LM2647MTC datasheet(11/25 Pages) NSC

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Operation Descriptions (Continued)

In a conventional converter, as the load is decreased to

about 10-30% of maximum load current, DCM (Discontinu-

ous 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 ampli-

fier 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, pass-

ing from Drain to Source of lower FET, see Channel 4 in

Figure 2). 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 as-

sured 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 inductor current

decays to zero (actually at V

SW_ZERO, see Electrical Char-

acteristics table). This would however still amount to conven-

tional 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 am-

plifier 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 circulat-

ing currents and energy associated with the FPWM mode.

See Figure 3 for a typical plot of SKIP mode at very light

loads. Note the bunching of several fixed-width pulses fol-

lowed by skipped pulses. The average frequency can actu-

ally 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.

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 ap-

plied 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 4. Since duty cycle is the same as

for conventional CCM, fromV=L*

∆I/ ∆t it can be seen that

∆I (or Ipp in Figure 4) 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.

20056311

CH1: HDRV, CH2: LDRV, CH3: SW, CH4: IL (0.2A/div)

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

FIGURE 3. Normal SKIP Mode Operation at Light

Loads

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