Any regulators feedback loop stability can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a sudden change in output load current or input line voltage. When a step-load occurs, VOUT shifts by an amount equal to ILOAD x the ESR of the output capacitor, where ESR is the effective series resistance. ILOAD also begins to charge or discharge COUT generating the feedback error signal that forces the regulator to adapt to the current change and return VOUT to its steady-state value. During this recovery time VOUT can be monitored for excessive overshoot or ringing, which would indicate a stability problem. Figure 4 below shows the transient response of the LTC3850 circuit in figure 1. The recovery is well damped and has less than a 50S settling time to a 50% load change. The output voltage maximum deviation from nominal can be reduced with the addition of more output capacitance.

Figure 4: LTC3850 Transient response of the circuit in Figure 1.
(Click this image to view a larger, more detailed version)

Light Load Operation

At heavy loads, the LTC3850 operates in constant frequency PWM mode. At light loads, it can switch in any of three modes: continuous, pulse skipping or Burst Mode' operation. Burst Mode operation switches in pulse trains of one cycle to several cycles, with the output capacitors supplying energy during intervening sleep periods. This provides the highest possible light load efficiency. Forced continuous mode offers PWM operation from no load to full load, providing the lowest possible output voltage ripple. Pulse skipping mode operates at a constant frequency, but always turns off the synchronous switch before the inductor current is allowed to reverse. This method reduces light load ripple compared to Burst Mode operation and improves light load efficiency compared to forced continuous mode.

Each channel also features a separate RUN pin with a precision 1.2V turn-on threshold. When the LTC3850's own current source is used to charge the soft-start capacitor, bringing a channel's RUN pin high causes its soft-start capacitor to begin charging within about 80s. As an alternative, either RUN pin can remain high while the TK/SS is held low, which keeps the internal 5V regulator enabled as a standby supply. This feature can be used to power a wake-up circuit, which controls the state of both TK/SS pins.

Fixed Operating Frequency/Minimum On-Time

The LTC3850's two channels run out of phase, which reduces the input RMS current ripple and thus the input capacitance requirement. Switching frequency can be adjusted from 250kHz to 780kHz, either set with a voltage on the FREQ/PLLFLTR pin, or synchronized to an external clock into the MODE/PLLIN pin using phase-locked loop. During high frequency operation, the LTC3850 can operate normally at low duty cycles due to its short top switch minimum on time. For example, a 15V to 1.1V converter operating at 700kHz requires a minimum on-time of less than 104ns (see equation below).

The LTC3850 can cycle its strong top gate drivers in just 90ns, making this low duty cycle application a reality.