Buck Regulator Architectures

4.6 Voltage Mode Buck Regulators

Voltage-Mode Regulator

2

VIN

COUT

RESR

RLOAD

L

RFB1

RFB2

VOUT

VREF

VFB

VP

ZC

+

-+

-T

VC

Output Filter

Error Amplifier

Modulator

Advantages and Disadvantages• Advantages

– Stable modulation/less sensitive to noise– Single feedback path– Can work over a wide range of duty cycles

• Disadvantages– Loop gain proportional to VIN

– LC double pole often drives Type III compensation– CCM and DCM differences - a compensation challenge– Slow response to input voltage changes– Current limiting must be done separately

3

Modulator Gain

4

VIN

VSW

+

-

T VRAMP

VP

VC

VC

P

INM V

VA

Output Filter

5

CO

RESR

RLOAD

LVOUTVSW

ZA

ZB

BA

B

SW

OUT

ZZZ

VV

* (Rx, Cy) indicate the components that drive the locations of the pole and the zero, detailed equations are in the notes

-40

-20

0

20

40

10 100 1,000 10,000 100,000 1,000,000

o

ESR

dB

Hz

(L, Co)

(Resr, Co)

Error Amplifier

The easiest place to compensate the entire loop is to adjust the compensation around the error amplifier. Several different approaches are possible.

6

RFB2VREF

VFB

ZF

+

-

VOUT

VC

ZI

I

F

OUT

C

ZZ

VV

Type II Compensation

7

RFB2

RFB1VREF

VFB

+

-

VOUT

VC

RCCC1

CC2ZF ZI

I

F

OUT

C

ZZ

VV

* (Rx, Cy) indicate the components that drive the locations of the pole and the zero (and k), detailed equations are in the notes

-20

0

20

40

60

10 100 1,000 10,000 100,000 1,000,000

pz k

ZF - ZI

dB

Hz

(Rc, Cc1) (Rc, Cc2)(Rc, RFB2)

Design Guidelines for Type II Compensation• Choose a large value for RFB2, between 2-200 kW• Set the mid-band gain k to give desired bandwidth• Set P equal to half the switching frequency:

P = 2p*Fsw/2

• Set z equal to the output filter double pole O

• Use the following equations to solve for the remaining variables

8

CPC2 Rω

1C Cz

C1 Rω1C FB2C kRR

Type III Compensation

9

RFB2

RFB1VREF

VFB

+

-

VOUT

VC

RC1CC1

CC2

CC3

RC2

ZFZI

I

F

OUT

C

ZZ

VV

* (Rx, Cy) indicate the components that drive the locations of the poles and zeros, detailed equations are in the notes

-20

0

20

40

60

10 100 1,000 10,000 100,000 1,000,000

1P1z

k

2P

2z

dB

Hz

(Rc1, Cc1)

(RFB2, Cc3)

(Rc1, Cc2) (Rc2, Cc3)

Design Guidelines for Type III Compensation• Choose a large value for RFB2, between 2-200 kW• Set the mid-band gain k to shift the open-loop gain up to give desired

bandwidth• Set P1 equal to half the switching frequency:

P1 = 2p * Fsw/2

• Set P2 equal to the output filter zero, ESR

• Set Z1 and Z2 equal to cancel out the output filter double pole

• Use the following equations to solve for the remaining variables

10

FB2Z2C3 Rω

1C C3P2

C2 Cω1R

FB2C1 kRR

3

C1Z1

FB2Z2C1 Rω

RωC cC

C1P1C2 Rω

1C

Internal Type III Voltage Mode Compensation (LM367x)

11

Typical Application CircuitInternal Block Diagram

Internal Type III Voltage Mode Compensation (LM285x)

12

PVINAVINENSSSGND PGND

SNS

SW

LM2852/3

+

Lo

Co

VIN = 3.3V

CIN VOUT = 2.5 V

Ramp and Clock

Generator

EN

PVIN

Gate Drive SW

PGND

Error Amp

PWM Comp

OscillatorReference

UVLO DAC

Zc2

400 kW

200

kW

200

kW

20 pF

Current Limit

+

-

+

-

Zc1

SNS

SS

Internally Compensated

Typical Application CircuitInternal Block Diagram

13

Thank you!