LM2647

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SNVS210E – MAY 2004 – REVISED JUNE 2005

In a conventional converter, as the load is decreased to about 10-30% of maximum load current, DCM

(Discontinuous Conduction Mode) occurs. In this condition the inductor current falls to zero during the OFF-time,

and stays there until the start of the next switching cycle. In this mode, if the load is decreased further, the duty

cycle decreases (pinches off), and ultimately may decrease to the point where the required pulse width becomes

less than the minimum ON-time achievable by the converter (controller + FETs). Then a sort of random skipping

behavior occurs as the error amplifier struggles to maintain regulation. This is not the most desirable type of

behavior. There are two ways out of this problem.

One way is to keep the lower FET ON until the start of the next cycle (as in the LM2647 operated in FPWM

mode). This allows the inductor current to drop to zero and then actually reverse direction (negative direction

through inductor, passing from Drain to Source of lower FET, see Channel 4 in Figure 6). Now the current can

continue to flow continuously till the end of the switching cycle. This maintains CCM and so the duty cycle does

not start to pinch off as in typical DCM. Nor does it lead to the undesirable random skipping described above.

Note that the pulse width (duty cycle) for CCM is virtually constant for any load and therefore does not usually

run into the minimum ON-time restriction. But it can happen, especially when the application consists of a very

high input voltage, a low output voltage rail, and also the switching frequency is set high. Let us check the

LM2647 to rule out this remote possibility. For example, with an input of 24V, an output of 1V, the duty cycle is

1/24 = 4.2%. This leads to a required ON-time of 0.042* 3.3 = 0.14 µs at a switching frequency of 300kHz (T=3.3

µs). Since 140ns exceeds the minimum ON-time of 30ns of the LM2647, normal constant frequency CCM mode

of operation is assured in FPWM mode, at virtually any load.

The second way out of the problems of discontinuous mode is the second operating mode of the LM2647, the

Pulse-skip (SKIP) Mode. In SKIP Mode, a zero-cross detector at the SW pin turns off the bottom FET when the

still amount to conventional DCM, with its attendant problems at extremely light loads as described earlier. The

LM2647 however avoids the random skipping behavior described earlier, and replaces it with a more defined or

formal SKIP mode. In conventional DCM, a converter would try to reduce its duty cycle from the CCM value as

the load decreases, as explained previously. So it would start with the CCM duty cycle value (at the CCM-DCM

boundary), but as the load decreases, the duty cycle would try to shrink to zero. However, in the LM2647, the

DCM duty cycle is not allowed to fall below 85% of the CCM value. So when the theoretically required DCM duty

cycle value falls below what the LM2647 is allowed to deliver (in this mode), pulse-skipping starts. It will be seen

that several of these excess pulses may be delivered, until the output capacitors charge up enough to notify the

error amplifier and cause its output to reverse. Thereafter several pulses could be skipped entirely until the

output of the error amplifier again reverses. The SKIP mode therefore leads to a reduction in the average

switching frequency. Switching losses and FET driver losses, both of which are proportional to frequency, are

significantly reduced at very light loads and efficiency is boosted. SKIP mode also reduces the circulating

currents and energy associated with the FPWM mode. See Figure 7 for a typical plot of SKIP mode at very light

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.

Copyright © 2004–2005, Texas Instruments Incorporated

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