Lecture-36

EE5325 Power Management Integrated Circuits

Dr. Qadeer Ahmad Khan

Integrated Circuits and Systems Group

Department of Electrical Engineering

IIT Madras

Buck-Boost Converter

Types of DC-DC converter

oI

D1

1

LI ,

inV

D1

1

outV

1

inV

outV

:BOOST

oI

LI,

inDV

outV

1

inV

outV

:BUCK

oI

D1

1

LI,

inV

D1

D

outV

1

inV

outV

:BOOST-BUCK

Also works as buck only or boost only

3EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Why Buck-Boost? Considering the Li-ion battery discharge profile, either buck or boost fails

to operate for the output voltage of 3.3V - 3.6V– Converter needs to be operated in buck-boost mode for most of the time

Ph

on

e P

ow

er

(V)

4EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Drawback of Conventional BB converter

Single Duty cycle, D, controls all the switches.

Switching losses are higher due to simultaneous operation of 4 switches

Conduction losses are higher due to larger Inductor current (nearly 2x when Vin ≈ Vout.

inV

D1

D

outV

oI

D1

1

LI

(1)

(2)

5EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Tri-Mode Operation of BB Converter

VIN > VOUT : Buck Mode

VIN < VOUT : Boost Mode

VIN ~ VOUT : Buck-Boost Mode

Buck

Boost

Buck-Boost

Inc

rea

sin

g V

IN

VOUT

6EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Conventional vs. Tri-Mode Efficiency

7EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

3.5 4 4 4.25 4.5 4.75 5 5.25 5.570

74

78

82

86

90

94Ef

fici

en

cy [

%]

Input Voltage Vin [V]

Proposed Buck-Boost (Measured)

Conventional Buck-Boost (Simulated)Tri-Mode Buck Boost

Conventional Buck Boost

Input Voltage

Vin = 2.7V to 5.5V, Vout = 3.3V, Iload = 500mA

Tri-Mode: Buck

Buck

Boost

Buck-Boost

Inc

rea

sin

g V

IN

VOUT

LOADL II INBUCKOUT VDV

PBO always ON

DBUCK controls buck switches

ILOAD

8EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Tri-Mode: Boost

Buck

Boost

Buck-Boost

Inc

rea

sin

g V

IN

VOUT

)D(1

VV

BOOST

INOUT

PBU always ON

DBOOST controls boost switches

)D(1

II

BOOST

LOADL

ILOAD

VOUT

9EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Tri-Mode: Buck-Boost

Buck

Boost

Inc

rea

sin

g V

IN

VOUT

IN

BOOST

BUCKOUT V

)D(1

DV

DBUCK controls buck switches

DBOOST controls boost switches

BOOST

LOADL

D1

II

Buck-Boost

VOUT

ILOAD

10EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Bat

tery

Vo

ltag

e (

V)

Issue with Tri-Mode Buck-Boost

Mode transition causes large voltage transient

Boundary condition must be satisfied

– Varies with load current and losses

Buck

Region

Buck-Boost

Region

VOUT

3.7V3.70.1)(1

0.9V

)D(1

DV

IN

BOOST

BUCK

OUT

)D(1-DD

D1-

DD

boost)-(buckV(buck)V

BOOST_minBUCK_maxBUCK

BOOST_min

BUCKBUCK_max

OUTOUT

Boundary Condition

Transient due to

mode-transitionBuck Region

Boost Region

Buck-Boost Region

1D:Mode Boost

0D :Mode Buck

BUCK

BOOST

3.3V

11EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Solutions for Mode Transitions

Appropriate Feed-forward voltage vf1 - 4 is subtracted to instantaneously change the duty cycles during mode transition.

Analog Implementation makes is susceptible to PVT and requires external compensation capacitor

S. Bang, D. Swank, A. Rao, W. McIntyre, Q. Khan and P. K. Hanumolu, 1.2A 2MHz tri-mode Buck-Boost LED

driver with feed-forward duty cycle correction, CICC, Sept. 2010.

400 425 450 475 500 525 550 575 600 625 6501.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

12EE5325 Power Management Integrated

Circuits Integrated Circuits and

Systems Group, Department of EE, IIT

Digital Constant ON/OFF Time Buck-Boost Converter

Uses constant ON/OFF technique

Enables High Switching Frequency Operation

All digital implementation eliminates the need of external compensation capacitor

Non-overlapping clock generator

All-digital fractional-N controller

DBUCK

PBU

NBU NBO

PBOVOUTVIN

L IL

ILOAD

C

DBOOST

VREF

BGR10MHz Clock

SW1 SW2

13

Q. Khan, et al, “A 3.3V 500mA Digital Buck-Boost Converter with 92% Peak Efficiency Using Constant ON/OFF Time

Delta-Sigma Fractional-N Control, Proc. ESSCIRC ‘11, Sept. 2011.

EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Constant ON/OFF Time Operation

Max ripple occurs at D=0.5 (Ton = Toff)

– The converter can be operated at high switching frequency when D=0.5

From eq. 1, D increases with Vin

– Fixing OFF time and making ON time function of Vin does not affect the inductor ripple

– Causes variable switching frequency

TDTTDT

TL

VVI

OFFON

ONOUTIN

L

)1(

)1(current, rippleInductor

Ton=137nS

Vin=4.5V

Ton=97nS

Vin=5.0V

Ton=75nS

Vin=5.5VToff=constant=50nS

Vo

ut

[V]

ΔIL

[m

A]

Ton=137nS

Vin=4.5V

Ton=97nS

Vin=5.0V

Ton=75nS

Vin=5.5VToff=constant=50nS

Vo

ut

[V]

ΔIL

[m

A]

14EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Fractional-N Control

. . . . . . . .

. . . . . . . .

50% Buck 100%

Buck

Buck Mode:

N cycles of 50% Buck : 1 cycle of 100% Buck

Buck-Boost Mode:

1 cycle of 50% Buck : 1 cycle of 50% Boost

Boost Mode:

N cycle of 50% Boost : 1 cycle of 0% Boost

50% Boost 0% Boost

15EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Fractional-N Control Logic

50%Boost:0%Boost

1:1

2:1

3:1

4:1

5:1

0111

1000

1001

1010

1011

ST7

ST8

ST9

ST10

ST11

50%Buck:50%Boost

1:10110ST6

50%Buck:100%Buck

5:1

4:1

3:1

2:1

1:1

0001

0010

0011

0100

0101

ST1

ST2

ST3

ST4

ST5

Fraction

N:1

Control CodeOperating

States

50%Boost:0%Boost

1:1

2:1

3:1

4:1

5:1

0111

1000

1001

1010

1011

ST7

ST8

ST9

ST10

ST11

50%Buck:50%Boost

1:10110ST6

50%Buck:100%Buck

5:1

4:1

3:1

2:1

1:1

0001

0010

0011

0100

0101

ST1

ST2

ST3

ST4

ST5

Fraction

N:1

Control CodeOperating

States

Buck

Mode

Buck-

Boost

Mode

Boost

Mode

De

cre

as

ing

VIN

5.5V

2.7V

Predefined states are stored in the lookup table providing the coarse voltages

Uses 18-bit acc for integrating the error (4 MSBs, 7 LSBs, 7 dropped bits.

Any intermediate states are resolved by ΔΣ Modulator

16EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Start-up Control

Startup time is the function of no. of bits dropped in the accumulator and converter resolution

No. of ACC bits dropped = 7

The startup time may be more than 10ms

Speeded up by dropping only 3 bits in accumulator and switch to 7 bits once the output settles

comparator o/p

Desired Vout

17EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Inductor Current Profile

[V]

ST1: ΔIL=340mA

DB

UC

K[V

]

DB

UC

K[V

]

ST2: ΔIL=314mA

ST5: ΔIL=165mAD

BU

CK

[V]

ST4: ΔIL=240mA

DB

UC

K[V

][V

]

ST1: ΔIL=340mA

DB

UC

K[V

]

DB

UC

K[V

][V]

ST1: ΔIL=340mA

DB

UC

K[V

]

DB

UC

K[V

]

ST2: ΔIL=314mA

ST5: ΔIL=165mAD

BU

CK

[V]

ST5: ΔIL=165mAST5: ΔIL=165mAD

BU

CK

[V]

ST4: ΔIL=240mA

DB

UC

K[V

]

ST4: ΔIL=240mA

DB

UC

K[V

]

18EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Controller Response

Settled LSB Code

Dithered Output

(Control Code)

Settled LSB Code

Dithered Output

(Control Code)

Settled LSB Code

Dithered Output

(Control Code)

Settled LSB Code

Dithered Output

(Control Code)

Settled LSB Code

Dithered Output

(Control Code)

Settled LSB Code

Dithered Output

(Control Code)

19EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Output Voltage RippleBuck Mode (Vin=5V) Buck-Boost Mode (Vin=3.6V)Buck Mode (Vin=5V) Buck-Boost Mode (Vin=3.6V)

20EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Mode Transition

DB

OO

ST

[V]

DB

UC

K[V

]

IL [

A]

VO

UT

[V]

buck-boostbuckbuck-boostbuck

DB

OO

ST

[V]

DB

UC

K[V

]

IL [

A]

VO

UT

[V]

buck-boostbuckbuck-boostbuck

21EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras

Hybrid PWM Fractional-N Control

Error Bit

(0 or 1)Dm<17:0>

Vref

Vout Gate

Driver

VFB

ACC

Mult

Logic

1x/16x

multiplier

RESET

CLK

Da<17:7>

Dc<6:0>

Vin

R1

R2

Cmult

MP1

MN1 MN2

MP2SW1 SW2

Dbuck

Dboost

DAC

0

Vm

Vc_min = 0.1Vm

Vc_max = 0.9Vm

Vc

Fraction-N

ControlDa<17:7>

VPWM

DBUCK

DBOOST

Buck Mode Buck-Boost Mode

(Fractional-N)

Boost Mode

DBOOST = 0

DBUCK = VPWM

90%Buck:100%Buck

90%Buck:10%Boost

10%Boost:0%Boost

DBOOST = VPWM

DBUCK = 1

22EE5325 Power Management Integrated Circuits Integrated Circuits and Systems Group, Department of EE, IIT Madras