線性穩壓器

(2) Linear Regulators (2) Instructor: Po-Yu Kuo (

郭柏佑

)

國立雲林科技大學 電子工程系

Structure of LDO

 A typical series regulator which consists of four main building blocks: Vin Vref

+

A(S)

-

Vn1 Power Transistor Vo R1 Vfb Re R L R2 Co 2

Structure of LDO

 Voltage Reference (V ref ): a very stable voltage with respect to temperature change and input voltage variations, usually of the bandgap type.

 Error Amplifier (A(s)): a very high (dc) gain opamp to achieve a close to zero error signal V err =V + - V .

 Feedback Network: R1 and R2 define the feedback factor and generate V fb to be compared with Vref to get the designed output voltage Vo.

 Series Pass/Power Transistor (Q1): power transistor configuration to pass high current from the source to output. As it handles large current, the size of pass transistor dominates the area of the whole series regulator.

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 Dropout Voltage (Vdo) is the minimum voltage difference between the input and output under which the regulator still able to maintain the output within the specification.

Vin Vref  With a Li-Ion battery as Vin, Vin varies from 2.7V to 4.2V.

V do,max =4.2-V ov,ML

Structure of LDO

+

A(S) Vn1 Power Transistor Vo R1 Vfb Re R L R2 Co 4

Structure of LDO

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Specifications of LDO

 Two Categories:  Regulating (accuracy) performance Line regulation, load regulation, temperature dependence, transient overshoot, transient recovery time, stability  Power Characteristics I o , Quiescent current I q , V in & V o ( ) 6

Efficiency

   Current Efficiency : 

I o

/(

I o



I q

) where I q is the quiescent current of LDO In LDO design for portable applications, I o is usually much larger than I q with > 99% efficiency When I o is 0, I q should be minimized (I by large values of R 1 and R 2 ) 3 should be small 7

Efficiency

 Smaller dropout voltage causes a higher power conversion efficiency especially Io >> Iq  In light-load condition (small Io), the efficiency is poorer as I 1 , I 2 , and I 3 are close to I o 8

Dropout Voltage and Power-Transistor Sizing

     V SD must be always larger than V ov at different conditions Design at the worst case: largest V ov at I o(max) the maximum temperature and μ p(min) at By using minimum L (the smallest transistor and hence parasitic capacitance), keep increasing W until meeting the dropout specification IR at the routing metals increase V DO Design margin by experience-generally the chosen W is 1.1 1.2 times of the theoretical W 9

Load Regulation

    Load Regulation (R): closed-loop output resistance of LDO R o is the open-loop output resistance of the pass transistor as R f1 , R f2 >> R o Better load regulation is achieved by smaller R minimum channel length of the pass transistor) and larger loop-gain magnitude o (using As R o 1/I o , high Io range gives better load regulation 10

Line Regulation

    g mp is the transconductance of power PMOS transistor Line regulation is independent of the gain of the power transistor Line regulation can be improved by a high-gain error amplifier Other error sources on line regulation are voltage reference and offset voltage of the error amplifier 11

Review on Voltage Gain

  G m and R o can be found individually Input-Output voltage gain can be found by the product of G m and R o 12

Line Regulation Including Other Errors

  Voltage gain of the error amplifier is not the only parameter to improve line regulation Good designs on supply independence of V ref and reducing systematic offset of error amplifier are very important 13

Temperature Coefficient

  Variation of Vo at different temperature depends on both voltage reference and error amplifier design R f1 and R f2 must be made by the same material and closely placed 14

Load Transient Response

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Load Transient Response

 Better load transient response by t resp L c ↓ .

↓, C o ↑, R e ↓ and 16

AC Design (1): Loop-Gain Analysis

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AC Design (2): Loop-Gain Analysis

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AC Design (3): Loop-Gain Analysis

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AC Design (4): Loop-Gain Analysis

     z e should cancel p2 within one decade of frequency for stability Parasitic pole(s), ppar, must be far away from the unity-gain frequency (UGF) Different UGFs are resulted from different Re values such as z e locating before or after p 2 p 2 locates at very low frequency as C pa and r a are large Required large C o and R e   Large C o is unfavorable in the cost consideration Low-frequency pole-zero cancellation is unfavorable to load transient recovery time 20

LDO with Voltage Buffer

   Smaller required R e can be achieved by inserting a low output-resistance (1/g mb ) voltage buffer One more pole (p 3 ) is created but is located at high frequency due to small output resistance of the voltage buffer p 2 (with voltage buffer) locates at a higher frequency than the one without voltage buffer (C b << C g ) 21

 Loop gain is larger when I o is smaller due to g mp r o  1/ √I o

Effect of Load Currents on Stability

 p smaller due to larger r of the power transistor (r 1 o is lower when I  1/I o ) o is o  Worst-case stability at maximum I o  Compensation at max. I o 22

Effect of Loop-Gain Magnitude on Stability

 Larger loop gain by increasing r a of the error amplifier  p 2 → p 2 ’  A larger Re is needed to create a zero at lower frequency (z e → z e ’)  Larger loop gain → more unstable as p gain 3 may be below the UGF of loop  A larger C o needed is generally 23

Loop Gain Simulation

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Summary of LDO Specifications

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Circuit Implementations

  Circuit of LDO consists of       R 1 C in and R 2 and C o V ref Error Amplifier Voltage Buffer Power Transistor V in,min = V ov,Me1 + V gs,Mb2 + V gs,Mp Low-voltage operation impossible!

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Circuit Implementations

    BJT has a small V BE drop (~0.7V) The circuit can operate at lower input supply compared to the previous case Smaller input capacitance for small V BE Base current introduces larger offset voltage and hence degrades accuracy of the output voltage 27