Buck Regulator Architectures 4 4 Constant On Time

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 • Disadvantages – Requires ripple at feedback comparator – Sensitive to output noise, because it translates to feedback ripple - - VREF + + – 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) RF 1 RF 2 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 LM 2695 • Ripple voltage at VOUT is the inductor’s ripple current x R 3 • 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 m. A 100 m. A 50 m. V Output Voltage LM 2695 Initial Circuit VIN = 12 V, VOUT = 10 V 7

Reduce the Ripple With One Capacitor! Intermediate Ripple Configuration LM 2695 Adding C 5 allows the ripple at FB to be same as at VOUT without the attenuation of R 1 & R 2. This reduces the ripple, but does not eliminate it 8

COT Transient Response With One Capacitor Added Load Transient Response 400 m. A 100 m. A 20 m. V Output Voltage LM 2695 Intermediate Ripple Configuration VIN = 12 V, VOUT = 10 V 9

How to Achieve Minimum Ripple LM 2695 10

Minimum Ripple-Circuit Transient Response Load Transient Response 400 m. A 10 m. V Output Voltage LM 2695 Minimum Ripple Configuration VIN = 12 V, VOUT = 10 V 11

Good To Know: What Happens if R 3 is Removed? The circuit regulates poorly with a lot of noise and jitter!! VSW t. ON VOUT t. OFF 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 VCC C 1 RON BST C 3 C 4 L 1 RON/SD SW LM 2695 VOUT D 1 Load R 1 R 3 RTN SGND FB C 2 Distributed capacitance around the PC board R 2 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