Buck Regulator Architectures

4.4 Constant On Time (COT) Buck Regulators

Constant ON-Time (COT) Hysteretic Regulator

• Advantages– Constant

frequency vs. VIN

– High Efficiency at light load

– Fast transient response

• Disadvantages– Requires ripple at

feedback comparator– Sensitive to output noise,

because it translates to feedback ripple

2

ON-time is constant, for a given VIN, as load current varies

Ripple is needed to properly switch the comparator!!

RF2

RF1

+

-

ErrorComparator

Modulator

VREF

+

-

RL

RC

(ESR)

VIN

VOUT

PowerStage

L

C

One-ShotInversely

Proportionalto VIN

VFB

Frequency of Operation (Continuous)

TON is the on-time and FS is the operating frequency. The constant on-time controller sets the on-time of the Buck switch.

3

K is a constant and RON is a programming resistor. VIN is in the denominator as expected, setting the on-time inversely proportional to VIN.

Rearrange and substitute TON into the first equation, then solve for FS

Constant ON-Time Achieves Nearly Constant Frequency

• Switching frequency is almost constant; the variations are due to effects of RDS-ON, diode voltage and input impedance of the RON pin

• Note: A resistor from VIN to RON sets the ON-time

4

Constant On-Time Regulator Waveforms (Discontinuous)

For a COT regulator, the constant frequency relationship holds true provided the inductor current remains continuous. At light loading conditions the current in the inductor will become discontinuous. Shown here are the switching waveforms for a Buck regulator controlled with constant on-time control in the discontinuous conduction mode, which means the ramping inductor current returns to zero every cycle.

5

Initial Configuration Circuit

• Ripple voltage at VOUT is the inductor’s ripple current x R3

• Since the inductor’s ripple current increases as VIN increases, the ripple voltage at VOUT increases along with it

6

FB

SW

L1

C2

R1

R2

BST

VCCC3

C4

D1VOUT

RON/SD

VIN

InputVoltage

C1

RTNSGND

RON

R3

Ripple here must be>25 mVp-p

Ripple here is greater thanthat at FB by the ratio of

(R1+R2)/R2.

LM2695

Initial Config. Transient Response

7

Output Voltage

LM2695 Initial Circuit VIN = 12V, VOUT = 10V

Load Transient Response

400 mA

100 mA

50 mV

Reduce the Ripple With One Capacitor!

8

Adding C5 allows the ripple at FB to be same as at VOUT without the attenuation of R1 & R2.

This reduces the ripple, but does not eliminate it

Intermediate Ripple Configuration

FB

SW

L1

C2

R1

R2

BST

VCCC3

C4

D1VOUT

RON/SD

VIN

InputVoltage

C1

RTNSGND

RON

R3

Ripple here must be>25 mVp-p

Ripple here cannow be a minimumof 25 mVp-p - same

as at FB.

C5

LM2695

COT Transient Response With One Capacitor Added

9

LM2695 Intermediate Ripple ConfigurationVIN = 12V, VOUT = 10V

400 mA

100 mA

Output Voltage

20 mV

Load Transient Response

How to Achieve Minimum Ripple

10

FB

SW

L1

C2R1

R2

BST

VCCC3

C4

D1

VOUTRON/SD

VIN

Input Voltage

C1

RTNSGND

RON

Ri ppl e here must be >25 mVp-p

Ri ppl e here depends on C2' s

ESR, and the i nductor ri ppl e

current.

C6

C7

R4

R3 has been removed.

LM2695

Minimum Ripple-Circuit TransientResponse

11

LM2695 Minimum Ripple ConfigurationVIN = 12V, VOUT = 10V

Load Transient Response

Output Voltage

400 mA

100 mA

10 mV

Good To Know:What Happens if R3 is Removed?

12

FB

SW

L1

C2R1

R2

BSTC4

D1VOUT

SGND

Ripple here must be>25 mVp-p

Going down when it should be going up!!

The circuit regulates poorly with a lot of noise and jitter!!

tON tOFF SW Pin

VOUT

VSW

VOUTRipple

Preferred waveform

Good To Know: Don’t Put Too Much Output Capacitance!

13

LM2695

FB

SW

L1

C2

R1

R2

BST

VCCC3

C4

D1VOUT

RON/SD

VINC1

RTNSGND

RON

R3

Load

Distributed capacitancearound the PC board

VIN

Other Items To Keep In Mind

• The flyback diode should be a Schottky, not an Ultra-fast!

• A 0.1 μF ceramic chip capacitor adjacent to the VIN pin is mandatory!

• PC board traces must be routed carefully!

14

Keep the loops physically small to minimize radiated EMI.

15

Thank you!