lm27241 National Semiconductor Corporation, lm27241 Datasheet - Page 12

• Manufacturer Part Number: lm27241
• Description: Synchronous Buck Regulator Controller For Mobile Systems
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Descriptions
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
FIGURE 4. Inductor Current in FPWM Mode
(Continued)
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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 (
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 .
CH1: PGOOD, CH2: Vo, CH3: LDRV, CH4: I
Output 1V
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 kΩ internal resistor (R
trical Characteristics table). This ensures that reset is ob-
tained. Then a charging current source I
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 kΩ 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).
@
FIGURE 5. SKIP to FPWM ’On The Fly’
0A, VIN = 10V, L = 10µH, f = 300kHz
<
L
(1A/div)
1.5V), a glitch may be
SS_DCHG
SS_CHG
, see Elec-
of 11µA is
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