LM27241 datasheet(12/27 Pages) NSC

12 / 27 page

Operation Descriptions (Continued)

STATE-TRANSITIONS AT LIGHT LOADS

A situation can occur if the converter transitions from SKIP

mode to FPWM mode under a light load condition (converter

is operating below the DCM boundary). This can occur after

startup if FPWM mode is selected for use in a light load

condition or if the FPWM pin is toggled high during normal

operation at light load. The problem occurs because in SKIP

mode the converter is operating at a set duty cycle and a

lower average frequency. When the converter is forced into

FPWM mode, this represents a change to the system. The

pulse widths and frequencies need to re-adjust suddenly and

in the process momentary imbalances can be created. Like

the case of a pre-biased load, there can be negative surge

current passing from drain to source of the lower FET. It must

be kept in mind that though the LM27241 has current limiting

for current passing in the ‘positive’ direction (i.e. passing

from source to drain of the lower FET), there is no 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 capacitances will lead

to higher peak reverse currents. The reverse swing will be

higher 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 or if FPWM mode

is required for a light load application, the low side FET and

inductor should be closely evaluated under this specific con-

dition. If the part is operated in FPWM mode with a light load

the user will experience lower efficiency and negative current

during the transition (as discussed). The user may also

experience a momentary drop on Vout when the transition is

made from SKIP to FPWM mode. This only occurs for no

load or very light load conditions (above the DCM boundary

there is no difference between SKIP FPWM modes).

In some cases, such as low Vout (< 1.5V), a glitch may be

present on PGOOD. If this is problematic, the glitch may be

eliminated by either operating in SKIP mode or using a small

sized soft start capacitor. See the following section for se-

lecting soft start capacitors .

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 table). This ensures that reset is ob-

tained. Then a charging current source I

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 al-

lowable) 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 discharged by the

same 1.8 k

Ω internal resistor and the controller 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).

20120112

FIGURE 4. Inductor Current in FPWM Mode

20120113

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’

LM27241 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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