Linear Voltage Regulators Power supplies and chargers SMM Alavi, SBU, Fall2017

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Clement Carroll
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1 Linear Voltage Regulator LVRs can be classified based on the type of the transistor that is used as the pass element. The bipolar junction transistor (BJT), field effect transistor (FET), or metal oxide semiconductor field-effect transistor (MOSFET) can be used in the output In this section, three of widely found LVRs are reviewed, which use the BJT in their output structure (pass element). 1. NPN-Darlington LVR Figure 11: Standard NPN Darlington LVR The output voltage is sensed by the resistive divider, and compared with the reference value. The error controls the base current for the PNP transistor, which in turn controls the drive current to the darlington pair, and thus the load current, maintaining the output voltage at the desired level. Vin increases Vout increases Vout-Vref increases the base voltage for Q3 increases bias for the PNP transistor reduces the reducing the base current for the darlington pair from beta=ic/ib, the load current reduces, then Vout decreases. This continues till Vout goes back to its desired value. There is a transient time. Vin decreases Vout decreases Vout-Vref decreases the base voltage for Q3 decreases bias for the PNP transistor increases load current increases, Vout increases till adjusted. Dropout voltage is defined as the minimum input-to-output difference of voltage required to keep a linear regulator s output in regulation. The smaller, the better. V D = 2V BE + V CE Page 1

2 Typical value for the V D for Darlington LVR is about 1.7V to 2.5V. The input voltage should be V D volt above V out. Ground Pin current (also sometimes called quiescent or no load current), I G, is the current used by the device which does not flow to the load. The smaller the better. For the darlington LVR, due to the large current gain of the darlington pair, the load current is controlled by a small I G. It is typically less than 10mA. The main advantage of the NPN darlington architecture is its ability to pass high currents, as the current gain for the darlington pair is high The simplified model A simplified model of the standard NPN darlington is shown in Figure 12. The darlington pair acts as a variable resistor with the output load. Figure 12: A simplified model for the standard NPN darlington LVR Advantages: - It is the least expensive of the three bipolar types. - Its output circuitry occupies the smallest area on chip. - It often only requires a small compensation capacitor which in most cases, is integrated on chip. (Occasionally an application with a rapidly changing dynamic load will require an external capacitor. In these cases, the capacitor damps the regulator s fast output response and prevents output voltage overshoot.) 1.3. Disadvantages - Large dropout voltage ( 2.0 V) - Lack of reverse battery protection. - If low dropout voltage is not a primary concern, and the system does not require reverse battery protection, the NPN output structure is the topology of choice in an application. Reverse battery protection: If battery is, by mistake, placed with revers polarity, the circuit should be able to protect itself. Page 2

3 1.4. Commercial Example Figure 13 shows the schematic of KIA7805, a 5V LVR, manufactured by KEC semiconductor, Korea. The standard NPN darlington structure is used for KIA7805. The PNP transistor Q11-1 drives the darlington pair Q15 and Q16. R20 and R21 are resistive dividers. If the output voltage rises or falls, the change propagates through Q6, and the voltage across R6 changes, causing the base voltage of Q7 rises or falls. Transistors Q7 and Q8 generate output error, which controls the emitter voltage of Q11-1 (i.e. the base voltage of Q15). They are inverting amplifier, thus the rise (fall) of the 5V output voltage, reduces (increases) the base voltage of Q15, which in turn, decreases (increases) the output current. Figure 13: The schematic of KIA7805, a 5V LVR, based on the standard NPN darlington structure. Page 3

4 2. PNP LVR (Low-Dropout or LDO LVR) The operation of the circuit is same as that of standard NPN-darlington, with a main difference that, the pass element is a PNP transistor that is driven directly by the output of the error amplifier, as shown in Figure 14. Figure 14: Schematic of a PNP (Low-dropout, LDO) LVR. The droupout voltage is the collector-emitter voltage of the PNP transistor, which results in less V D compared to that of the standard NPN-darlington. The typical value for V D is between 100mV and 600mV. The ground pin current is larger in comparison with that of the standard NPN darlington. The reason is simply because of that in the standard NPN darlington, the output current/voltage is controlled by a darlington pair with a high current gain β. The PNP transistor, with a very small base current, can cause a very large current to flow through the NPN transistor that is connected to the load. In contrast, for the LDO LVR, the PNP transistor supplies the output current/voltage, which requires larger ground pin current that that of the standard NPN-darlignton or the quasi-ldo that is discussed in the next section Advantages: - The dropout voltage is low, between 100mV and 600mV. If the input voltage is supplied by a battery, the battery life time will increase. - The base-emitter junction protects the LDO against the reverse battery condition Disadvantage - The LDO leads to a relatively high quiescent current - It requires an external large output capacitor (about 10uF) to ensure stability - Compared to NPN transistors, PNP transistors occupy more die area to pass the same amount of current. Page 4

5 2.3. Commercial example LP2950 is a LDO based LVR, which can provide 3V, 3.3V, and 5V output voltage. A functional block diagram of LP2950 is shown in Figure Figure 15: Functional block diagram of LP2950 Most manufacturers provide graphs of the dropout voltage as a function of load current and temperature, as show in Figure 16. It is seen that the dropout voltage increases by the load current and temperature. Figure 16: Dropout voltage as a function of load current and temperature for LP2950. Page 5

6 3. PNP-NPN LVR (Quasi-LDO) The structure of the PNP-NPN (also known as quasi-ldo) LVR is very similar to the standard NPN darlington, with a main difference that it uses only one NPN transistor instead of a darlington pair. Figure 17: The PNP-NPN (also known as quasi-ldo) LVR. The voltage dropout includes the emitter-collector voltage of the PNP transistor plus the base emitter voltage of the NPN transistor. Therefore, its V D is larger than the LDO, but less than the standard NPN darlington LVR. The graphs of V D as functions of the load current and temperature are typically given by manufacturer. For this structure, the PNP transistor drives the base current for the NPN transistor, and because of the large current gain β, the ground pin current is relatively less than the LDO, and almost at the same level as that used of the standard NPN darlington LVR Disadvantage - It requires an external large output capacitor (about 10uF) to ensure stability - It does not have inherent protection against the reverse battery condition. Figure 18 summarizes the comparisons between the aforementioned LVRs Page 6

7 Figure 18: Comparisons between three LVR types 4. Internal voltage reference 4.1. Using Zener diode Temperature-compensated zener diodes are normally used for the reference. However, these have breakdown voltages between 7V and 9V, which put a lower limit on the input voltage to the regulator Using the bandgap voltage The reference voltage can be developed by using the energy-band-gap voltage of the semiconductor material. In this method, the reference does not impose minimum input voltage limitations on the regulator. The figure below, shows a circuit, which provides a temperature-compensated voltage reference, V ref, based on the energy-band-gap voltage. Operating conditions: - Q1 is operated at a relatively high current density. The current density of Q2 is about 10 times lower. - If the transistors have high current gains, the voltage across R 2 will also be proportional to ΔV BE, V(R2)=R 2*Ic=R2* ΔV BE/R 3. Page 7

8 Figure 19: Silicon bandgap voltage reference V BE for Q3 is given by: V go is the extrapolated energy-band-gap voltage for the semiconductor material at absolute zero q is the charge of an electron n is a constant which depends on how the transistor is made (approximately 1.5 for double-diffused, NPN transistors) k is Boltzmann's constant, T is absolute temperature I C is collector current VBE0 is the emitter-base voltage at T 0 and I C0. ΔV BE is given by Where J 1 and J 2 are the current densities for Q1 and Q2, respectively. The voltage reference is then given by: Page 8

9 For the temperature-compensated voltage reference,, which yields or Thus, if the sum of and is equal to the energy-band-gap voltage of the semiconductor, the reference will be temperature-compensated. Note that the emitter base voltage of Q3 has a negative temperature coefficient while the ΔV BE component across R 2 has a positive temperature coefficient in the equation of each other in order to provide a temperature-compensated voltage reference V ref., compensating 4.3. Commercial example of bandgap reference In the design of KIA7805 (Figure 13), the reference voltage is created by using the bandgap reference method. The part of the circuit, associated with the bandgap reference is Q2 and Q3 are transistors with high current densities. Q4 and Q5 are transistors with low current density. V BE(Q3) is approximately equal to V BE(Q2) V BE(Q4) is approximately equal to V BE(Q5) By defining ΔV BE = V BE (Q3) V BE (Q4), V R6=V BE(Q3)+V BE(Q2)-V BE(Q4)-V BE(Q5)-V R3 V R6 = 2ΔV BE V R3 Transistors have high current gain, therefore, I E(Q5) is approximately equal to I E(Q6). Therefore, V R7 = 16.5V R6 = 16.5 (2ΔV BE V R3 ) = 33ΔV BE 16.5V R3 The bandgap voltage reference V ref is then computed as follows: V ref = V BE (Q6) + V R7 + V BE (Q7) + V BE (Q8) The base-emitter voltages of Q6, Q7 and Q8 provide the negative-temperature-coefficient component for V ref, and V R7 provide the positive-temperature-coefficient component for V ref. Thus, a temperature Page 9

10 compensated V ref is obtained, which is equal to three times the bandgap voltage. Note that the change on the output voltage does not affect V R3, and thus it can be considered as a constant value. Transistors Q2, Q3, Q4, and Q5 are so that 33ΔV BE 16.5V R3 is positive. Thus, V ref = = 3.75 V Figure 20: The bandgap voltage reference for KIA References [1] Linear Regulator Output Structures, Rev. 1, Technical note # SR004AN/D, On Semiconductor, April [2] AN-42 IC provides On-Card Regulation for Logic Circuits, Technical note # SNVA512B, May [3] C. Simpson, Linear and Switching Voltage Regulator Fundamentals, National Semiconductor. Page 10

11 Problems 1- For the following circuits, - Describe how the output voltage is regulated if V in increases. - Determine the voltage dropout. Page 11

12 2- The following figure shows the functional block diagram of the CB8129 LVR. A) Determine the type of LVR. B) How the output voltage is regulated if the input voltage decreases suddenly. C) List some of advantages and disadvantages of the chip. 3- The following figure shows a schematic of LM7805. a- Determine the type of the linear voltage regulator. b- Determine and describe the part of the circuit which generates the bandgap voltage. c- Determine the value of the bandgap voltage. d- Determine the error amplifying transistors. e- Describe the voltage regulation (negative feedback) procedure by the assumption that V out increases or decreases from its desired level. Page 12

13 Page 13

14 4- Repeat question 3 for the following circuit Texas Instruments LM109/309. Page 14

Linear Regulator APPLICATION NOTE

Linear Regulator APPLICATION NOTE Kieran O Malley ON Semiconductor 2000 South County Trail East Greenwich, RI 02818 APPLICATION NOTE Choosing a linear regulator for an application involves more than looking for the part with the lowest