Buck Regulator Architectures

4.4 Constant On Time (COT) Buck Regulators

Constant ON-Time (COT) Hysteretic Regulator ON-time is constant, for a given V IN , as load current varies

• Advantages – Constant frequency vs. V IN – High Efficiency at light load – Fast transient response • Disadvantages – Requires ripple at feedback comparator – Sensitive to output noise, because it translates to feedback ripple

V REF + Modulator V FB R F2 + Error Comparator One-Shot Inversely Proportional to V IN R F1 V IN Power Stage L C R C (ESR) V OUT Ripple is needed to properly switch the comparator!!

R L

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Frequency of Operation (Continuous)

T ON is the on-time and F S is the operating frequency. The constant on-time controller sets the on-time of the Buck switch.

K is a constant and R ON is a programming resistor. V IN is in the denominator as expected, setting the on-time inversely proportional to V IN .

Rearrange and substitute T ON into the first equation, then solve for F S 3

Constant ON-Time Achieves Nearly Constant Frequency

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

Note

: A resistor from V IN to R ON 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

Input Voltage VIN VCC C3 C1 R ON BST C4 L1 RON/SD SW V OUT D1 LM2695 R1 RTN SGND FB R3 C2 Ripple here is greater than that at FB by the ratio of (R1+R2)/R2.

R2 Ripple here must be >25 mVp-p • Ripple voltage at V OUT is the inductor’s ripple current x R3 • Since the inductor’s ripple current increases as V IN voltage at V OUT increases along with it increases, the ripple 6

Initial Config. Transient Response Load Transient Response 400 mA 100 mA 50 mV Output Voltage LM2695 Initial Circuit V IN = 12V, V OUT = 10V

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Reduce the Ripple With One Capacitor!

Intermediate Ripple Configuration

Input Voltage C1 VIN VCC R ON BST RON/SD LM2695 SW C4 D1 C3 L1 V OUT C5 RTN SGND FB R1 R2 R3 C2 Ripple here can now be a minimum of 25 mVp-p - same as at FB.

Ripple here must be >25 mVp-p

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

This reduces the ripple, but does not eliminate it

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COT Transient Response With One Capacitor Added Load Transient Response 400 mA 100 mA 20 mV Output Voltage LM2695 Intermediate Ripple Configuration V IN = 12V, V OUT = 10V

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How to Achieve Minimum Ripple

Input Voltage C1 R ON VIN VCC C3 BST RON/SD LM2695 SW D1 FB RTN SGND C4 L1 R3 has been removed.

R4 C7 C6 Ripple here must be >25 mVp-p R1 C2 V OUT R2 Ripple here depends on C2's ESR, and the inductor ripple current.

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Minimum Ripple-Circuit Transient Response Load Transient Response 400 mA 100 mA 10 mV Output Voltage LM2695 Minimum Ripple Configuration V IN = 12V, V OUT = 10V

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Good To Know: What Happens if R3 is Removed?

BST C4 L1 SW D1 R1 SGND FB Ripple here must be >25 mVp-p R2 C2 V OUT

V SW t ON

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

t OFF

SW Pin

V OUT

Preferred waveform V OUT Ripple Going down when it should be going up!!

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

V IN VIN VCC C1 R ON BST RON/SD SW

LM2695

RTN SGND FB C3 C4 L1 D1 V OUT Load R1 R3 C2 Distributed capacitance around the PC board R2 13

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 V IN pin is mandatory! • PC board traces must be routed carefully!

Keep the loops physically small to minimize radiated EMI.

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Thank you!

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