电源设计手册.pdf

HB206/D ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC SCILLC. SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual perance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. GLOBAL Literature Fulfillment Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone 303-675-2175 or 800-344-3860 Toll Free USA/Canada Fax 303-675-2176 or 800-344-3867 Toll Free USA/Canada Email ONlit N. American Technical Support 800-282-9855 Toll Free USA/Canada JAPAN ON Semiconductor, Japan Customer Focus Center 4-32-1 Nishi-Gotanda, Shinagawa-ku, Tokyo, Japan 141-0031 Phone 81-3-5740-2700 Email r14525 ON Semiconductor Website For additional ination, please contact your local Sales Representative PUBLICATION ORDERING INATION HB206/D Rev. 4, Feb-2002 ON Semiconductor Linear instead it varies with changes in supply voltage VCC and with changes in IC junction temperature TJ induced by changes in ambient temperature and power dissipation. Also, the regulator output voltage VO is affected by the voltage drop across ZO, caused by the output current IO. In the following text, the reference and amplifier sections will be described, and their contributions to the changes in the output voltage analyzed. B. Voltage Reference Naturally, the major requirement for the reference is that it be stable; variations in supply voltage or junction temperature should have little or no effect on the value of the reference voltage Vref. 1. Zener Diode Reference The simplest of a voltage reference is shown in Figure 1–3a. It consists of a resistor and a zener diode. The zener voltage VZ is used as the reference voltage. In order to determine VZ, consider Figure 1–3b. The zener diode VR1 of Figure 1–3a has been replaced with its equivalent circuit model and the value of VZ is therefore given by at a constant junction temperature VZ VBZ IZZZ VBZ VCC – VBZ R Zz ZZ1 whereVBZ zener breakdown voltage IZ zener current ZZ zener impedance at IZ. Note that changes in the supply voltage give rise to changes in the zener current, thereby changing the value of the reference voltage VZ. 7 Figure 1–1. Voltage Regulator Functional Block Diagram VCC VO Reference Error Amplifier Vref Figure 1–2. Voltage Regulator Equivalent Circuit Model VCC ZO IO VO V f VCC, Tj Figure 1–3. Zener Diode Reference VCC R VZ VR1 ab VBZ ZZ VZ IZ R VCC 8 2. Constant Current Zener Reference The effect of zener impedance can be minimized by driving the zener diode with a constant current as shown in Figure 1–4. The value of the zener current is largely independent of VCC and is given by IZ VBEQ1 RSC 2 where VBEQ1 base–emitter voltage of Q1. This gives a reference voltage of Vref VZ VBEQ1 VBZ IZZZ VBEQ13 where IZ is constant and given by Equation 2. The reference voltage about 7.0 V of this configuration is therefore largely independent of supply voltage variations. This configuration has the additional benefit of better temperature stability than that of a simple resistor–zener reference. Referring back to Figure 1–3a, it can be seen that the reference voltage temperature stability is equal to that of the zener diode, VR1. The stability of zener diodes used in most integrated circuitry is about 2.2 mV/C or 0.04/C for a 6.2 V zener. If the junction temperature varies 100C, the zener or reference voltage would vary 4. A variation this large is usually unacceptable. However, the circuit of Figure 1–4 does not have this drawback. Here the positive 2.2 mV/C temperature coefficient TC of the zener diode is offset by the negative 2.2 mV/C TC of the VBE of Q1. This results in a reference voltage with very stable temperature characteristics. Figure 1–4. Constant Current Zener Reference VCC Q2 R Q1 VZ VBEQ1RSC VR1 IZ Vref 9 3. Bandgap Reference Although very stable, the circuit of Figure 1–4 does have a disadvantage in that it requires a supply voltage of 9.0 V or more. Another type of stable reference which requires only a few volts to operate was described by Widlar1 and is shown in Figure 1–5. In this circuit Vref is given by Vref VBEQ3 I2R2 whereI2 VBEQ1 – VBEQ2 R1 neglecting base currents 4 The change in Vref with junction temperature is given by ∆ Vref ∆ VBE3 ∆VBEQ1 – ∆VBEQ2 R1 R25 7 It can be shown that, ∆ VBEQ1 ∆ TJK ln I1 and,∆ VBEQ2 ∆ TJK ln I2 ∆ TJ change in junction temperature ∆ Vref ∆ VBEQ3 ∆ TJK where K a constant and, I1 I2 Combining 5, 6, and 7 6 R2 R1 I1 ln I2 8 Since ∆ VBEQ3 is negative, and with I1 I2, ln I1/I2 is positive, the net change in Vref with temperature variations can be made to equal zero by appropriately selecting the values of I1, R1, and R2. Figure 1–5. Bandgap Reference VCC Vref Q3 R2 R3 I1 Q1 VBEQ1 VBEQ2 R1 Q2 I2 VBEQ3 10 C. The Error Amplifier Given a stable reference, the error amplifier becomes the determining factor in integrated circuit voltage regulator perance. Figure 1–6 shows a typical differential error amplifier in a voltage regulator configuration. With a constant supply voltage VCC and junction temperature, the output voltage is given by VO AVOL υi – ZOL IO AVOL {Vref VIO – VO β} – ZOL IO9 whereAVOL amplifier open loop gain VIO offset voltage ZOL open loop output impedance β feedback ratio β is always ≤1 R1 R1 R2 IO output current υi true differential voltage Manipulating Equation 9 Vref VIO – VO β AVOL ZOL IO AVOL 1 10 Note that if the amplifier open loop gain is infinite, this expression reduces to Vref VIO Vref VIOVO β 1 111 R2 R1 The output voltage can thus be set any value equal to or greater than Vref VIO. Note also that if AVOL is not infinite, with constant output current a non–varying output load, the output voltage can still be “tweaked–in” by varying R1 and R2, even though VO will not exactly equal that given by Equation 11. Assuming a stable reference and a finite value of AVOL, inaccuracy of the output voltage can be traced to the following amplifier characteristics 1. Amplifier Offset Voltage Drift The transistors of integrated circuit amplifiers are usually not perfectly matched. As in operational amplifiers, this is expressed in terms of an offset voltage VIO. At a given temperature, this effect can be nulled out of the desired output voltage by adjusting Vref or 1/β. However, VIO drifts with temperature, typically 5.0 V/C to 15 V/C, causing a proportional change in the output voltage. Closer matching of the internal amplifier transistors minimizes this effect, as does selecting a feedback ratio β to be close to unity. 2. Amplifier Power Supply Sensitivity Changes in regulator output voltage due to power supply voltage variations can be attributed to two amplifier perance parameters power supply rejection ratio PSRR and common mode rejection ratio CMRR. In modern integrated circuit regulator amplifiers, the utilization of constant current sources gives such large values of PSRR that this effect on VO can usually be neglected. However, supply voltage changes can affect the output voltage since these changes appear as common mode voltage changes, and they are best measured by the CMRR. 11 Figure 1–6. Typical Voltage Regulator Configuration VCC IO VO R2 R1 AVOLυi υi ZOL Vref - VIO The definition of common mode voltage VCM, illustrated by Figure 1–7a, is 12 VCMV1 V2 2 – V V– 2 where V1 voltage on amplifier noninverting V2 voltage on amplifier inverting V positive supply voltage V– negative supply voltage Figure 1–7. Definition of Common Mode Voltage Error VO AVOLυi VO V V- V2 V1 V V- V2 V1 VCM CMRR υi ab 12 Figure 1–8. Common Mode Regulator Effects AVOLυi VO VCM CMRR υi VCC Vref R2 R1 In an ideal amplifier, only the differential voltage V1 – V2 has any effect on the output voltage; the value of VCM would not effect the output. In fact, VCM does influence the amplifier output voltage. This effect can be modeled as an additional voltage offset at the amplifier equal to VCM/CMRR as shown in Figures 1–7b and 1–8. The latter figure is the same configuration as Figure 1–6, with amplifier offset voltage and output impedance deleted for clarity and common mode voltage effects added. The output voltage of this configuration is given by VO AVOL υi AVOL13 Manipulating, β VO AVOL 1 14 where VCM Vref – and, CMRR common mode rejection ratio 2 VCC 15 Vref– VCM CMRR – VO Vref– VCM CMRR It can be seen from Equations 14 and 15 that the output can vary when VCC varies. This can be reduced by designing the amplifier to have a high AVOL, a high CMRR, and by choosing the feedback ratio β to be unity. 13 3. Amplifier Output Impedance Referring back to Equation 9, it can be seen that the equivalent regulator output impedance ZO is given by ZO ∆VO ∆IOβAVOL ZOL 16 This impedance must be as low as possible, in order to minimize load current effects on the output voltage. This can be accomplished by lowering ZOL, choosing an amplifier with high AVOL, and by selecting the feedback ratio β to be unity. A simple way of lowering the effective value of ZOL is to make an impedance transation with an emitter follower, as shown in Figure 1–9. Given a change in output current ∆IO the amplifier will see a change of only ∆IO/hFEQ1 in its output current IO. Therefore, ZOL in Equation 16 has been effectively reduced to ZOL/hFEQ1, reducing the overall regulator output impedance ZO. D. The Regulator within a Regulator Approach In the preceding text, we have analyzed the sections of an integrated circuit voltage regulator and determined how they contribute to its non–ideal perance characteristics. These are shown in Table 1–1 along with procedures which minimize their effects. It can be seen that in all cases regulator perance can be improved by selecting AVOL as high as possible and β 1. Since a limit is soon approached in how much AVOL can be practically obtained in an integrated circuit amplifier, selecting a feedback ratio β equal to unity is the only viable way of improving total regulator perance, especially in reducing regulator output impedance. However, this presents a basic problem to the regulator designer. If the configuration of Figure 1–6 is used, the output voltage cannot be adjusted to a value other than Vref. The solution is to utilize a different regulator configuration known as the regulator within a regulator approach.2 Its greatest benefit is in reducing total regulator output impedance. Figure 1–9. Emitter Follower Output VCC Vref R2 R1 IO′ Q1IO ZOL VO 14 Table 1–1 VO Changes Section Effect Can Be Induced ByMinimized By Selecting Reference VCC Constant current–zener Bandgap reference Reference TJ Bandgap reference TC compensated zener VCC High CMRR amplifier High AVOL amplifier β 1 Amplifier TJ Low VIO drift amplifier High AVOL amplifier β 1 IO Low ZOL amplifier High AVOL amplifier Additional emitter follower output β 1 As shown in Figure 1–10, amplifier A1 sets up a voltage V1 given by V1 Vref171 R2 R1 V1 now serves as the reference voltage for amplifier A2, whose output voltage VO is given by 18V1 VrefVO 1 R2 R1 Note that the output impedance of A2, and therefore the regulator output impedance, has been minimized by selecting A2’s feedback factor to be unity; and that output voltage can still be set at voltages greater than Vref by adjusting R1 and R2. Figure 1–10. The “Regulator within a Regulator” Configuration R2 R1 VO A1 A2 V1 ZOL Vref 1Widlar, R. J., New Developments in IC Voltage Regulators, IEEE Journal of Solid State Circuits, Feb.1971, Vol. SC–6, pgs. 2–7. 2Tom Fredericksen, IEEE Journal of Solid State Circuits, Vol. SC–3, Number 4, Dec. 1968, A Monolithic High Power Series Voltage Regulator. 15 SECTION 2 SELECTING A LINEAR IC VOLTAGE REGULATOR A. Selecting the Type of Regulator There are five basic linear regulator types; positive, negative, fixed output, tracking and floating regulators. Each has its own particular characteristics and best uses, and selection depends on the designer’s needs and trade–offs in perance and cost. 1. Positive Versus Negative Regulators In most cases, a positive regulator is used to regulate positive voltages and a negative regulator negative voltages. However, depending on the system’s grounding requirements, each regulator type may be used to regulate the “opposite” voltage. Figures 2–1a and 2–1b show the regulators used in the conventional and obvious mode. Note that the ground reference for each indicated by the heavy line is continuous. Several positive regulators could be used with the same supply to deliver several voltages with common grounds; negative regulators may be utilized in a similar manner. If no other common supplies or system components operate off the supply to the regulator, the circuits of Figures 2–1c and 2–1d may be used to regulate positive voltages with a negative regulator and vice versa. In these configurations, the