Datasheet

( )
f
IN OUT OUT
OUT(LL)
IN
1
I
2 L
V V V
V
´
´ ´
- ´
=
TPS51123A
SLUSAA6C –APRIL 2011–REVISED SEPTEMBER 2012 www.ti.com
Light Load Condition in Auto-Skip Operation
The TPS51123A automatically reduces switching frequency at light load conditions to maintain high-efficiency.
This reduction of frequency is achieved smoothly and without increase of V
OUT
ripple. Detail operation is
described as follows. As the output current decreases from heavy load condition, the inductor current is also
reduced and eventually comes to the point that its valley touches zero current, which is the boundary between
continuous conduction and discontinuous conduction modes. The rectifying MOSFET is turned off when this zero
inductor current is detected. As the load current further decreased, the converter runs in discontinuous
conduction mode and it takes longer and longer to discharge the output capacitor to the level that requires next
ON cycle. The ON time is kept the same as that in the heavy load condition. In reverse, when the output current
increase from light load to heavy load, switching frequency increases to the preset value as the inductor current
reaches to the continuous conduction. The transition load point to the light load operation I
OUT(LL)
(i.e. the
threshold between continuous and discontinuous conduction mode) can be calculated as follows;
(2)
where f is the PWM switching frequency.
Switching frequency versus output current in the light load condition is a function of L, V
IN
and V
OUT
, but it
decreases almost proportional to the output current from the I
OUT(LL)
shown in Equation 2. For example, it ise 60
kHz at I
OUT(LL)
/5 if the frequency setting is 300 kHz.
Out-of-Audio™ Light-Load Operation
Out-of-Audio™ (OOA) light-load mode is a unique control feature that keeps the switching frequency above
acoustic audible frequencies toward virtually no load condition while maintaining best of the art high conversion
efficiency. When the Out-of-Audio™ operation is selected, OOA control circuit monitors the states of both
MOSFET and force to change into the ON state if both of MOSFETs are off for more than 32 μs. This means that
the top MOSFET is turned on even if the output voltage is higher than the target value so that the output
capacitor is tends to be overcharged.
The OOA control circuit detects the over-voltage condition and begins to modulate the on time to keep the output
voltage regulated. As a result, the output voltage becomes 0.5% higher than normal light-load operation.
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