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Buck Regulator Architectures 4.4 Constant On Time (COT) Buck Regulators Constant ON-Time (COT) Hysteretic Regulator ON-time is constant, for a given VIN, as load current varies • Advantages + + VREF • Disadvantages – Requires ripple at feedback comparator – Sensitive to output noise, because it translates to feedback ripple - – Constant frequency vs. VIN – High Efficiency at light load – Fast transient response VIN Modulator Error Comparator VFB One-Shot Inversely Proportional to VIN Power Stage L VOUT C RL RC (ESR) RF1 RF2 Ripple is needed to properly switch the comparator!! 2 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 3 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 Input Voltage VCC VIN C1 C3 RON BST C4 L1 RON/SD SW VOUT D1 LM2695 R1 RTN SGND R3 C2 FB R2 Ripple here is greater than that at FB by the ratio of (R1+R2)/R2. Ripple here must be >25 mVp-p • 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 Initial Config. Transient Response Load Transient Response 400 mA 100 mA 50 mV Output Voltage LM2695 Initial Circuit VIN = 12V, VOUT = 10V 7 Reduce the Ripple With One Capacitor! Intermediate Ripple Configuration Input Voltage VCC VIN C1 C3 RON BST C4 L1 RON/SD SW VOUT D1 LM2695 C5 RTN SGND R1 R3 C2 FB R2 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 VOUT without the attenuation of R1 & R2. This reduces the ripple, but does not eliminate it 8 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 9 How to Achieve Minimum Ripple Input Voltage VCC VIN C1 C3 RON BST R3 has been removed. C4 L1 RON/SD VOUT SW LM2695 D1 R4 C6 C2 C7 FB RTN R1 SGND Ripple here must be >25 mVp-p Ripple here R2 depends on C2's ESR, and the inductor ripple current. 10 Minimum Ripple-Circuit Transient Response Load Transient Response 400 mA 100 mA 10 mV Output Voltage LM2695 Minimum Ripple Configuration VIN = 12V, VOUT = 10V 11 Good To Know: What Happens if R3 is Removed? BST C4 The circuit regulates poorly with a lot of noise and jitter!! L1 SW VOUT D1 R1 SGND C2 VSW FB R2 tON Ripple here must be >25 mVp-p VOUT tOFF SW Pin Preferred waveform VOUT Ripple Going down when it should be going up!! 12 Good To Know: Don’t Put Too Much Output Capacitance! VIN VIN VCC C3 C1 RON BST C4 L1 RON/SD SW LM2695 VOUT D1 Load R1 R3 RTN SGND FB 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 VIN pin is mandatory! • PC board traces must be routed carefully! Keep the loops physically small to minimize radiated EMI. 14 Thank you! 15