Analog Circuits and Systems Prof. K Radhakrishna Rao

Lecture 20: DC Voltage Regulators

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Review

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DC Voltage Power Supply

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AC to DC converter

Full wave rectifier with capacitive filter

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i

i

I T C 2

IT 20ms

⎛ ⎞⎜ ⎟⎝ ⎠

=

Peak - to - peak ripple at the output =

where is the average current deliveredby

AC toDC converter and

Voltage Regulator

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Characteristics of a Power Supply

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( )

==

η =

− = = −

−

−

; ;

i i i

o o o

o o

i i

i o d i i o o

i o o o i

i o

P VI = P V I

V I=( )

VI

P P P VI V I(V V )I I I

V V

Input Power =

Output Power

Efficiency

Power dissipated in the voltage regulator

as except for bias current

should be kept to minimum (known as

dropout voltage) in order to improve the efficiency.

Characteristics of a Power Supply (contd.,)

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100∂

o i

o

o

o o

V VVx

VV I

Line regulation : Percentage change in as the input voltage changes

from its maximum to minimum

Load regulation : Percentage change in as the output current

is changed from its mini 100∂ o

o

Vx

Vmum to maximum

Temperature coefficient of the output voltage : Change in output voltage

for a small change in input voltage at a specified output voltage

expressed in terms of parts per millio( )∂ ∂

°o

o

V / Tppm/ C

Vn =

Characteristics of a Power Supply (contd.,)

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⎛ ⎞∂⎜ ⎟⎜ ⎟∂⎝ ⎠

oo

o

VI

I

Output impedance : Change in output voltage for a small

change in output current at a given load current

Ripple rejection factor : Percentage change in output voltage

for a small change

⎛ ⎞∂−⎜ ⎟⎜ ⎟∂⎝ ⎠

o i

i o

V VV V

in input voltage around the nominal value

of input voltage. PSRR (Power Supply Rejection Ratio)

Load/Line Transient Regulation

�  Voltage at the output terminal is likely to have undesirable transients at the time of switching on the input power or when step load changes occur

�  The output impedance of the voltage regulator is complex �  The presence of feedback loop with large loop gain and the multiple

poles associated with this loop makes the output impedance complex

�  Compensation has to be provided to make the output impedance resistive

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VCVS as voltage regulator

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2reg Z

1

RV 1 VR

⎛ ⎞= +⎜ ⎟⎝ ⎠

Voltage regulator as a feedback system

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2reg Z

1

RV 1 VR

⎛ ⎞= +⎜ ⎟⎝ ⎠

VCVS as voltage regulator

Voltage regulator as a feedback system (contd.,)

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10 10 11 1 2

1 2 2

21

10

1

2

12

1 2

G GG =s s s1 1 s 1

GGBGss 1

RGR R

ωω <<ω

⎛ ⎞ ⎛ ⎞ ⎛ ⎞+ + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟ω ω ω⎝ ⎠ ⎝ ⎠ ⎝ ⎠

ωω

⎛ ⎞+⎜ ⎟ω⎝ ⎠

+

;

;

;

;

Forward path for

In internally compensated Op Amps

Hence

Feedback path

Voltage regulator as a feedback system (contd.,)

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Voltage regulator as a feedback system (contd.,)

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2

1

2 2 2

1 1 1

22 2 2

1 1 1

2 2

2

12

22 2

1

1

1 1 1

11 1 1 11 1 1

1

1 1

⎛ ⎞+⎜ ⎟

⎝ ⎠⎛ ⎞ ⎛ ⎞ ⎛ ⎞

+ + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠= = =⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞

+ + + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎛ ⎞⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠+ + + +⎜ ⎟ω ω⎝ ⎠⎛ ⎞

+⎜ ⎟⎝ ⎠= =

⎛ ⎞+ + + ω⎜ ⎟ω ω ⎝ ⎠

o

i

o

i

l

n n

V RV R

R R RR R RV

V R R Rs s sg R R Rs

GB GB GB

RR GBwhere Qs s RQ R

;

Voltage regulator as a feedback system (contd.,)

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�  High rate of rise and least amount of ringing occurs at Q = 1 �  is determined by the required output voltage

�  As w2 and G10 are fixed for a given Op Amp w1 is chosen to make Q = 1

�  In internally compensated Op Amps like 741 and TL081 w1 is already chosen to make Q = 1 for Vo = VZ, i.e., G2 = 1

= =+

Z 12

o 1 2

V RGV R R

2 21

10 1

R1G R

⎛ ⎞ωω = +⎜ ⎟

⎝ ⎠

21

10Gω

ω =

Voltage regulator as a feedback system (contd.,)

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�  For any other G2 < 1; Q will always be correspondingly less than1resulting in unsatisfactory transient response.

�  It is therefore necessary to use uncompensated Op Amps like LM748 to design voltage regulators with good transient response.

Design of Voltage Regulator using 741/TL081

�  Design voltage regulator with a reference of 1.2 V for a regulated output of 10 V for a maximum load current 15 mA.

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Design of Voltage Regulator using 741/TL081 (contd.,)

�  Voltage Regulator using Voltage reference IC for 1.2 V: REF3012

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1

2

1

2

R1 10 1.2VR

R = 1.2 k and R = 8.8 k

⎛ ⎞+ =⎜ ⎟

⎝ ⎠ΩΩ

The Op Amp used is 741

Simulation

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Simulation

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Current boosting pass transistor

�  Super Transistor using opamp

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Current boosting pass transistor (contd.,)

�  using n-channel pass transistor �  using p-channel pass transistor - Load Dropout Regulator (LDO)

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Voltage Regulator with Current Boosting

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LM 2940: 1 Amp Low Drop Out Regulator

�  Input Voltage Range = 6 V to 26 V �  Output Current in Excess of 1 A �  Dropout Voltage Typically 0.5 V at IOUT = 1 A �  Output Voltage Trimmed before Assembly (5, 8, 9, 10, 12 and 15

volts) �  Load regulation: 80 to 130 mV from Io = 50 mA to 1amp �  Line regulation: 50 to 80 mV for Vi = 2 V to 26 V �  Output impedance: 35 to 55 mW at Io = 100 mA Vo = 5 and 8 V �  Ripple rejection: 72 to 66 dB for Io = 100 mA Vo = 5 and 8 V �  Temperature coefficient of the output voltage: 33 ppm Vo = 5

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Schematic diagram of LM 2940

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It uses a PNP transistor for current boosting

Using LM2940

* C1 is needed if regulator is located far (more than a few inches) from power supply filter

** COUT must be at least 22 μF to maintain stability. It may be increased without bound to maintain regulation during transients. Locate as close as possible to the regulator. This capacitor must be rated over the same operating temperature range as the regulator and the ESR (Equivalent Series Resistance) is critical.

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Conclusion

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