Technical Review of Low
`Dropout Voltage Regulator
`Operation and Performance
`
`Application
`Report
`
`August 1999
`
`Mixed Signal Products
`SLVA072
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 1 of 30
`
`

`

`IMPORTANT NOTICE
`
`Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
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`to verify, before placing orders, that information being relied on is current and complete. All products are sold
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`pertaining to warranty, patent infringement, and limitation of liability.
`
`TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
`accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
`TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily
`performed, except those mandated by government requirements.
`
`CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF
`DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL
`APPLICATIONS”). TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, AUTHORIZED, OR
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`
`In order to minimize risks associated with the customer’s applications, adequate design and operating
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`
`TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
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`
`Copyright  1999, Texas Instruments Incorporated
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 2 of 30
`
`

`

`Contents
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`1 Dropout Voltage
`1.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2 Quiescent Current or Ground Current
`2.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3 LDO Topologies
`3.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4 Efficiency
`4.1 Application Implications—An Example
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`1
`4
`
`5
`6
`
`7
`9
`
`9
`9
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5 Load Regulation
`5.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`10
`11
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`6 Line Regulation
`6.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`12
`13
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`7 Transient Response
`7.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`14
`14
`
`8 Frequency Response
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`16
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`9 Range of Stable ESR (Tunnel of Death)
`9.1 Application Implications
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`18
`20
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`10 Accuracy
`10.1 Reference Voltage Drift
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`10.2 Error Amplifier Voltage Drift
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`10.3 Tolerance of External Sampling Resistors
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`10.4 Application Implications—an Example
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`20
`20
`22
`22
`23
`
`11 References
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`25
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`iii
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 3 of 30
`
`

`

`Figures
`
`List of Figures
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`1. LDO Voltage Regulator
`1
`2. Series Pass Element I-V Characteristic and LDO Equivalent Circuits
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2
`3. NMOS Operation With LDO in Saturation Region
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3
`4. NMOS Operation With LDO in Dropout Region
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3
`5. Typical Input/Output Voltage Characteristics of a Linear Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4
`6. Dropout Region of TI TPS76333 (3.3-V LDO)
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4
`7. I-V Characteristic of Bipolar Transistors
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5
`8. I-V Characteristic of MOS Transistors
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`6
`9. Quiescent Current and Output Current
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`6
`10. Linear Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`7
`11. Pass Element Structures
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`7
`12. PMOS Voltage Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`10
`13. Load Transient Response of TPS76350
`11
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`14. TPS76350 Output Voltage With Respect to Output Current
`11
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`15. Line Transient Response of TPS76333
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`13
`16. TPS76333 Output Voltage With Respect to Input Voltage
`13
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`17. 1.2-V, 100-mA LDO Voltage Regulator With Output Capacitor of 4.7 m F
`14
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`18. Transient Response of Step Load Change of 1.2-V, 100-mA LDO Voltage Regulator
`With an Output Capacitor Co=4.7 m F
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`19. AC Model of a Linear Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`20. Frequency Response of the LDO Voltage Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`21. LDO Frequency Response Without Compensation
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`22. LDO Frequency Response With External Compensation
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`23. Unstable Frequency Response of LDO With too High ESR
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`24. Unstable Frequency Response of LDO With too Low ESR
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`25. Range of Stable ESR Values
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`26. LDO With Reference Voltage Drift
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`27. LDO With Error Amplifier Voltage Drift
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`28. LDO With Sampling Resistors
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`29. LDO Regulator
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`15
`16
`17
`18
`18
`19
`19
`20
`21
`22
`23
`24
`
`1 Comparison of Pass Element Structures
`
`List of Tables
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`9
`
`iv
`
`SLVA072
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 4 of 30
`
`

`

`Technical Review of Low Dropout Voltage Regulator Operation
`and Performance
`
`Bang S. Lee
`
`ABSTRACT
`This application report provides a technical review of low dropout (LDO) voltage
`regulators, and describes fundamental concepts including dropout voltage, quiescent
`current, and topologies. The report also includes detailed discussions of load/line
`regulation, efficiency, frequency response, range of stable ESR, and accuracy of LDO
`voltage regulators.
`
`1 Dropout Voltage
`Dropout voltage is the input-to-output differential voltage at which the circuit
`ceases to regulate against further reductions in input voltage; this point occurs
`when the input voltage approaches the output voltage. Figure 1 shows an
`example of a simple NMOS low dropout (LDO) voltage regulator.
`
`Series Pass
`Element
`_
`Vds
`
`+
`
`S
`
`D
`
`G
`
`Control
`Circuit
`
`Id
`
`RO
`
`+
`
`VO
`_
`
`+
`
`Vi
`
`_
`
`Figure 1. LDO Voltage Regulator
`
`LDO operation can be explained using the NMOS series pass element I-V
`characteristics shown in Figure 2. NMOS devices are not widely used in LDO
`designs, but they simplify the explanation of LDO performance. Figure 2 (a)
`shows the two regions of operation—linear and saturation. In the linear region,
`the series pass element acts like a series resistor. In the saturation region, the
`device becomes a voltage-controlled current source. Voltage regulators usually
`operate in the saturation region.
`
`1
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 5 of 30
`
`

`

`Dropout Voltage
`
`Linear Region
`
`Saturation Region
`
`Id
`
`(Operation Like a Resistor)
`
`(Operation Like a Current Source)
`
`Slope = Ri
`
`Vgs5
`
`Vgs4
`
`Vgs3
`
`Vgs2
`
`Vgs1
`
`0
`
`Vds(sat)
`
`Vds = VI – VO
`
`(a) I-V Characteristic of n-channel MOSFET
`
`Series Pass Element
`
`+
`
`VI
`
`_
`
`S
`
`D
`
`Ri
`
`Id
`
`RO
`
`+
`
`VO
`
`_
`
`Series Pass Element
`
`Rds
`
`b V2
`gs
`
`S
`
`G
`
`+
`
`_
`Vgs
`
`Id
`
`D
`
`RO
`
`+
`
`VO
`_
`
`+
`
`VI
`_
`
`(b) LDO Equivalent Circuit in The Linear Region
`
`(c) LDO Equivalent Circuit in The Saturation Region
`
`Figure 2. Series Pass Element I-V Characteristic and LDO Equivalent Circuits
`
`Figures 2 (b) and (c) show the LDO equivalent circuits for the two operating
`regions. The control circuit is not shown. Figure 2 (c) shows the LDO equivalent
`circuit in the saturation region (assume threshold voltage is zero). There is a
`constant current source between the drain and source, which is a function of
`gate-to-source voltage, Vgs. The drain current (load current) is given by
`Id + bV2
`gs
`Where b is a current gain.
`From equation (1), the series pass element acts like a constant current source
`in the saturation region in terms of gate-to-source voltage. Under varying load
`conditions, Vgs controls the LDO regulator to supply the demand output load.
`Figure 3 illustrates the LDO operation in the saturation region. When load current
`increases from Id2 to Id3, the operating point moves from Po to P2, and the
`input-to-output voltage differential, Vds, is given by
`Vds + VI* VO
`
`(1)
`
`(2)
`
`2
`
`SLVA072
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`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 6 of 30
`
`

`

`Dropout Voltage
`
`From equation (2) and Figure 3, as the input voltage decreases, the voltage
`regulator pushes the operating point toward P1 (toward the dropout region). As
`the input voltage nears the output voltage, a critical point exists at which the
`voltage regulator can not maintain a regulated output. The point at which the LDO
`circuit begins to lose loop control is called the dropout voltage. Below the dropout
`voltage, the LDO regulator can no longer regulate the output.
`Ids
`
`Id3
`
`Id2
`
`Id1
`
`P2
`
`P1
`
`P0
`
`Operation Within
`Regulation
`
`Vgs4
`
`Vgs3
`
`Vgs2
`
`Vgs1
`
`Vgs
`
`Vgs3Vgs2
`
`Vgs1
`
`0
`
`Vds1
`V(Dropout)
`
`Vds0
`
`Vds2
`
`Vds = VI – VO
`
`Figure 3. NMOS Operation With LDO in Saturation Region
`
`IO
`
`Linear Region
`
`Ri(min)
`
`Operation
`Within
`Regulation
`
`P2
`
`P1
`
`P3
`
`Ri(max)
`
`Pto
`
`Vgs7
`
`Vgs6
`
`Vgs5
`
`Vgs4
`
`Vgs3
`
`Vgs2
`
`Vgs1
`
`V3
`
`V2
`
`V1
`
`Vds = VI – VO
`
`IO1
`
`0
`
`Figure 4. NMOS Operation With LDO in Dropout Region
`In the dropout region, the series pass element limits the load current like a
`resistor—as shown in Figure 2 (b). Figure 4 shows NMOS operation with the LDO
`regulator in the dropout region and decreasing input voltage. The equivalent
`resistors Ri(max) and Ri(min) are the maximum and minimum values respectively
`of the series pass element in the linear region. When the input voltage decreases
`to near the output voltage, the operating point P1 moves to the operating point
`P2 that is the minimum regulating point at the specific load condition (Io1) (i.e.,
`dropout voltage). Within the dropout region, Vgs is not a function of the control
`loop, but of the input voltage. In other words, the regulator control loop cut off and
`Vgs begins to depend on the decreasing input voltage. Thus when the input
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`3
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 7 of 30
`
`b
`

`

`Dropout Voltage
`
`voltage decreases further, the control voltage (Vgs) also decreases in proportion
`to the decreasing input voltage. The operating point moves down to P3 from P2.
`Finally, the regulator reaches the turnoff point, Pto.
`
`[V]
`
`Off Region
`
`Dropout
`Region
`
`VI
`
`VO
`
`Regulation
`Region
`
`Vds
`
`V(dropout)
`
`Figure 5. Typical Input/Output Voltage Characteristics of a Linear Regulator
`
`Figure 5 shows the dropout region in relation to the off and regulation regions.
`Below V(dropout) , the output voltage drops with decreasing input voltage.
`
`1.1 Application Implications
`In dropout region, the magnitude of the dropout voltage depends on the load
`current and the on resistance (Ron) of the series pass element. It is given by
`Vdo + ILoadRon
`Throughout the dropout region, the output voltage is not maintained any more by
`the control loop since the control loop is electrically disconnected at the output
`of the controller (Figure 1) and then the pass device acts like a resistor. Therefore,
`the output voltage can be pulled down to ground by the load.
`
`Figure 6 shows the input-output characteristics of the TPS76333 3.3-V LDO
`regulator. The dropout voltage of the TPS76333 is typically 300 mV at 150 mA.
`The LDO regulator begins dropping out at 3.6-V input voltage; the range of the
`dropout region is between 3.6 V and 2.0 V input voltage.
`
`(3)
`
`Regulation
`Region
`
`Dropout
`Region
`
`3.3
`
`Off
`Region
`
`– Output Voltage – V
`
`VO
`
`0
`
`2.0
`
`3.6
`
`10
`
`VI – input voltage – V
`
`Figure 6. Dropout Region of TI TPS76333 (3.3-V LDO)
`
`4
`
`SLVA072
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`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 8 of 30
`
`

`

`2 Quiescent Current or Ground Current
`Quiescent current, or ground current, is the difference between input and output
`currents. Minimum quiescent current is necessary for maximum current
`efficiency. Quiescent current is defined by
`
`Quiescent Current or Ground Current
`
`Iq + II* IO
`Quiescent current consists of bias current (such as band-gap reference,
`sampling resistor and error amplifier) and drive current of the series pass
`element, which do not contribute to output power. The value of quiescent current
`is mostly determined by the series pass element, topologies, ambient
`temperature, etc.
`Linear voltage regulators usually employ bipolar or MOS transistors as the series
`pass element. The collector current of bipolar transistors is given by
`
`Ic+ bIb
`where b is forward current gain and typically ranges from 20-500, Ic is the collector
`current, and Ib is the base current. Figure 7 shows the I-V characteristic of bipolar
`transistors.
`
`Ic
`
`Ic3
`
`Ic2
`
`Ic1
`
`Ib
`
`Ib3 Ib2
`
`Ib1
`
`0
`
`Ib4
`
`Ib3
`
`Ib2
`
`Ib1
`
`Vce
`
`Figure 7.
`
`I-V Characteristic of Bipolar Transistors
`
`Equation (5) and Figure 7 show that the base current of bipolar transistors is
`proportional to the collector current. As load current increases, base current also
`increases. Since base current contributes to quiescent current, bipolar
`transistors intrinsically have high quiescent currents. In addiiton, during the drop
`out region the quiescent current can increase due to the addiitional parasitic
`current path between the emitter and the base of the bipolar transistor, which is
`caused by a lower base voltage than that of the output voltage.
`The drain-source current of MOS transistors is given by
`
`2
`
`Ids+ b1ǒVgs* VtǓ
`where b 1 is a MOS transistor gain factor, Vgs is the gate-to-source voltage, and
`Vt is the device threshold. Figure 8 shows the I-V characteristic of MOS
`transistors.
`
`(4)
`
`(5)
`
`(6)
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`5
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 9 of 30
`
`b
`

`

`Quiescent Current or Ground Current
`
`1b
`
`Ids
`
`Ids4
`
`Ids3
`
`Ids2
`
`Vgs
`
`Vgs2
`Vgs4
`Vgs3
`
`0
`
`Vgs4
`
`Vgs3
`
`Vgs2
`
`Vgs1
`
`Vds
`
`Figure 8.
`
`I-V Characteristic of MOS Transistors
`
`The drain-to-source current is a function of the gate-to-source voltage, not the
`gate current. Thus, MOS transistors maintain a near constant gate current
`regardless of the load condition.
`
`2.1 Application Implications
`Figure 9 shows the quiescent current of both transistors with respect to the load
`current.
`
`Iq
`
`Bipolar Transistor
`
`Standby
`Current
`
`MOS Transistor
`
`0
`
`IO
`
`Figure 9. Quiescent Current and Output Current
`
`For bipolar transistors, the quiescent current increases proportionally with the
`output current because the series pass element is a current-driven device. For
`MOS transistors, the quiescent current has a near constant value with respect to
`the load current since the device is voltage-driven. The only things that contribute
`to the quiescent current for MOS transistors are the biasing currents of band-gap,
`sampling resistor, and error amplifier. In applications where power consumption
`is critical or where small bias current is needed in comparison with the output
`current, an LDO voltage regulator employing MOS transistors is essential.
`
`6
`
`SLVA072
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 10 of 30
`
`

`

`3 LDO Topologies
`The regulator circuit can be partitioned into four functional blocks: the reference,
`the pass element, the sampling resistor, and the error amplifier as shown in
`Figure 10.
`
`LDO Topologies
`
`_
`
`+
`
`VDO
` Pass
`Element
`
`Idrv
`
`IO
`
`+
`
`VO
`
`_
`
`+ _
`
`II
`
`Vref
`
`+
`
`VI
`
`_
`
`Ia
`
`Ir
`
`Is
`
`Figure 10. Linear Regulator
`
`Figure 11 shows that linear voltage regulators can be classified based on pass
`element structures: NPN-Darlington, NPN, PNP, PMOS, and NMOS regulators.
`The bipolar devices can deliver the highest output currents for a given supply
`voltage. The MOS-based circuits offer limited drive performance with a strong
`dependence on aspect ratio (width to length ratio) and to voltage-gate drive. On
`the positive side, however, the voltage-driven MOS devices minimize quiescent
`current flow.
`
`+
`
`VDO
`
`–
`
`Idrv
`
`(c) PNP
`
`––
`
`VBE
`
`+
`
`VDO
`
`+ Vsat
`
`+
`
`Ib
`
`(b) NPN
`
`+
`
`VDO –
`–
`
`2VBE
`
`+
`
`+ Vsat
`
`(a) Darlington
`
`––
`
`VGS
`+
`
`+
`
`VDO
`
`+
`
`Vsat –
`
`+
`
`VDO
`
`–
`
`(d) PMOS
`
`(e) NMOS
`
`Figure 11. Pass Element Structures
`
`The Darlington requires at least 1.6 V of dropout voltage to regulate, while the
`LDO will typically work with less than 500 mV of input-to-output voltage
`differential. The dropout voltage of NPN-Darlington is given by
`
`V(dropout) + VCE(sat) ) 2VBE ^ 1.6 X 2.5 V
`
`for Darlington
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`(7)
`
`7
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 11 of 30
`
`

`

`LDO Topologies
`
`The NPN regulator is comprised of an NPN and a PNP transistor. The base
`potential of the NPN transistor should always be higher than the emitter potential
`to ensure proper operation of the pass element. When the input-output differential
`voltage is high, there is no problem. When the input voltage approaches the
`output voltage, the control circuit pushes the pass element toward saturation to
`ensure proper operation of the regulator, and the value of the transistor
`equivalent variable resistor decreases. However, the equivalent variable resistor
`can not decrease to zero because the transistor NPN needs to maintain a
`necessary Vbe level. Below a certain level of input voltage, the regulator cannot
`maintain the regulation.
`The minimum voltage difference between the input and output required to
`maintain regulation (dropout voltage) is given by
`V(dropout) + VCE(sat) ) VBE w 0.9 V
`The NPN transistor receives its drive current from the input rail through the PNP
`transistor. The base drive circuit contributes its emitter current (Idrv) to output
`current (IO). Therefore, the quiescent current of the NPN regulator is small. The
`quiescent current for the NPN regulator is defined as follows:
`
`for NPN regulator
`
`Iq + Ibias
`Where:
`
`for NPN regulator
`
`Iq = quiescent current
`Ibias = total bias current (Ibias = Ia + Ir + Is)
`The PNP regulator shown in Figure 11 (c) operates the same as the NPN with the
`exception that the NPN pass transistor has been replaced by a single PNP
`transistor. The big advantage of the PNP regulator is that the PNP pass transistor
`can maintain output regulation with very little voltage drop across it.
`V(dropout) + VCE(sat) ^ 0.15 X 0.4 V
`By selecting a high-gain series transistor, dropout voltages as low as 150 mV at
`100 mA are possible. However, the base drive current flows to ground and no
`longer contributes to the output current. The value of this ground current directly
`depends on the pass element transistor’s gain. Thus, the quiescent current of the
`PNP regulator is higher than the NPN regulator. The quiescent current is defined
`as follows:
`
`for PNP regulator
`
`(8)
`
`(9)
`
`(10)
`
`Iq + Idrv) Ibias ^ 0.8 X 2.6 mA
`Where:
`
`for PNP regulator
`
`(11)
`
`Idrv = base drive current of PNP
`Figures 11(d) and 11(e) show the P-MOS and N-MOS voltage regulators
`respectively, which employ MOSFETs as the pass element. The PMOS devices
`have very low dropout voltages. The NMOS can have a low dropout voltage with
`a charge pump. The dropout voltage is determined by saturation voltage across
`the pass element, and the dropout voltage is proportional to the current flowing
`through the pass element.
`
`V(dropout) + IoRon ^ 35 X 350 mV
`
`for PMOS regulator
`
`(12)
`
`8
`
`SLVA072
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 12 of 30
`
`

`

`where Ron is the on-resistance of the pass element
`At light load, the dropout voltage is only a few millivolts. At full load, the typical
`dropout voltage is 300 mV for most of the families. The MOSFET pass element
`is a voltage controlled device and, unlike a PNP transistor, does not require
`increased drive current as output current increases. Thus, very low quiescent
`current is obtained (less than 1 mA).
`
`3.1 Application Implications
`Table 1 summarizes the differences of these pass element devices.[2]
`Table 1. Comparison of Pass Element Structures
`
`PARAMETER
`Io,max
`Iq
`Vdropout
`Speed
`
`DARLINGTON
`High
`Medium
`Vsat+2Vbe
`Fast
`
`NPN
`High
`Medium
`Vsat+Vbe
`Fast
`
`PNP
`High
`Large
`Vce(sat)
`Slow
`
`NMOS
`Medium
`Low
`Vsat+Vgs
`Medium
`
`PMOS
`Medium
`Low
`VSD(sat)
`Medium
`
`Traditionally, the PNP bipolar transistor has been applied to low dropout
`applications, primarily because it easily enables a low drop out voltage. However,
`it has a high quiescent current and low efficiency, which are not ideal in
`applications where maximizing efficiency is a priority. The NMOS pass element
`is most advantageous due to its low on resistance. Unfortunately, the gate drive
`difficulties make it less than ideal in applicaitons and as a result there are few
`NMOS LDOs available. PMOS devices have been highly developed and now
`have performance levels exceeding most bipolar devices.
`
`Efficiency+
`
`4 Efficiency
`The efficiency of a LDO regulator is limited by the quiescent current and
`input/output voltage as follows:
`IoVo
`ǒIo) IqǓVI
`To have a high efficiency LDO regulator, drop out voltage and quiescent current
`must be minimized. In addition, the voltage difference between input and output
`must be minimized since the power dissipation of LDO regulators accounts
`for the efficiencyǒPower Dissipation+ ǒVI* VOǓIOǓ. The input/output voltage
`difference is an intrinsic factor in determining the efficiency regardless of the load
`condition.
`
` 100
`
`Efficiency
`
`(13)
`
`4.1 Application Implications—An Example
`What is the efficiency of the TPS76333 3.3-V LDO regulator with the following
`operating conditions?
`•
`Input voltage range is 3.8 V to 4.5 V.
`• Output current range is 100 mA to 150 mA.
`• Maximum quiescent current is 140 m A.
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`9
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 13 of 30
`
`

`

`(14)
`
`(15)
`
`(16)
`
`(17)
`
`(18)
`
`Load Regulation
`
`Efficiency+
`
`150 mA 3.3
`ǒ150 mA) 140 mAǓ4.5 V 100 + 73.2%
`
`5 Load Regulation
`Load regulation is a measure of the circuit’s ability to maintain the specified output
`voltage under varying load conditions. Load regulation is defined as
`Load regulation; DVo
`DIo
`Figure 12 shows a PMOS voltage regulator. The output voltage change for a
`given load change (D Vo/D
`Io) under constant input voltage VI can be calculated as
`follows:
`Q1 is the series pass element, and b is the current gain of Q1. ga is the
`transconductance of the error amplifier at its operating point.
`
`Q1 (b )
`
`IO
`
`P1
`
`+
`
`R1
`
`VO ± D
`
` VO
`
`RL
`
`R2
`
`–
`
`Vs
`
`Vr
`
`+ _
`
`ga
`
`Vref
`
`+
`
`VI
`
`–
`
`Figure 12. PMOS Voltage Regulator
`Assume that there is a small output current change (D
`Io). The change of output
`current causes the output voltage change. Thus,
`
`DVo+ DIoReq
`Where Req is the equivalent output resistor at P1 (Req = (R1 + R2)RL ≈ RL).
`The change of sensed voltage multiplied by ga of the error amplifier input
`difference and b of the PMOS current gain (Figure 12) must be enough to achieve
`the specified change of output current. Thus,
`
`DIo+ b gaDVs + b gaǒ R2
`R1 ) R2ǓDVo
`
`Then, the load regulation is obtained from equation (17).
`
`1
`
`bga ǒR1 ) R2
`R2 Ǔ
`
`DVo
`DIo
`Since load regulation is a steady-state parameter, all frequency components are
`neglected. The load regulation is limited by the open loop current gain of the
`system. As noted from the above equation, increasing dc open-loop current gain
`improves load regulation.
`
`+
`
`10
`
`SLVA072
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 14 of 30
`
`

`

`5.1 Application Implications
`The worst case of the output voltage variations occurs as the load current
`transitions from zero to its maximum rated value or vice versa. Figure 13 shows
`that the load regulation is determined by the D VLDR. Figure 14 shows the output
`voltage variation with respect to the output current with the TPS76350 5-V LDO
`regulator.
`
`Load Regulation
`
`D VLDR
`
`50
`
`100
`t – Time – m s
`
`150
`
`200
`
`100
`
`0
`
`150
`
`100
`
`0
`
`–100
`
`–150
`
`0
`
`– mA
`
`IO
`
`Output Current
`
`Change – mV
`Output Voltage
`
`Figure 13. Load Transient Response of TPS76350
`
`0
`
`25
`
`50
`
`Output Voltage Variation – mV
`
`0
`
`30
`
`60
`
`90
`
`120
`
`150
`
`180
`
`IO – Output Current – mA
`
`Figure 14. TPS76350 Output Voltage With Respect to Output Current
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance
`
`11
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 15 of 30
`
`

`

`Line Regulation
`
`6 Line Regulation
`Line regulation is a measure of the circuit’s ability to maintain the specified output
`voltage with varying input voltage. Line regulation is defined as
`
`Line regulation;
`
`DVo
`DVI
`The output voltage change for a given input voltage change (D Vo / D VI) can be
`calculated from Figure 12 as follows;
`
`* DVo+
`
`VIReq
`Rds) Req
`
`* DIoReq
`
`VIReq
`Rds) Req
`VIReq
`Rds) Req
`where the open loop current gain G = b × ga, and Rds is the equivalent resistor
`between drain and source of the series pass element. Req is the equivalent output
`resistor at the point P1 (Req = (R1 + R2)RL ≈ RL). The sensed voltage Vs is given
`by
`
`Vo +
`
`+
`
`* GǒVs* VrǓReq
`
`Vs+
`
`R2
`R1 ) R2
`Substituting (21) into equation (20),
`
`Vo
`
`ReqǒR1)R2Ǔ
`VI) ǒR1 ) R2ǓGVrReq
`Rds)Req
`R1 ) R2 ) GR2Req
`Now in the usual case,
`
`Vo+
`
`GVsơ 1
`From equations (22) and (23), the output voltage is
`
`Vo+
`
`VI)
`
`ǒR1 ) R2Ǔ
`R2
`
`Vr
`
`ǒR1 ) R2Ǔ
`GR2ǒRds) ReqǓ
`Now, the right hand side of the equation can be split into two parts. One is the
`steady state average output voltage and the another is the function of input
`voltage. The average steady state output voltage is then given by
`
`Vo+
`
`ǒR1 ) R2Ǔ
`R2
`Thus, the line regulation is obtained from equation (24).
`
`Vr
`
`DVo
`DVI
`
`+
`
`1
`ǒRds) RLǓ
`
`ǒR1 ) R2Ǔ
`GR2
`
`12
`
`SLVA072
`
`(19)
`
`(20)
`
`(21)
`
`(22)
`
`(23)
`
`(24)
`
`(25)
`
`(26)
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1027
`Page 16 of 30
`
`

`

`Or substituting the open loop current gain G into equation (26), the line regulation
`can be
`
`Line Regulation
`
`
`
`ǒR1 ) R2R2 Ǔ
`
`(27)
`
`ȣȤ
`
`
`Like load regulation, line regulation is a steady state parameter—all frequency
`components are neglected. Increasing dc open loop current gain improves the
`line regulation.
`
`1
`
`ȡȢ
`
`DVo
`DVI
`
`+ȧǒRds) RLǓbgaȧ
`
`6.1 Application Implications
`Figure 15 shows the input voltage transient response; the line regulation is
`determined by D VLR.
`
`D VLR
`
`50
`
`100
`
`150
`
`t – Time – m s
`
`567
`
`40
`
`20
`
`0
`
`–20
`
`–40
`
`0
`
`– V
`
`
`
`VI
`
`Input Voltage
`
`Change – mV
`Output Voltage
`
`Figure 15. Line Transient Response of TPS76333
`Figure 16 shows the circuit performance of the TPS76333 3.3-V LDO regulator
`with respect to the input voltages. The broken line shows the range of output
`voltage variation resulting from the input voltage change (1.244 mV to 18.81 mV).
`18.81mV
`1.244mV
`
`Output Voltage
`Variation
`
`3.3
`
`VO– Output Voltage – V
`
`0
`
`2.0
`
`3.6
`VI – Input Voltage – V
`
`10
`
`Figure 16. TPS76333 Output Voltage With Respect to Input Voltage
`
` Technical Review of Low Dropout Voltage Regulator Operation and Performance