LM2647 datasheet(12/25 Pages) NSC

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Operation Descriptions (Continued)

Note: A common question is: can one change from FPWM to SKIP Mode ‘on

the fly’? That means that the voltage on the FPWM pin would be

changed while the converter is operating normally (with outputs in

regulation). This is generally not recommended. The designer must

realize that doing so would in essence represent a fundamental

change applied to the system. The pulse widths would need to re-

adjust suddenly and in the process momentary imbalances can be

created. For example, there is an observed negative surge current

passing from Drain to Source of the lower FET. It must be kept in mind

that though the LM2647 has current limiting for current passing in the

‘positive’ direction (positive with regards to the inductor, i.e. passing

from Source to Drain of the lower FET), there is no set limit for reverse

currents. The amount of reverse current when the FPWM pin is toggled

‘on the fly’ can be very high. This current is determined by several

factors. One key factor is the output capacitance. Large output capaci-

tances will lead to higher peak reverse currents. The reverse swing will

be worse for lighter loads because of the bigger difference between the

duty cycles/average frequency in the two modes. See Figure 5 for a

plot of what happened in going from SKIP to FPWM mode at 0A load

(worst case). The peak reverse current was as high as 3A, lasting

about 0.1ms. The inductor could also saturate severely at this point if

designed for light loads. In general, if the designer wants to toggle the

FPWM pin while the converter is operating, both the low side FET

rating and the inductor peak current rating must be closely evaluated

under this condition.

SOFT-START

The maximum output voltage of the error amplifier is limited

during start-up by the voltage on the 0.1µF capacitor con-

nected between the SS pin and ground. When the controller

is enabled (by taking EN pin high) the following steps may

occur. First the SS capacitor is discharged (if it has a pre-

charge) by a 1.8 k

Ω internal resistor (R

SS_DCHG, see Elec-

trical Characteristics). This ensures that reset is obtained.

Note that reset is said to occur only when the voltage on both

the SS pins falls below 100mV (V

SS_RESET, see Electrical

Characteristics table). Then a charging current source

SS_CHG of 11µA is applied at this pin to bring up the voltage

of the Soft-start capacitor voltage gradually. This causes the

(maximum allowable) duty cycle to increase slowly, thereby

limiting the charging current into the output capacitor and

also ensuring that the inductor does not saturate. The Soft-

start capacitor will eventually charge up close to the 5V input

rail. When EN is pulled low the Soft-start capacitor is dis-

charged by the same 1.8 k

Ω internal resistor and the con-

troller is shutdown. Now the sequence is allowed to repeat

the next time EN is taken high.

The above Soft-start sequence is actually initiated not only

whenever EN is taken high, but also under a normal

power-up or during recovery from a fault condition (more on

this later).

As mentioned in the section ‘Forced-PWM Mode and Pulse-

skip Mode’ under startup, since the currents are high until the

output capacitors have charged up, there will be no observ-

able 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.

SHUTDOWN/SOFT-SHUTDOWN

When the EN pin is driven low, the LM2647 initiates shut-

down by turning OFF both upper and lower FETs completely

(this occurs irrespective of FPWM or SKIP modes). See

Figure 6 for a typical shutdown plot and note that the LDRV

20056312

FIGURE 4. Inductor Current in FPWM Mode

20056313

CH1: PGOOD, CH2: Vo, CH3: LDRV, CH4: IL (1A/div)

Output 1V @ 0A, VIN = 10V, L = 10µH, f = 300kHz

FIGURE 5. SKIP to FPWM ’On The Fly’

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