User’s Manual Usage of Three-Terminal Regulators Document No. G12702EJ8V0UM00 (8th edition) Date Published May 2000 N CP(K) © Printed in Japan 2000 [MEMO] 2 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. 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M7D 98. 12 User’s Manual G12702EJ8V0UM00 3 CONTENTS 1. INTRODUCTION ................................................................................................................................ 5 2. BASIC STRUCTURE OF A POWER SUPPLY IC....................................................................... 5 2.1 2.2 Structure of a Bipolar IC ........................................................................................................................... About Power Supply IC Equivalent Circuits.............................................................................................. 5 6 3. BASIC CIRCUITS OF A POWER SUPPLY IC ............................................................................ 7 3.1 3.2 3.3 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 4 Absolute Maximum Ratings...................................................................................................................... Recommended Operating Conditions ...................................................................................................... Electrical Specifications............................................................................................................................ Design Methods ....................................................................................................................................... User’s Manual G12702EJ8V0UM00 24 24 25 28 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 Separation region Separation region Collector p PNP transistor n+ Base p Emitter n+ n p n+ Separation region Base p Collector p n n+ Emitter p p n+ P-type substrate User’s Manual G12702EJ8V0UM00 5 Figure 2-2. Structure of Resistor Base diffused resistor Base pinch resistor Separation region Separation region Resistor electrode + P-type diffusion n diffusion layer Resistor Pinch layer electrode region P-type diffusion Resistor layer electrode Separation region n+ n p Epitaxial layer Resistor electrode electrode n p n+ p n+ P-type substrate Figure 2-3. Structure of Capacitor MOS capacitor Junction capacitor − + Separation region Oxide layer Separation region AI electrode n n+ p n+ p p n Separation region p n+ 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. 6 User’s Manual G12702EJ8V0UM00 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 Startup circuit Reference voltage Protection circuit Series bus transistor + − Error amplification circuit OUTPUT RB Split resistor RA GND User’s Manual G12702EJ8V0UM00 7 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 µPC7800A Series. Figure 3-2. Band Gap Reference Circuit V+ VREF = VBE3 + R2 ( KT ln R2 ) R3 q R1 I R1 VREF R2 ∆VBE R3 R2 Q3 Q2 VBE Q1 ∆VBE R3 GND The reference voltage is as follows. VREF = VBE3 + (IC2 + IB3) • R2 = VBE3 + R2 R3 (∆VBE) + IB3 • R2 R2 . =. VBE3 + R3 KT q ln R2 R1 ................................................................................................................ (3 - 3) The temperature coefficient is as follows. ∂VREF ∂T = ∂VBE3 ∂T + K q • R2 R3 ln R2 .............................................................................................................. (3 - 4) R1 By optimally choosing the ratio of obtained. 8 R2 R3 • R2 , a temperature compensated reference voltage is known to be R1 User’s Manual G12702EJ8V0UM00 Figure 3-3. (Simplified) Band Gap Reference Circuit of µPC7800A 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 β (1 + A) VREF ........................................................................................................................ (3 - 1) Here, A is the open loop gain of the error amplifier and β = 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/ β 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. User’s Manual G12702EJ8V0UM00 9 <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 (µPC7800A Series) Limiting circuit for securing SOA INPUT Q16 Q17 Output stage transistor Q15 : Current limiting transistor R11 : Current detection resistor OUTPUT 10 User’s Manual G12702EJ8V0UM00 <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 (µPC7800A 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 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 (µPC317, µPC337) 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 11 Figure 3-6. Adjustable Output Type Block Diagram INPUT Current source Protection circuit + − Startup circuit Reference voltage source OUTPUT VREF ADJ RB RA Output voltage setting circuit VO 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 OUT IN IN OUT 0.7 V 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. 12 User’s Manual G12702EJ8V0UM00 Figure 3-8. Low Saturation Type Block Diagram INPUT Startup circuit Limiting circuit for securing SOA Reference voltage circuit Error amplifier Drive circuit Series path transistor (PNP type) OUTPUT Overheat protection circuit 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 D1 three-terminal regulator is long, there may be INPUT OUTPUT Three-terminal regulator CIN CO + oscillation. Therefore, add a 0.1 to 0.47 µF capacitor with superior voltage and temperature D2 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 µF 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. User’s Manual G12702EJ8V0UM00 13 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 +VOUT Positive voltage 3-terminal regulator +VIN CIN Load A Co Di1 Load GND Di1' CIN' Co' Negative voltage 3-terminal regulator −VIN Load B −VOUT 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. 14 User’s Manual G12702EJ8V0UM00 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 n p n p p p p n n 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 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 15 Figure 4-4. Example of Typical Connection Circuit (Adjustable Output Power Supply) Input INPUT CIN 0.1µF OUTPUT µ PC317 ADJ + CADJ R1 240 Ω Output VO D1 + CO 1µF R2 Note This example is for a positive voltage. For a negative voltage (µPC337), 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 µF or more capacitor for output voltage transient response improvement (and add 10 µF to CADJ at the same time). CADJ : Connecting a 10 µF 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) Note R2 Setting 2.5 240 5.0 720 12 2064 24 4368 30 5520 (Ω) 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. 16 User’s Manual G12702EJ8V0UM00 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) VIN Drives the base of an external transistor using a threeterminal regulator. Here R1 is determined as follows. IOUT Q1 IO VOUT IN R1 OUT GND C2 0.1 µ F 2. High output current circuit (with short circuit protection) R2 IO VOUT IREG 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. VBE2 I1(MAX.) = R2 the output current is as follows. Q2 IN OUT GND C1 0.1 µ F IO(MAX.) = I1(MAX.) + IREG(MAX.) R1 0.4 Ω 2Ω IO VOUT R1 = I1 IREG IO(MAX.) = IN C1 0.1 µ F R2 IREG R3 6 Ω D1 + IREG(MAX.) …..………..……….….…. (4.3) R2 D1 cancels VBE at Q1. Q1 and three-terminal current distribution is determined by R1 and R2. I1 Q1 VBE2 = C2 0.1 µ F 3. High output current circuit (with short circuit protection) R2 hFEI(MIN.) This is an expansion of circuit 1. Current detection is performed using R2. Therefore, since the current at Q1 is restricted by I1 Q1 R1 VIN …………………….…. (4.1) IOUT IO = hFE1(MIN.) IREG(MAX.) − C1 0.1 µ F R1 6Ω IREG(MAX.) − IREG 6Ω VIN VBE1 R1 = OUT GND ……………………….…………….…… (4.4) R1 + R2 R1 • IREG(MAX.) …………………..….. (4.5) C2 0.1 µ 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. User’s Manual G12702EJ8V0UM00 17 4. High input voltage circuit R1 = IOUT Q1 VIN This circuit can be used when the input voltage exceeds the rating. IN R1 VOUT OUT GND C1 0.1 µ F VIN − VZD IOUT(MAX.) ………………………………….… (4.6) hFE1(MIN.) Moreover, if the load current changes little, a resistor can be used. C2 0.1 µ F ZD 5. High input, high output voltage circuit (without short circuit protection) VIN IN C1 VOUT OUT GND 0.1 µ F R1 C2 0.1 µF 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 ) VIN R1 4.7 kΩ Q2 IN R2 1 kΩ Q1 ZD 18 VOUT OUT GND C1 0.1 µ F D1 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. C2 0.1 µF VOUT = VO(REG) + VZD ..…………………………....... (4.7) D2 protects against reverse bias in the GND and OUT pins on a load short circuit. D2 Note 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. User’s Manual G12702EJ8V0UM00 7. Remote shutdown circuit D1 VIN Q1 IN R1 VOUT OUT GND C1 0.1µ F R2 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. C2 0.1 µ F Control Q2 R3 8. Slow startup circuit (without short circuit protection) VIN IN VOUT OUT GND R2 IBIAS D1 C1 0.1µ F R3 R1 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) Q1 C2 The output voltage after stabilization is VOUT VO2 = VO(REG) + R1 VO final value R2 …………. (4.9) T .=. −CR ln 0.01 [s] ........……………………….... (4.10) Delay time T Power on Time 9. Adjustable output voltage circuit (without load short circuit protection) IN C1 VO(REG) Furthermore, the delay can be represented as follows if expecting up to 99% of the final value. VO(REG) VIN IBIAS + The Zener diode in the circuit shown in 5 is replaced by a resistor. VOUT OUT GND 0.1 µ F IBIAS C2 0.1µF R2 D1 R1 VOUT = VO(REG) + R1 IBIAS + VO(REG) R2 ………… (4.11) 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 µPC317 are superior in output voltage precision and stability. User’s Manual G12702EJ8V0UM00 19 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 µ F + µ PC7805A +VIN VOUT IN OUT GND R4 0.1 µ F VO(REG) 910 Ω C1 R5 9.1 kΩ − R1 + R2 µ PC741 −VIN R3 10 kΩ VOUT = R4 R4 + R5 × VO(REG) × R1 + R2 R1 …………. (4.12) C2 0.1 µ F 10 µ F + RD6.2EB 11. Adjustable output voltage circuit (7 to 30 V) VIN IN VOUT OUT GND − C1 0.1 µ F µ PC741 R1 10 kΩ + This is similar to the circuits shown in 5 and 8. Since it uses op amplifier µPC741 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 µPC741 and the output voltage of the three-terminal regulator. C2 0.1 µ F R2 12. Tracking regulator circuit +VIN IN +VOUT OUT GND 0.1 µ F C2 C1 0.1 µ F 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. R1 − VOUT = C3 0.1 µ F µ PC741 • VOUT ……………………………. (4.13) D1 protects against reverse bias between the base and emitter of the transistor at power-on. R2 D1 −VOUT Tf1 20 R1 − + −VIN R2 User’s Manual G12702EJ8V0UM00 13. Tracking regulator circuit + VIN + VOUT IN C1 OUT GND 0.1 µ F 0.1 µ F C2 C3 0.1 µ F C4 + − VIN 0.1 µ F GND IN OUT R1 µPC741 − R1 R2 R2 − VOUT 14. Positive and negative dual power supply circuit (using positive voltage three-terminal regulators) IN 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. + VOUT OUT GND 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. D1 IN GND OUT GND D2 − VOUT 15. Wired OR D1 VIN1 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 D3 VIN2 D4 User’s Manual G12702EJ8V0UM00 21 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 IN OUT VOUT + OUT GND + 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 shortcircuited or on an overload. Be particularly careful when using a socket. 22 User’s Manual G12702EJ8V0UM00 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) + VIN (b) IN R OUT L + VO IN GND GND D1 C OUT C ZD 5.4 Reverse Bias Between Output Pin and GND Pin Figure 5-3 (a) + VIN IN (b) VOUT OUT GND IBIAS 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. User’s Manual G12702EJ8V0UM00 23 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 I BIAS(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. 24 User’s Manual G12702EJ8V0UM00 Think of this as a 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 µPC7805AHF) Guaranteed Range Inside Broken Lines 5.4 Output voltage VO (V) 5.2 5.0 4.8 VIN = 10 V IO = 5 mA −50 0 50 100 150 Junction temperature TJ (°C) <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. User’s Manual G12702EJ8V0UM00 25 Figure 6-2. Line Regulation REGIN Conceptualization (For µPC7805AHF, VIN = 10 V Standard) +30 +10 REGIN TYP. REGIN MAX. Input stability REGIN (mV) +20 0 −10 TA = 25°C 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). 26 User’s Manual G12702EJ8V0UM00 Figure 6-3. Load Regulation Conceptualization (For µPC7805AHF, IO = 500 mA) −10 REGL MAX. 0 REGL TYP. Load stability REGL (mV) +10 −20 −30 TA = 25°C VIN = 10 V 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. 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 • 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 27 <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 µPC7800A 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 (µPC7800A Series) IOpeak- (VIN − VO) characteristic Peak output current IOpeak (A) 3.0 2.5 2.0 TJ = 1.5 1.0 0°C 75° 25° C C 125 °C 0.5 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). 28 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) + θC-HS + θHS) • PD + TA ............................................................................................... (6.1) Rth(J-C): Thermal resistance (junction to case) θC-HS: Contact thermal resistance (includes thermal resistance of insulation sheet when using insulation sheet) θHS: 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 µPC78L00 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 θHS 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 Thermal resistance of heatsink θ HS (°C/W) 100 50 20 t = 1.5 mm t = 3 mm 10 5 2 1 10 20 50 100 200 500 1000 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. User’s Manual G12702EJ8V0UM00 29 <1> Design objectives Positive power supply using µPC7805AHF Maximum output current I O max. = 0.6 (A) Maximum voltage difference between input and output VDIF max. = 6 (V) Maximum operating ambient temperature T A 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) + θC-HS + θHS) • PD + TA ...................................................................................................... (6.3) Rth(J-C): Thermal resistance (junction to case) θC-HS: Thermal resistance (case to heatsink) θHS: Thermal resistance of heatsink PD: Power dissipation Here, TJ max. = 100 (°C), TA max. = 60 (°C), θC-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 θHS needed in the heatsink. θHS = TJ – T A PD – Rth(J-C) – θC-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. 30 User’s Manual G12702EJ8V0UM00 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 torque (N•m) TO-126 2.0 × 10− to 4.1 × 10− TO-220 3.1 × 10− to 5.1 × 10− MP-45G 3.1 × 10− to 5.1 × 10− 3 3 3 3 3 3 Figure 6-6. Standard Installation Method for Heatsink Insulation 3 M screw Flat washer 3 M screw Flat washer Insulating board Insulating bushing Spring washer 3 M nut Heatsink Heatsink Flat washer Spring washer 3 M nut MP-45G TO-220 Table 6-2. Recommended Insulating Bushings and Insulating Board Code No. Product Name Quality of Materials Material Color Insulating bushing B-24 25K bushing U Gelanex 3310 Light brown Insulating board S-7 MP-25 insulating board A Polyester Colorless, transparent User’s Manual G12702EJ8V0UM00 Incombustibility Grade UL 94V-0 − 31 [MEMO] 32 User’s Manual G12702EJ8V0UM00 [MEMO] User’s Manual G12702EJ8V0UM00 33 [MEMO] 34 User’s Manual G12702EJ8V0UM00 Facsimile Message From: Name Company Tel. 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. FAX Address Thank you for your kind support. North America Hong Kong, Philippines, Oceania NEC Electronics Inc. NEC Electronics Hong Kong Ltd. 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