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User's Manual
Usage of Three-Terminal Regulators
Document No. G12702EJ8V0UM00 (8th edition) Date Published May 2000 N CP(K)
(c)
Printed in Japan
2000
[MEMO]
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User's Manual G12702EJ8V0UM00
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.
* The information in this document is subject to change without notice. Before using this document, please confirm that this is the latest version. * Not all devices/types available in every country. Please check with local NEC representative for availability and additional information. * No part of this document may be copied or reproduced in any form or by any means without the prior written consent of NEC Corporation. NEC Corporation assumes no responsibility for any errors which may appear in this document. * NEC Corporation does not assume any liability for infringement of patents, copyrights or other intellectual property rights of third parties by or arising from use of a device described herein or any other liability arising from use of such device. No license, either express, implied or otherwise, is granted under any patents, copyrights or other intellectual property rights of NEC Corporation or others. * Descriptions of circuits, software, and other related information in this document are provided for illustrative purposes in semiconductor product operation and application examples. The incorporation of these circuits, software, and information in the design of the customer's equipment shall be done under the full responsibility of the customer. NEC Corporation assumes no responsibility for any losses incurred by the customer or third parties arising from the use of these circuits, software, and information. * While NEC Corporation has been making continuous effort to enhance the reliability of its semiconductor devices, the possibility of defects cannot be eliminated entirely. To minimize risks of damage or injury to persons or property arising from a defect in an NEC semiconductor device, customers must incorporate sufficient safety measures in its design, such as redundancy, fire-containment, and anti-failure features. * NEC devices are classified into the following three quality grades: "Standard", "Special", and "Specific". The Specific quality grade applies only to devices developed based on a customer designated "quality assurance program" for a specific application. The recommended applications of a device depend on its quality grade, as indicated below. Customers must check the quality grade of each device before using it in a particular application. Standard: Computers, office equipment, communications equipment, test and measurement equipment, audio and visual equipment, home electronic appliances, machine tools, personal electronic equipment and industrial robots Special: Transportation equipment (automobiles, trains, ships, etc.), traffic control systems, anti-disaster systems, anti-crime systems, safety equipment and medical equipment (not specifically designed for life support) Specific: Aircraft, aerospace equipment, submersible repeaters, nuclear reactor control systems, life support systems or medical equipment for life support, etc. The quality grade of NEC devices is "Standard" unless otherwise specified in NEC's Data Sheets or Data Books. If customers intend to use NEC devices for applications other than those specified for Standard quality grade, they should contact an NEC sales representative in advance.
M7D 98. 12
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CONTENTS
1. INTRODUCTION ................................................................................................................................ 2. BASIC STRUCTURE OF A POWER SUPPLY IC.......................................................................
2.1 2.2 Structure of a Bipolar IC ........................................................................................................................... About Power Supply IC Equivalent Circuits..............................................................................................
5 5
5 6
3. BASIC CIRCUITS OF A POWER SUPPLY IC ............................................................................
3.1 3.2 3.3
7
Basic Circuits ........................................................................................................................................... 7 Operating Principles of Adjustable Output Types..................................................................................... 11 Operating Principles of Low Saturation Types ......................................................................................... 12
4. POWER SUPPLY IC APPLICATION CIRCUITS .......................................................................... 13
4.1 4.2 Typical Circuit Connection........................................................................................................................ 13 Application Circuit Set .............................................................................................................................. 17
5. PRECAUTIONS ON APPLICATION ............................................................................................... 22
5.1 5.2 5.3 5.4 5.5 5.6 Shorting Input Pins and Ground Pins ....................................................................................................... Floating Ground Pins ............................................................................................................................... Applying Transient Voltage to Input Pins.................................................................................................. Reverse Bias Between Output Pin and GND Pin ..................................................................................... Precautions Related to Low Saturation Types ......................................................................................... Thinking on Various Protection Circuits.................................................................................................... 22 22 23 23 24 24
6. POWER SUPPLY IC DATA SHEET APPEARANCE AND DESIGN METHODS..................... 24
6.1 6.2 6.3 6.4 Absolute Maximum Ratings...................................................................................................................... Recommended Operating Conditions ...................................................................................................... Electrical Specifications............................................................................................................................ Design Methods ....................................................................................................................................... 24 24 25 28
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User's Manual G12702EJ8V0UM00
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
NPN transistor
PNP transistor
Separation region Collector p n+ Base p n+
Separation region Emitter n+ n p
n+
Separation region Base p Collector p Emitter p p n+
n
P-type substrate
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Figure 2-2. Structure of Resistor
Base diffused resistor Separation region Resistor electrode p Separation region P-type diffusion Resistor layer electrode n n+ n+ p
Base pinch resistor
+ P-type diffusion n diffusion layer Resistor Pinch layer electrode region
Epitaxial layer Resistor electrode electrode
Separation region
n n+
p
P-type substrate
Figure 2-3. Structure of Capacitor
Junction capacitor Separation region p n+ -+ Separation region
MOS capacitor Oxide layer
AI electrode n n+ n+ p p
Separation region
n
p
P-type substrate
P-type substrate
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.
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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
Current source
Protection circuit
Startup circuit
Reference voltage
+ - Error amplification circuit
Series bus transistor OUTPUT RB Split resistor RA GND
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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+ I VREF = VBE3 + R2 ( KT ln R2 ) R3 q R1 R2 R2 VBE R3 Q3 Q2 VBE Q1 VBE R3 GND
VREF
R1
The reference voltage is as follows. VREF = VBE3 + (IC2 + IB3) * R2 = VBE3 + R2 R3 (VBE) + IB3 * R2 KT q ln R2 R1 ................................................................................................................ (3 - 3)
R2 . . = VBE3 + R3
The temperature coefficient is as follows.
VREF T
=
VBE3 T
+
K q
*
R2 R3
ln
R2 .............................................................................................................. (3 - 4) R1 R2 R3 R2 R1
By optimally choosing the ratio of obtained.
*
, a temperature compensated reference voltage is known to be
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Figure 3-3. (Simplified) Band Gap Reference Circuit of PC7800A Series
VIN
VREF
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 = A VREF ........................................................................................................................ (3 - 1)
(1 + A)
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.
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<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. 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 Since the larger the voltage difference between input and output the more the base current of the output stage power transistor is limited,
(PC7800A Series)
Limiting circuit for securing SOA INPUT
Q16
Q17 Output stage transistor
Q15 : Current limiting transistor R11 : Current detection resistor
OUTPUT
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<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 150C to 200C 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 (PC7800A Series)
INPUT
Q16
Q17
OUTPUT Q12 : Overheat protection transistor
GND
The overheat protection circuit is designed to operate at temperatures exceeding the absolute maximum rating (generally 150C). 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.
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Figure 3-6. Adjustable Output Type Block Diagram
INPUT Current source Protection circuit
Startup circuit
+ - Reference voltage source
OUTPUT RB Output voltage setting circuit
VREF
ADJ
VO
RA
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 x 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 0.7 V
OUT
IN
OUT
0.7 V
GND
GND
Configurations other than this are nearly identical to a general power supply IC. Figure 3-8 shows a block diagram.
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Figure 3-8. Low Saturation Type Block Diagram
INPUT
Startup circuit
Reference voltage circuit Overheat protection circuit
Limiting circuit for securing SOA Error amplifier Drive circuit
Series path transistor (PNP type) OUTPUT
Overcurrent restriction circuit GND
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
INPUT Three-terminal regulator CIN CO + D2 OUTPUT
D1
oscillation.
Therefore, add a 0.1 to 0.47 F
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 threeterminal 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.
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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)
Di2
+VIN CIN
Positive voltage 3-terminal regulator Di1
+VOUT
Co Load Di1'
Load A
GND CIN' -VIN Negative voltage 3-terminal regulator Co' -VOUT
Load B
Di2'
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.
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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)
Separation Output region NPN transistor NPN transistor np p n p n p np p
<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 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 = 5.2 mA flows to OUTPUT.
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Figure 4-4. Example of Typical Connection Circuit (Adjustable Output Power Supply)
Input INPUT
PC317
ADJ
OUTPUT R1 240
Output VO D1 + CO 1F
CIN 0.1F
+ CADJ
R2
Note This example is for a positive voltage. For a negative voltage (PC337), D1 and capacitor polarity are reversed. CIN CO CADJ : 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. : 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). : 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 + R2 R1 * VREF + IADJ * R2
R2 . =. 1 + * VREF R1 Table 4-1 shows the relationship between typical output voltages and R2. Table 4-1. Settings of Output Voltage Setting Resistor R2
Output Voltage VO (V) 2.5 5.0 12 24 30 R2 Setting
Note
()
240 720 2064 4368 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.
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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) Drives the base of an external transistor using a threeterminal regulator. Here R1 is determined as follows.
IO VOUT IREG IN OUT GND
VIN 6 R1 C1 0.1 F
Q1
IOUT
R1 =
VBE1 IREG(MAX.) - IOUT hFEI(MIN.)
............................. (4.1)
IO = hFE1(MIN.) IREG(MAX.) -
C2 0.1 F
VBE1 R1
+ IREG(MAX.) ... (4.2)
In this circuit, the output current has an actual range that is 5 to 6 times the three-terminal regulator rating.
2. High output current circuit (with short circuit protection)
This is an expansion of circuit 1. Current detection is performed using R2. Therefore, since the current at Q1 is restricted by
I1 IO VOUT IREG
VIN
R2
Q1
I1(MAX.) =
VBE2 R2
the output current is as follows. IO(MAX.) = I1(MAX.) + IREG(MAX.)
R1 6
Q2 IN R1 C1 0.1 F OUT GND
C2 0.1 F
=
VBE2 R2
+ IREG(MAX.) .................................. (4.3)
3. High output current circuit (with short circuit protection)
D1 cancels VBE at Q1. Q1 and three-terminal current distribution is determined by R1 and R2.
I1 IO VOUT IREG
VIN
R1 0.4 2
R2 R1
Q1
=
I1 IREG
.................................................. (4.4)
R2
R3 6 IN OUT GND
IO(MAX.) =
R1 + R2 R1
* IREG(MAX.) ............................ (4.5)
D1 C1 0.1 F
C2 0.1 F
Caution Absolutely do not connect output pins in parallel to increase the current capacity of a threeterminal 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.
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4. High input voltage circuit
This circuit can be used when the input voltage exceeds the rating. R1 =
IN OUT GND VOUT
VIN R1
IOUT Q1
VIN - VZD IOUT(MAX.) hFE1(MIN.)
........................................... (4.6)
C1 0.1 F ZD
C2 0.1 F
Moreover, if the load current changes little, a resistor can be used.
5. High input, high output voltage circuit (without short circuit protection)
VIN
IN C1 0.1 F
OUT GND R1 C2 0.1 F
VOUT
D
Using the fact that the current flowing out from the GND pin of the three-terminal regulator is practically constant, add Zener Di to the GND pin to raise only the Zener portion of the voltage. R1 supplies idling to the Zener. It also is possible to use a resistor, but this is inferior to the Zener from a stability standpoint. D is needed as load short circuit protection. In addition, the input voltage must be set within a range that holds the voltage difference between input and output to the ratings even on a short circuit.
ZD
6. High input, high output voltage circuit Note (with short circuit protection )
This circuit combines circuits 4 and 5. The circuit made up of Q1, Q2, and D1 is a preregulator. The output voltage is as follows. VOUT = VO(REG) + VZD ....................................... (4.7) D2 protects against reverse bias in the GND and OUT pins on a load short circuit.
D2
VIN
Q2 R2 1 k
IN
OUT GND C2 0.1 F
VOUT
R1 4.7 k
Q1
C1 0.1 F
Note
D1 ZD
D1 or ZD must be selected so that the voltage difference between input and output of the threeterminal regulator is kept within ratings even on a load short circuit. In addition, D2 must have low forward voltage.
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7. Remote shutdown circuit
D1
VIN R1
Q1
IN
OUT GND
VOUT
Control the output voltage using a preregulator set up ahead of the three-terminal regulator. The control input is as follows. At "H" level: Normal output At "L" level: Output interruption In addition, D1 is added to prevent reverse bias between the input and output pins of the three-terminal regulator.
R2 Control Q2 R3
C1 0.1 F
C2 0.1 F
8. Slow startup circuit (without short circuit protection)
VIN
IN IBIAS C1 0.1 F
OUT GND
VOUT R2 D1 R3
This circuit moderates the rise time of the output voltage. At power-on, this is the three-terminal regulator's specific output voltage, after which it gradually rises to its final value. The initial output voltage is VO1 = VO(REG) ................................................ (4.8) The output voltage after stabilization is
R1
Q1 C2
VOUT
VO final value
VO2 = VO(REG) + R1
IBIAS +
VO(REG) R2
............. (4.9)
VO(REG) Delay time T
Furthermore, the delay can be represented as follows if expecting up to 99% of the final value. T .= -CR ln 0.01 [s] ....................................... . (4.10)
Time
Power on
9. Adjustable output voltage circuit (without load short circuit protection)
The Zener diode in the circuit shown in 5 is replaced by a resistor. VO(REG) R2
VIN
IN C1
OUT GND C2 0.1F R1 R2
VOUT
VOUT = VO(REG) + R1
IBIAS +
............ (4.11)
0.1 F IBIAS
D1
Use a voltage difference between input and output that is within the three-terminal regulator ratings. For a load short circuit or capacity load, the diode shown using dashed lines is needed and in particular a low forward voltage is needed.
Note that applications using the adjustable output three-terminal regulator PC317 are superior in output voltage precision and stability.
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10. Adjustable output voltage circuit (0.5 to 10 V, without short circuit protection)
Splits the fixed output voltage VO(REG) of the threeterminal regulator using R4 and R5 and compares with the output voltage VOUT value split using R1 and R2. The output voltage can be represented as follows.
10 F
PC7805A
+ VOUT
+VIN
VOUT =
R4 R4 + R5
x VO(REG) x
R1 + R2 R1
............. (4.12)
C1
OUT GND R4 0.1 F VO(REG) 910 R5 9.1 k -
IN
-VIN R3 10 F + RD6.2EB
PC741
R1 + R2
10 k
C2 0.1 F
11. Adjustable output voltage circuit (7 to 30 V)
VIN
IN
OUT GND -
VOUT
This is similar to the circuits shown in 5 and 8. Since it uses op amplifier PC741 with a single power supply, the lowest value of the output voltage can be no lower than the sum of the output saturated voltage of the PC741 and the output voltage of the three-terminal regulator.
C1
0.1 F
PC741
+
R1 10 k R2
C2 0.1 F
12. Tracking regulator circuit
+VIN
IN C1 0.1 F
OUT GND
+VOUT 0.1 F C2 R1
A tracking regulator is configured using a power transistor with one positive voltage three-terminal regulator. The positive voltage is the fixed voltage of the threeterminal regulator. The negative voltage can be changed arbitrarily by the split ratio of R1 and R2. Thus the negative voltage output is as follows. R2 R1
- VOUT =
C3 0.1 F
* VOUT .................................. (4.13)
PC741
- + R2
D1 protects against reverse bias between the base and emitter of the transistor at power-on.
D1 -VIN Tf1 -VOUT
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13. Tracking regulator circuit
+ VIN
C1
OUT GND 0.1 F 0.1 F
IN
+ VOUT R1
C2
PC741 -
+ R1 R2
This power supply has superior tracking characteristics due to using an op amplifier and one positive and one negative voltage three-terminal regulator. The GND pin of each three-terminal regulator is driven in common by the op amplifier output. Favorable tracking characteristics are obtained by making R1 = R2. Moreover, bias current errors also can be canceled if the resistor R1//R2 is added between the non-inverting pin of the op amplifier and GND.
C3 - VIN
0.1 F
0.1 F GND IN OUT
C4
R2 - VOUT
14. Positive and negative dual power supply circuit (using positive voltage three-terminal regulators)
This is a positive and negative dual power supply that uses two positive voltage three-terminal regulators. D1 and D2 are low forward voltage diodes that are absolutely necessary. They prevent output voltage pulldown due to discrepancies in the startup timing of each regulator.
IN
OUT GND D1
+ VOUT
IN
OUT GND D2
GND
- VOUT
15. Wired OR
VIN1
D1
VOUT
When connecting the outputs of two or more threeterminal regulators, do it so that voltage from outside is not added to the regulator output at D1 and D3. D2 and D4 are connected to compensate for the lowering of output by D1 and D3.
D2
VIN2
D3
D4
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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)
Discharge current
VOUT
IN
OUT VOUT + GND
IN GND
OUT +
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 shortcircuited or on an overload. Be particularly careful when using a socket.
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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)
+ VIN R C ZD
IN GND
OUT
+ VO
L IN GND D1 C OUT
5.4 Reverse Bias Between Output Pin and GND Pin
Figure 5-3 (a) (b)
+ VIN
IN GND IBIAS
OUT
VOUT
External protection diode
ZD VZ
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 threeterminal regulator toward the output pin.
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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. criterion for selecting a power supply IC. Think of this as a
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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 PC7805AHF) Guaranteed Range Inside Broken Lines
5.4
5.2
Output voltage VO (V)
5.0
4.8
VIN = 10 V IO = 5 mA -50 0 50 Junction temperature TJ (C) 100 150
<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 62, 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.
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Figure 6-2. Line Regulation REGIN Conceptualization (For PC7805AHF, VIN = 10 V Standard)
+30
+20 Input stability REGIN (mV) REGIN MAX. REGIN TYP.
+10
0
-10 TA = 25C IO = 500 mA -20 0 5 10 15 20 25
Input voltage VIN (V)
<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).
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Figure 6-3. Load Regulation Conceptualization (For PC7805AHF, IO = 500 mA)
+10
Load stability REGL (mV)
-10
-20
-30
REGL TYP.
REGL MAX. TA = 25C VIN = 10 V
0
0
0.5
1.0
1.5
Output current IO (A)
<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. 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 * 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). Applications that adjust output voltage by inserting a resistor in GND take this item into
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<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 onchip 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 (PC7800A Series)
IOpeak- (VIN - VO) characteristic 3.0 2.5 2.0
Peak output current IOpeak (A)
TJ =
1.5 1.0 0.5
0C
75 25 C C 125 C
0 5 10 15 20 25 30 35 Voltage difference between input and output VIN - VO (V)
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).
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User's Manual G12702EJ8V0UM00
(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. (C) Radiation design 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:
PD: TA:
Thermal resistance of heatsink Internal power dissipation of IC (PD = (VIN - VO) * IO + VIN * IBIAS) 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
Thermal resistance of heatsink HS (C/W)
50 20 10 5 2 1 10 20 50 100 200 500 1000 t = 1.5 mm t = 3 mm
Surface area of heatsink A (cm2)
If TJ is not within the design values, return to (A) or (B) and recalculate. An example of heatsink design is shown next.
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<1> Design objectives Positive power supply using PC7805AHF Maximum output current Maximum voltage difference between input and output Maximum operating ambient temperature Maximum junction temperature <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) IO max. = 0.6 (A) VDIF max. = 6 (V) TA max. = 60 (C) TJ max. = 100 (C)
C-HS: Thermal resistance (case to heatsink) HS:
PD: Thermal resistance of heatsink 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. x IO max. = 3.6 (W) in expression (6.3), find the thermal resistance HS needed . in the heatsink.
HS =
TJ - TA PD
- 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: TA: Power dissipation Operating ambient temperature
Setting TJ to 100C or less in the used state is recommended.
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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 TO-126 TO-220 MP-45G Tightening torque (N*m) 2.0 x 10-3 to 4.1 x 10-3 3.1 x 10-3 to 5.1 x 10-3 3.1 x 10-3 to 5.1 x 10-3
Figure 6-6. Standard Installation Method for Heatsink Insulation
3 M screw Flat washer
3 M screw Flat washer Insulating board
Insulating bushing Heatsink MP-45G TO-220 Spring washer 3 M nut Heatsink Flat washer Spring washer 3 M nut
Table 6-2. Recommended Insulating Bushings and Insulating Board
Code No. Product Name Quality of Materials Material Insulating bushing Insulating board B-24 S-7 25K bushing U MP-25 insulating board A Gelanex 3310 Polyester Color Light brown Colorless, transparent Incombustibility Grade UL 94V-0 -
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