7.3.1 Hysteretic Control Circuit Overview

The LM5007 is a buck DC/DC converter that uses a constant on-time (COT) control scheme. The on-time is programmed by an external resistor and varies inversely with line input voltage (V_IN). The core regulation elements of the LM5007 are the feedback comparator and the programmed on-time one-shot. The regulator output voltage is sensed at the feedback pin (FB) and compared to an internal reference voltage (2.5 V). If the FB voltage is below the reference voltage, the buck switch is turned on for a fixed time interval determined by the input voltage and a programming resistor (R_ON). Following the on period, the switch remains off for at least the minimum off-time interval of 300 ns. If the FB voltage is still below the reference after the 300-ns off-time, the switch turns on again for another on-time interval. This switching behavior continues until the FB voltage reaches the reference voltage level.

The LM5007 operates in discontinuous conduction mode (DCM) at light load currents and continuous conduction mode (CCM) at heavier load currents. In DCM, current through the output inductor starts at zero and ramps up to a peak value during the buck switch on-time and then back to zero during the off-time. The inductor current remains at zero until the next on-time interval begins when FB falls below the internal reference voltage. The operating frequency in DCM is relatively low and varies with load. Thus, the conversion efficiency is maintained at light loads, since the switching losses decrease with the reduction in load current and switching frequency. Calculate the approximate switching frequency in DCM with Equation 1.

Equation 1.

In CCM, current flows continuously through the inductor and never ramps down to zero. The switching frequency in CCM is greater than that in DCM and remains relatively constant with load and line variations. Calculate the approximate switching frequency in CCM with Equation 2.

Equation 2.

The output voltage (V_OUT) can be programmed by two external resistors as shown in Figure 4. Calculate the output voltage setpoint using Equation 3.

Equation 3.

The feedback comparator in hysteretic regulators depend upon the output ripple voltage to switch the power MOSFET on and off at regular intervals. In order for the internal comparator to respond quickly to changes in output voltage, proportional to inductor current, a minimum amount of capacitor Equivalent Series Resistance (ESR) is required. A ripple voltage of 25 mV to 50 mV is recommended at the feedback pin (FB) for stable operation. In cases where the intrinsic capacitor ESR is too small, additional series resistance may be added.

For applications where lower output voltage ripple is required, the load can be connected directly to the low ESR output capacitor as shown in Figure 4. The series resistor (R) will degrade the load regulation. Another technique for enhancing the ripple voltage at FB is to place a capacitor in parallel with the upper feedback resistor, R1. The addition of this feedforward capacitor reduces the attenuation of the ripple voltage from the feedback divider.