LM2647

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SNVS210F – JUNE 2003 – REVISED APRIL 2013

loads. Note the bunching of several fixed-width pulses followed by skipped pulses. The average frequency can

actually fall very low at very light loads. Note however that when this happens the inductor core is seeing only

very mild flux excursions, and so no significant audible noise is created. But if EMI is a particularly sensitive issue

for the particular application, the user can simply opt for the slightly less efficient, though constant frequency

FPWM mode.

Output 1V @ 0.04A, VIN = 10V, SKIP, L = 10µH, f = 300kHz

Figure 11. Normal SKIP Mode Operation at Light Loads

The SKIP mode is enabled when the FPWM pin is held low (or left floating). Note that at higher loads, and under

steady state conditions (above CCM-DCM boundary), there will be absolutely no difference in the behavior of the

LM2647 or the associated converter waveforms based on the voltage applied on the FPWM pin. The differences

show up only at light loads.

Under startup too, since the currents are high until the output capacitors have charged up, there will be no

observable difference in the shape of the ramp-up of the output rails in either SKIP mode or FPWM mode. The

design has thus forced the startup waveforms to be identical irrespective of whether the FPWM mode or the

SKIP mode has been selected.

The designer must realize that even at zero load condition, there is circulating current when operated in FPWM

mode. This is illustrated in Figure 12. Since duty cycle is the same as for conventional CCM, from V = L*

ΔI / Δt it

can be seen that

ΔI (or Ipp in Figure 12) must remain constant for any load, including zero. At zero load, the

average current through the inductor is zero, so the geometric center of the sawtooth waveform (the center being

always equal to load current) is along the x-axis. At critical conduction (boundary between conventional CCM and

what should have been DCM were it not in FPWM mode), the load current is equal to Ipp/2. Note that

excessively low values of inductance will produce much higher current ripple and this will lead to higher

circulating currents and dissipation.

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