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Buck Regulator Architectures. 4.4 Constant On Time (COT) Buck Regulators. V. Power. IN. Modulator. Stage. One-Shot. +. Inversely. +. V. Proportional. -. REF. L. V. -. to V IN. OUT. Error. Comparator. C. R. L. R. C. V. (ESR). FB. R. F1. R. F2.
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Buck Regulator Architectures 4.4 Constant On Time (COT) Buck Regulators
V Power IN Modulator Stage One-Shot + Inversely + V Proportional - REF L V - to VIN OUT Error Comparator C R L R C V (ESR) FB R F1 R F2 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 ON-time is constant, for a given VIN, as load current varies Ripple is needed to properly switch the comparator!!
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. 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
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.
LM2695 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
Initial Config. Transient Response Load Transient Response 400 mA 100 mA 50 mV Output Voltage LM2695 Initial Circuit VIN = 12V, VOUT = 10V
LM2695 Reduce the Ripple With One Capacitor! Intermediate Ripple Configuration 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
COT Transient Response With One Capacitor Added Load Transient Response 400 mA 100 mA 20 mV Output Voltage LM2695 Intermediate Ripple Configuration VIN = 12V, VOUT = 10V
LM2695 How to Achieve Minimum Ripple
Minimum Ripple-Circuit TransientResponse Load Transient Response 400 mA 100 mA 10 mV Output Voltage LM2695 Minimum Ripple Configuration VIN = 12V, VOUT = 10V
V SW t t SW Pin ON OFF V OUT Preferred waveform VOUT Ripple Good To Know:What Happens if R3 is Removed? The circuit regulates poorly with a lot of noise and jitter!! Going down when it should be going up!!
Good To Know: Don’t Put Too Much Output Capacitance! VIN VCC VIN C3 C1 BST R C4 ON L1 RON/SD SW V OUT D1 Load LM2695 R1 R3 Distributed capacitance RTN around the PC board C2 FB SGND R2
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! Keep the loops physically small to minimize radiated EMI.
