limiting

transistor

: Current detection resistor

INPUT

Limiting circuit for securing SOA

OUTPUT

Output stage transistor

User’s Manual G12702EJ8V0UM00 11

<8> Overheat protection circuit

The overheat protection circuit prevents destruction of the IC by cutting off output if the temperature of the

chip itself increases too much.

Figure 3-5 shows the parts of an overheat protection circuit. Q12, which is biased to the extent that it is not

ON in a normal operating state, is completely ON at 150°C to 200°C accompanying a decrease in VBE when

the temperature of the chip increases. When Q12 is ON, it cuts off the output voltage by absorbing the base

current of the output stage power transistor.

Figure 3-5. Example of Overheat Protection Circuit (

µµ

PC7800A Series)

: Overheat protection

transistor

OUTPUT

INPUT

GND

The overheat protection circuit is designed to operate at temperatures exceeding the absolute maximum

rating (generally 150°C). Therefore, if the overheat protection circuit has operated, the IC should be

considered to have been exposed to an abnormal state and positive use of the overheat protection circuit

should be avoided (so a separate circuit is needed to perform power supply overheat protection).

3.2 Operating Principles of Adjustable Output Types

An adjustable output type (

PC317,

PC337) differs from a fixed output voltage type in that it uses a method for

configuring an output voltage setting voltage circuit externally so that an arbitrary output voltage can be set

externally.

Figure 3-6 is the block diagram of a variable output voltage type. The output voltage is controlled by comparing

the voltage between external resistors RA and RB and the reference voltage VREF in the error amplifier.

Moreover, each block is connected between INPUT and OUTPUT and the current needed in each block (circuit

operating current) is output from the OUTPUT pin. Therefore, the outflow current from the ADJ pin becomes

negligible and its affect on the output voltage value can be ignored.

User’s Manual G12702EJ8V0UM00

Figure 3-6. Adjustable Output Type Block Diagram

REF

Output

voltage

setting

circuit

INPUT

OUTPUT

Protection

circuit

Current

source

Startup

circuit

Reference

voltage source

ADJ

−

3.3 Operating Principles of Low Saturation Types

All of the power supply ICs discussed so far use Darlington connected NPN type transistors in the output stage.

Therefore, the voltage difference between input and output that is needed to operate these power supply ICs cannot

be lower than the voltage between the base and emitter of the Darlington connected output stage transistor (0.7 V ×

2 = 1.4 V). A low saturation type power supply IC makes it possible to operate with a small voltage difference

between input and output by using a PNP transistor as the output stage transistor (refer to Figure 3-7).

Figure 3-7. Differences Between General Power Supply IC and

Low Saturation Type Output Stage Configurations

(a) General power supply IC (b) Low saturation type power supply IC

IN OUT

GND

0.7 V

IN OUT

GND

Configurations other than this are nearly identical to a general power supply IC. Figure 3-8 shows a block

diagram.

User’s Manual G12702EJ8V0UM00 13

Figure 3-8. Low Saturation Type Block Diagram

INPUT

Series path transistor

(PNP type)

OUTPUT

Limiting circuit

for securing

SOA

Overcurrent

restriction

circuit

Drive

circuit

Overheat

protection

circuit

Reference

voltage

circuit

Startup

circuit

GND

Error

amplifier

4. POWER SUPPLY IC APPLICATION CIRCUITS

4.1 Typical Circuit Connection

<1> Fixed output voltage type

Figure 4-1 shows an example of a typical circuit connection. Check the data sheet for each product type for

the values of input and output capacitors.

Figure 4-1. Example of Typical Circuit Connection (Single Power Supply Output)

CIN :If the wiring from a smoothing circuit to the

three-terminal regulator is long, there may be

oscillation. Therefore, add a 0.1 to 0.47

capacitor with superior voltage and temperature

characteristics near the input pin.

CO: This always must be added for oscillation

prevention in the case of a negative voltage three-

terminal regulator. For an application in which

the load current changes suddenly, also add 10

to 100

F of electrical capacitors for output voltage

transient response improvement.

D1: Although not needed for standard applications, this

is necessary when the time constant on the load

side is long and there is a residual voltage in CO for

some time after the power supply is cut and

backward voltage is applied to the regulator IC.

D2: Needed when there is a possibility of OUTPUT

being lower potential than GND.

OUTPUTINPUT

Three-terminal regulator

User’s Manual G12702EJ8V0UM00

Figure 4-2 is an example of a typical connection for obtaining a positive and negative power supply. The diodes

between output and GND are for preventing latchdown at startup and are absolutely necessary in the case of loads

shown by solid lines. Without the diodes, current flows in the separation regions between elements as described in

chapter 2 and the output voltage does not rise (refer to Figure 4-3).

Figure 4-2. Example of Typical Circuit Connection (Dual Power Supply Output)

−V

OUT

Positive voltage

3-terminal

regulator

Negative voltage

3-terminal

regulator

Load

Load A

Load B

GND

−V

CIN, CO, CIN', CO': As in the sample circuit for a single power supply load, these sometimes are needed depending

on circuit conditions.

Di1, Di1':

Absolutely necessary for loads shown by solid lines, in which a load current flows from

+VOUT toward -VOUT.

This is to prevent regulator output on either side from being latched down by differences

occurring in the rise of regulator output voltage due to smoothing circuit capacitor capacity

differences or the like.

Note that these are not specifically needed in the case of only those loads shown by dashed

lines.

Di2, Di2':

As in the sample circuit for a single power supply load, these sometimes are needed

depending on the application circuit.

User’s Manual G12702EJ8V0UM00 15

If the output pin becomes a lower potential than GND, the P type separation region and n type output pin (NPN

transistor) enter a forward bias state and the "parasitic transistor" shown with dashed lines is formed. When this

occurs, it is connected to the adjacent transistors and does not operate normally.

Figure 4-3. Example of Power Supply IC Cross Section Diagram (Latchdown)

Output

Separation

region

NPN transistor NPN transistor

<2> Adjustable output voltage type

When a voltage not included in a fixed output voltage type is needed or the output voltage is to be adjusted

and used, even a fixed output voltage type can be used by floating the GND as described later, but voltage

precision and drift become a problem. An adjustable output voltage type is useful in such cases.

Figure 4-4 shows an example of the typical connection. Since a bias current for the operation of each block

inside the IC flows from INPUT to OUTPUT as described in section 3.2, be careful of the load current. By

selecting 240 Ω as R1 as in the sample typical connection even when there is no load, no problems arise

since a current of

1.25 V / 240 Ω = 5.2 mA

flows to OUTPUT.

User’s Manual G12702EJ8V0UM00

Figure 4-4. Example of Typical Connection Circuit (Adjustable Output Power Supply)

ADJ

PC317

240 Ω

OUTPUTINPUTInput Output

1 F

0.1 F

Note This example is for a positive voltage.

For a negative voltage (

PC337), D1 and capacitor polarity are reversed.

CIN : Since there may be oscillation if the wire leading from a smoothing circuit to a three-terminal regulator is

long (15 cm or more), add a capacitor near the input pin.

CO: For an application in which the load current changes suddenly, add a 10

F or more capacitor for output

voltage transient response improvement (and add 10

F to CADJ at the same time).

CADJ : Connecting a 10

F capacitor parallel to R2 can improve the ripple rejection rate (approximately 20 dB)

and increase oscillation stability.

In this case, diode D1 is needed for to prevent application of backward voltage on an output short circuit.

R1, R2: These are resistors for setting the output voltage. The output voltage VO is determined as follows.

VO= 1 + • VREF + IADJ • R2

= 1 + • VREF

Table 4-1 shows the relationship between typical output voltages and R2.

Table 4-1. Settings of Output Voltage Setting Resistor R2

Output Vol tage VO (V) R2 SettingNote (Ω)

2.5 240

5.0 720

12 2064

24 4368

30 5520

Note TYP. values

<3> Low saturation type

The standard method of use is the same as for a general fixed output voltage type (see Figure 4-1).

However, the capacitor connected to the output must have a greater capacity than in a general power supply

IC. In addition, note that the output voltage cannot be adjusted by inserting a resistor or the like in the GND

pin as described later.

. .

User’s Manual G12702EJ8V0UM00 17

4.2 Application Circuit Set

This circuit set mainly is filled in for positive output voltage three-terminal regulators. However, the circuits also

can be applied to negative voltage three-terminal regulators by changing the polarity of parts employed.

1. High output current circuit

(without short circuit protection)

OUT

6 Ω

0.1 F

REG

0.1 F

IN OUT

GND

Drives the base of an ex t ernal transistor using a three-

terminal regul ator.

Here R1 is determi ned as follows .

In this circuit , the output current has an ac t ual range

that is 5 to 6 times the three-termi nal regul ator rating.

2. High output current circuit

(with short circuit protection)

OUT

0.1 F

REG

0.1 F

IN OUT

GND

6 Ω

This is an expansion of circuit 1. Current detection is

performed usi ng R2.

Therefore, si nce the current at Q1 is restricted by

I1(MAX.) =

the output current is as follows .

IO(MAX.) = I1(MAX.) + IREG(MAX.)

= + IREG(MAX.) …..………..……….….…. (4. 3)

3. High output current circuit

(with short circuit protection)

OUT

0.1 F

REG

0.1 F

IN OUT

GND

2 ΩR

6 ΩR

0.4 Ω

D1 cancels VBE at Q1.

Q1 and three-terminal c urrent distri bution is det erm i ned

by R1 and R2.

Caution Absolutely do not connect output pins in parallel to increase the current capacity of a three-

terminal regulator. If the output voltage becomes unbalanced, certain ICs operate in a restricted

current vicinity and current hardly flows in certain ICs, and furthermore the current may flow in

reverse. Also refer to 15 Wired OR.

VBE1

R1=IOUT …………………….…. (4.1)

IREG(MAX.) −hFEI(MIN.)

VBE1

IO=h

FE1(MIN.) IREG(MAX.) −R1 + IREG(MAX.) … (4.2)

R2I1

R1=IREG ……………………….…………….…… (4.4)

R1 + R2

IO(MAX.) =R1• IREG(MAX.) …………………..…. . (4.5)

VBE2

User’s Manual G12702EJ8V0UM00

4. High input voltage circuit

VIN Q1

IOUT VOUT

0.1 F

IN OUT

GND

0.1 F

This ci rcuit can be used when the input voltage exceeds

the rating.

Moreover, i f the load current changes lit tle, a resi stor

can be used.

5. High input, high output voltage circuit

(without short circuit protection)

OUT

0.1 F

IN OUT

GND

0.1

Using the f act that the current fl owi ng out from the GND

pin of the t hree-terminal regulat or i s practically constant,

add Zener Di to the GND pi n t o rai se only the Zener

portion of t he voltage. R1 s uppl i es i dl i ng to the Zener. I t

also is possible t o use a resistor, but thi s is inf eri or to

the Zener from a stability standpoint .

D is needed as l oad short circuit protection. I n addi tion,

the input v ol tage must be set withi n a range that holds

the volt age di fference bet ween i nput and output to the

ratings even on a short c i rc ui t.

This ci rcuit com bi nes circui ts 4 and 5. The c i rcuit made

up of Q1, Q2, and D1 is a preregulator.

The output vol tage is as follows.

VOUT = VO(REG) + VZD ..…………………………....... (4.7)

D2 protects agai ns t reverse bias in the GND and OUT

pins on a load s hort circui t.

6. High input, high output voltage circuit

(with short circuit protectionNote)

OUT

0.1 F

IN OUT

GND

4.7 kΩ

1 kΩ

0.1

F D

Note D1 or ZD must be s el ected so t hat the voltage

differenc e between input and output of the three-

terminal regul ator is kept within rat i ngs even on a

load short circuit .

In additi on, D2 must have low forward voltage.

VIN − VZD

R1 = IOUT(MAX.) ………………………………….… (4.6)

hFE1(MIN.)

User’s Manual G12702EJ8V0UM00 19

7. Remote shutdown circuit

OUT

0.1 F

IN OUT

GND

0.1 F

Control

Control the out put voltage using a preregulator s et up

ahead of the three-terminal regulat or.

The control i nput i s as foll ows.

At "H" l evel: Normal out put

At "L" level: Output interruption

In additi on, D1 is added to prevent reverse bias bet ween

the input and out put pins of the three-termi nal regul ator.

8. Slow startup circuit (without short circuit protection)

OUT

0.1 F

IN OUT

GND R

BIAS

OUT

O(REG)

final value

Delay time T

Power on Time

This ci rcuit moderat es the rise time of t he out put

voltage.

At power-on, this is the three-termi nal regul ator's

specific output voltage, after which i t gradually rises to

its f i nal value.

The initial output volt age i s

VO1 = VO(REG) .........…………………………......... (4.8)

The output voltage after stabiliz at ion is

Furthermore, t he del ay can be represent ed as follows i f

expect i ng up t o 99% of the f i nal value.

T = −CR ln 0.01 [s] ........………………………....

(4.10)

9. Adjustable output vol tage ci rcui t

(without load short circuit protection)

OUT

IN OUT

GND

0.1 F R

BIAS

0.1 F

Note that applications using the adjustable output three-terminal

regulator

PC317 are superior in output voltage precision and

stability.

The Zener diode in the c i rcuit shown i n 5 i s replaced by

a resistor.

Use a voltage differenc e between input and output that

is wit hi n t he three-terminal regul at or ratings.

For a load short circuit or capacity load, the di ode

shown using dashed lines is needed and in partic ul ar a

low forward vol t age i s needed.

VO(REG)

VO2 = VO(REG) + R1 IBIAS + R2 …………. (4.9)

VO(REG)

VOUT = VO(REG) + R1 IBIAS + R2 ………… (4.11)

. .

User’s Manual G12702EJ8V0UM00

10. Adjustable output vol tage ci rcui t

(0.5 to 10 V, without short circuit protection)

+VIN VOUT

PC7805A

IN OUT

GND

R1C2

RD6.2EB

10 F

0.1 F R4

910 Ω

9.1 kΩ

VO(REG)

PC741

−

0.1 F

kΩ

−VIN R3

10 F

Splits the fixed output v ol tage VO(REG) of the three-

terminal regul ator using R4 and R5 and compares wi t h

the output voltage VOUT val ue split us i ng R1 and R2.

The output vol tage can be represent ed as follows .

11. Adjustable output voltage circuit (7 to 30 V)

0.1 F

VIN VOUT

IN OUT

GND

R1C2

−

0.1 F

10 kΩ

PC741

This is similar t o the circui ts shown in 5 and 8. Since i t

uses op ampl i fier

PC741 with a s i ngl e power supply,

the lowest value of the output voltage can be no lower

than the sum of the output saturat ed voltage of t he

PC741 and the output voltage of the three-termi nal

regulator.

12. Tracking regulator ci rcui t

0.1 F

+VIN +VOUT

IN OUT

GND

−VIN −VOUT

0.1 F

Tf1

PC741

−

A tracking regulator is configured using a power

transis tor with one posi tive vol tage three-termi nal

regulator.

The positi ve voltage i s the fix ed voltage of the three-

terminal regul ator. The negative voltage can be

changed arbitrari l y by the spli t ratio of R1 and R2.

Thus the negati ve voltage output is as follows .

D1 protects agai ns t reverse bias between the bas e and

emitt er of the transistor at power-on.

R4R1 + R2

VOUT = R4 + R5× VO(REG) ×R1 …………. (4.12)

− VOUT = R1 • VOUT ……………………………. (4.13)

User’s Manual G12702EJ8V0UM00 21

13. Tracking regulator ci rcui t

0.1 F

+ V

OUT

IN OUT

GND

− V

OUT

0.1 FC

IN OUT

GND

0.1 F

−

PC741

This power suppl y has superior tracking characteristics

due to using an op am pl i fier and one posit i ve and one

negative voltage three-t erm i nal regul ator.

The GND pin of each t hree-terminal regulator is driv en

in common by the op ampli f i er output.

Favorable trac king charac t eri stics are obtained by

making R1 = R2. Moreov er, bias current errors also can

be canceled i f the resistor R1//R2 is added between the

non-inverti ng pi n of the op amplif i er and GND.

14. Positive and negative dual power supply circuit

(using positive voltage three-term i n al regulators)

+ VOUT

IN OUT

GND D1

GND

IN OUT

GND D2

− VOUT

This is a positiv e and negative dual power s uppl y that

uses t wo pos i tive vol tage three-termi nal regul ators.

D1 and D2 are low forward voltage diodes that are

absolutely necessary. They prev ent output vol tage

pulldown due to discrepancies in the startup tim i ng of

each regulator.

15. Wired OR

VIN1 VOUT

VIN2 D3

When connect i ng t he outputs of t wo or m ore three-

terminal regul ators, do it so that voltage from outside is

not added to the regul ator output at D1 and D3.

D2 and D4 are connected to c om pensate for t he l oweri ng

of output by D1 and D3.

User’s Manual G12702EJ8V0UM00

5. PRECAUTIONS ON APPLICATION

Do not use a three-terminal regulator under temperature conditions or voltage conditions that exceed the ratings.

Other precautions that are specific to three-terminal regulators are shown below.

5.1 Shorting Input Pins and Ground Pins

When a capacitor with a large capacity is connected to the load of a three-terminal regulator, if the input pin is

shorted to GND or the power supply is turned OFF, the voltage of the capacitor connected to the output pin is applied

between the output and input pins of the three-terminal regulator.

Figure 5-1

(a) (b)

OUT

OUTIN

GND

Discharge current

OUTIN

GND +

The withstand voltage between the output and input pins of a three-terminal regulator is approximately 0.7 V for a

low current with the output transistor base-emitter voltage.

Therefore, a diode like the one in Figure 5-1 (b) is effective against the reverse bias of the input and output pins.

Figure 5-1 (b) is for a positive voltage regulator. The diode direction is reversed for negative voltage.

5.2 Floating Ground Pins

Do not make the GND pin of a three-terminal regulator floating in the operating state. If it is made floating, an

input voltage that has not been stabilized is output unchanged. This is because the output stage power transistor is

biased by an overvoltage protection Zener or current mirror transistor leakage current. Since IC internal overheat

protection and the like do not operate normally in this case, there is a possibility of destruction if the load is short-

circuited or on an overload.

Be particularly careful when using a socket.

User’s Manual G12702EJ8V0UM00 23

5.3 Applying Transient Voltage to Input Pins

A three-terminal regulator is destroyed if a higher voltage than the rating or a voltage more than 0.5 V lower than

the GND pin is applied to the input line. In cases in which such voltages are superimposed on the line, add a surge

suppressor using a Zener diode or the like.

Figure 5-2

(a) (b)

IN OUT

GND

+ V

ZDC

IN OUT

GND

5.4 Reverse Bias Between Output Pin and GND Pin

Figure 5-3

(a) (b)

IN OUT

GND

+ V

OUT

BIAS

External protection diode

In the sample application shown in Figure 5-3 (a), the voltage of the Zener diode is applied between the output

and GND pins of the three-terminal regulator when the load is short-circuited.

Inside the three-terminal regulator, a diode like that shown in Figure 5-3 (b) apparently is formed, but if a current

flows in this part, the three-terminal regulator is sometimes destroyed. Therefore, when using a GND like that shown

in Figure 5-3 (a) in a floating state, it is necessary to add a low forward voltage diode from the GND pin of the three-

terminal regulator toward the output pin.

User’s Manual G12702EJ8V0UM00

5.5 Precautions Related to Low Saturation Types

Since a low saturation type of power supply IC uses a PNP transistor in the output stage, particular care is

needed. In a low input state before the output voltage enters regulation state (such as at startup), a large circuit

current flows because the output stage transistor is saturated. Depending on the product, the circuit current is

decreased at startup by an on-chip rushing current prevention circuit, but even in this case a relatively large circuit

current flows compared to normal operation (For details, refer to the "Circuit operating current at startup IBIAS(S)" rating

of each product). Thus, care is needed in the following matters.

•On startup, be careful of the output capacity of the power supply on the input side and the output impedance,

since a circuit operating current flows in the input superimposed on the load current.

•It is not possible to adjust the output voltage by inserting a resistor or the like in the GND. This is because the

circuit operating current increases at startup.

Be sure to connect a low impedance type capacitor to the output to increase stability against abnormal oscillation.

5.6 Thinking on Various Protection Circuits

NEC power supply ICs, which have on-chip overcurrent protection circuits, limiting circuits for securing SOA, and

overheat protection circuits, are very difficult to destroy in their normal operating state.

Nonetheless, you should not design circuits that put too much confidence in these protection circuits. These

protection circuits are for protection against sudden accidents. To the best of your ability, avoid operating protection

circuits for long stretches of time. In particular, be careful using the overheat protection circuit since this is like

operating at a temperature exceeding the absolute maximum rating.

6. POWER SUPPLY IC DATA SHEET APPEARANCE AND DESIGN METHODS

6.1 Absolute Maximum Ratings

This item shows values that must not be exceeded even momentarily under any usage conditions or test

conditions. Moreover, it is a mistake to think that use at the absolute maximum ratings is possible. Design should be

performed so that even in an abnormal state the equipment being considered leaves room for the absolute maximum

ratings.

In addition, it is assumed that GND is the lowest potential in the case of a positive output power supply and that

INPUT is the lowest potential in the case of a negative output power supply (see chapter 2).

6.2 Recommended Operating Conditions

If used under these conditions, it is possible to obtain output voltage precision as expected. Think of this as a

criterion for selecting a power supply IC.

User’s Manual G12702EJ8V0UM00 25

6.3 Electrical Specifications

NEC guarantees the minimum values and maximum values of electrical characteristics at the time of shipment.

Therefore, whether or not it is possible to satisfy the specifications of the power supply to be designed must be

determined by adequately investigating each rating and condition in each item of the electrical characteristics. Each

item of the electrical characteristics is described below (Since the explanations below are mainly for positive output

power supply ICs, reread them while reversing polarities for negative power supply ICs).

<1> Output voltage VO

This item is the most important rating in using a power supply IC. Pay attention to measurement conditions.

If power supply specifications are within this range of conditions, the expected precision (for example ±5%) is

obtained (see Figure 6-1).

Figure 6-1. Output Voltage Conceptualization (For

µµ

PC7805AHF) Guaranteed Range Inside Broken Lines

5.4

5.2

5.0

4.8

−50 0 50 100 150

= 10 V

= 5 mA

Junction temperature T

(°C)

Output voltage V

(V)

<2> Line regulation REGIN

When the input voltage increases, the output voltage also increases. This item shows how much the output

voltage changes when the input voltage VIN is varied within the measured conditions. As shown in Figure 6-

2, output voltage changes nearly linearly with respect to input voltage. Therefore, it is possible to infer how

much the output voltage will change from the initial period when the initial input voltage is changed to a given

input voltage.

User’s Manual G12702EJ8V0UM00

Figure 6-2. Line Regulation REGIN Conceptualization (For

µµ

PC7805AHF, VIN = 10 V Standard)

+30

+20

+10

−10

−20 5010152025

TA = 25°C

IO = 500 mA

Input voltage VIN (V)

Input stability REGIN (mV)

REGIN MAX.

REGIN TYP.

<3> Load regulation REGL

Whereas REGIN is the change in output voltage with respect to input voltage, load regulation REGL shows the

change in output voltage with respect to load current (output current). When load current increases, output

voltage decreases nearly linearly. The output voltage for an arbitrary load current can be inferred in the same

way as REGIN (see Figure 6-3).

User’s Manual G12702EJ8V0UM00 27

Figure 6-3. Load Regulation Conceptualization (For

µµ

PC7805AHF, IO = 500 mA)

TA = 25°C

VIN = 10 V

REGL TYP.

REGL MAX.

+10

−10

−20

−30

0 0.5 1.0 1.5

Output current IO (A)

Load stability REGL (mV)

<4> Quiescent current IBIAS

This is the bias current needed for each internal block of a power supply IC to operate. It flows from input

toward GND. Applications that adjust output voltage by inserting a resistor in GND take this item into

account.

<5> Quiescent current change

∆

∆∆

∆

IBIAS

This shows the change in IBIAS when the input voltage or load current changes.

<6> Ripple rejection rate R •

••

• R

The ripple voltage that appears in the output when a 120 Hz sine wave (minimum value and maximum value

of sine wave are noted in measured conditions) is input in the input is represented by the following

expression.

• R = 20 log (VIN/VOripple) [dB]

If the frequency increases, R • R decreases mainly due to the frequency characteristics of the internal error

amplifier of the IC.

<7> Output noise voltage Vn

This shows the noise that occurs inside a power supply IC (mainly thought to be thermal noise).

User’s Manual G12702EJ8V0UM00

<8> Peak output current IOpeak

This is the current at which the overcurrent protection circuit operates. It is defined as the output current

when the output voltage is lowered by 2% from its initial value.

As described in chapter 3, the overcurrent protection circuit operates together with the stable operation area.

Moreover, note that IOpeak decreases as temperature increases (negative temperature characteristic). Figure

6-4 shows the IOpeak-VIN-VO characteristics of the

PC7800A Series. For a nonlinear load such as a motor or

lamp, select a power supply IC that has sufficient leeway (50% or less of normal characteristic graph).

<9> Output short circuit current IOshort

This is the current that flows when output is short-circuited. Since most NEC power supply ICs have an on-

chip limiting circuit for securing SOA, the following relation holds.

IOshort < IOpeak

Like IOpeak, IOshort displays a negative temperature characteristic. Refer to Figure 6-4 for temperature

characteristics of the output short circuit current and changes with respect to input voltage.

Figure 6-4. Example of IOpeak Characteristics (

µµ

PC7800A Series)

3.0

2.5

2.0

1.5

1.0

0.5

Opeak

- (V

− V

) characteristic

Voltage difference between input and output V

− V

(V)

Peak output current I

Opeak

(A)

= 0°C

125°C

75°C25°C

0 5 10 15 20 25 30 35

6.4 Design Methods

(A) Input circuit design

Determine the capacity of a smoothing capacitor of an input circuit using an O.H. Shade graph or simulator

so that the minimum value of the input voltage is not lower than the measurement conditions of output

voltage.

At this time, connect a film capacitor between input and GND of the power supply IC separate from the

smoothing capacitor to prevent abnormal oscillation (refer to the data sheet of each product type for capacitor

values).

User’s Manual G12702EJ8V0UM00 29

(B) Output circuit design

Check whether the load current used is a current no greater than the peak output current.

Connect a capacitor for abnormal oscillation prevention between output and GND of the power supply IC. If

transient load stability becomes a problem, make sure the capacitor is connected in parallel.

The junction temperature can be calculated using the following expression.

TJ = (Rth(J-C) +

C-HS +

HS) • PD + TA............................................................................................... (6.1)

Rth(J-C): Thermal resistance (junction to case)

C-HS: Contact thermal resistance (includes thermal resistance of insulation sheet when using

insulation sheet)

HS: Thermal resistance of heatsink

PD: Internal power dissipation of IC (PD = (VIN - VO) • IO + VIN • IBIAS)

TA: Operating ambient temperature

Expression (6.1) is the calculation expression when using a heatsink. When not using a heatsink, such as in

the

PC78L00 Series, use the following expression.

TJ = Rth(J-A) • PD + TA ...................................................................................................................... (6.2)

Rth(J-A): Thermal resistance (junction to ambient air)

Use the values in the data sheets for Rth(J-C) and Rth(J-A) in expressions (6.1) and (6.2).

Since TJ, Rth(J-C), PD, and TA are given, find the thermal resistance of the heatsink

HS from them using

expression (6.1). Figure 6-5 shows the thermal resistance of an aluminum board. Since the heatsink

manufacturer produce heatsinks suited to power supply ICs, also consult the heatsink manufacturer.

Figure 6-5. Thermal Resistance of Aluminum Board

100

10 20 50 100 200 500 1000

Surface area of heatsink A (cm

)

Thermal resistance of

heatsink

(°C/W)

t = 1.5 mm

t = 3 mm

If TJ is not within the design values, return to (A) or (B) and recalculate. An example of heatsink design is

shown next.

User’s Manual G12702EJ8V0UM00

<1> Design objectives

Positive power supply using

PC7805AHF

Maximum output current IO max. = 0.6 (A)

Maximum voltage difference between input and output VDIF max. = 6 (V)

Maximum operating ambient temperature TA max. = 60 (°C)

Maximum junction temperature TJ max. = 100 (°C)

<2> Heatsink thermal resistance calculation

In a used state, the junction temperature TJ is the following.

TJ = (Rth(J-C) +

C-HS +

HS) • PD + TA ...................................................................................................... (6.3)

Rth(J-C):Thermal resistance (junction to case)

C-HS: Thermal resistance (case to heatsink)

HS: Thermal resistance of heatsink

PD: Power dissipation

Here, TJ max. = 100 (°C), TA max. = 60 (°C),

C-HS << 1 (°C/W), and Rth(J-C) = 5.0 (°C/W)

By substituting PD max. = VDIF max. × IO max. = 3.6 (W) in expression (6.3), find the thermal resistance

HS needed

in the heatsink.

HS = – Rth(J-C) –

C-HS

= 6.1 (°C/W) .................................................................................................................................... (6.4)

<3> Determination of size of heatsink

From expression (6.4), the design objectives can be satisfied using a heatsink of 6.1 (°C/W).

Figure 6-5 shows the relationship between the thickness, surface area, and thermal resistance of an

aluminum board.

By using a 3 mm thick 60 cm2 aluminum board here, it can be seen that the heatsink will have the necessary

thermal resistance.

(Use example without heatsink)

The junction temperature TJ in the used state when not installing a heatsink is the following.

TJ = Rth(J-A) • PD + TA ........................................................................................................................ (6.5)

Rth(J-A): Thermal resistance (junction to ambient air) (free air)

PD: Power dissipation

TA: Operating ambient temperature

Setting TJ to 100°C or less in the used state is recommended.

. .

TJ – TA

User’s Manual G12702EJ8V0UM00 31

Precautions when installing in a heatsink

•

••

•Make the convexity or concavity of the part installation surface of the heatsink 0.05 mm

or less.

•

••

•Spread silicon grease to a uniform thickness between the heatsink and part. Determine

the kind of grease on consulting the maker of the heatsink.

•

••

•Painting the heatsink black increases its effectiveness in radiating heat. However, if it is

close to a heat source, it has the reverse effect of absorbing heat.

•

••

•Use one of the insulating board bushings shown in Table 6-2.

•

••

•Cut a screw in a heatsink and absolutely do not use self-tapping screws to install one.

When installing a heatsink, if the tightening torque of a screw is too great, the fins can be distorted and the IC

damaged. Drive screws using a torque driver that can manage the tightening torque.

Table 6-1. Three-Terminal Regulator Tightening Torque

Markings Tightening t orque (N•m)

TO-126 2.0 × 10−3 to 4.1 × 10−3

TO-220 3.1 × 10−3 to 5.1 × 10−3

MP-45G 3.1 × 10−3 to 5.1 × 10−3

Figure 6-6. Standard Installation Method for Heatsink Insulation

3 M screw

Flat washer 3 M screw

Flat washer

Spring washer

3 M nut

Insulating board

Insulating bushing

Spring washer

3 M nut

MP-45G TO-220

Heatsink Heatsink

Table 6-2. Recommended Insulating Bushings and Insulating Board

Quality of MaterialsCode No. Product Nam e

Material Color

Incombustibility

Grade

Insulat i ng bushing B-24 25K bushing U Gelanex 3310 Light brown UL 94V-0

Insulat i ng board S-7 MP-25 insulating board A Poly ester Colorless , transparent −

User’s Manual G12702EJ8V0UM00

[MEMO]

User’s Manual G12702EJ8V0UM00 33

[MEMO]

User’s Manual G12702EJ8V0UM00

[MEMO]

Although NEC has taken all possible steps

to ensure that the documentation supplied

to our customers is complete, bug free

and up-to-date, we readily accept that

errors may occur. Despite all the care and

precautions we've taken, you may

encounter problems in the documentation.

Please complete this form whenever

you'd like to report errors or suggest

improvements to us.

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