Automatic Operation Mode Control
In nominal currents the synchronous buck converter
operates in continuous-conduction constant-frequency
mode. This mode of operation achieves higher efficiency
due to the substantially lower voltage drop across the
synchronous MOSFET compared to a Schottky diode.
In contrast, continuous-conduction operation with load
currents lower than the inductor critical value results in lower
efficiency. In this case, during a fraction of a switching cycle,
the direction of the inductor current changes to the opposite,
actively discharging the output filter capacitor.
FIGURE 4. OUTPUT VOLTAGE PROGRAM
To maintain the output voltage in regulation, the discharged
energy should be restored during the consequent cycle of
operation by the cost of increased circulating current and
losses associated with it.
The critical value of the inductor current can be estimated by
the following expression:
2 • FSW • LO • VIN
To improve converter efficiency at loads lower than critical,
the switch-over to variable frequency hysteretic operation
with diode emulation is implemented into the PWM scheme.
The switch-over is provided automatically by the mode
control circuit that constantly monitors the inductor current
and alters the way the PWM signal is generated.
The voltage across the synchronous MOSFET at the
moment of time just before the upper-MOSFET turns on is
monitored for purposes of mode change. When the
converter operates at currents higher than critical, this
voltage is always negative. In currents lower than critical, the
voltage is always positive. The mode control circuit uses a
sign of voltage across the synchronous devices to determine
if the load current is higher or lower than the critical value.
To prevent chatter between operating modes, the circuit
looks for eight contiguous signals of the same polarity before
it makes the decision to perform a mode change. The same
algorithm is true for both CCM-hysteretic and hysteretic-
When the critical inductor current is detected, the converter
enters hysteretic mode. The PWM comparator and the error
amplifier that provided control in the CCM mode are inhibited
and the hysteretic comparator is now activated. A change is
also made to the gate logic. In hysteretic mode the
synchronous rectifier MOSFET is controlled in diode
emulation mode, hence conduction in the second quadrant
1 2 345 6 7 8
FIGURE 5. CCM - HYSTERETIC TRANSITION
1 2 3 45 6 7 8
FIGURE 6. HYSTERETIC - CCM TRANSITION
December 28, 2004