Datasheet

( )
f
IN OUT OUT
OUT(LL)
IN
1
I
2 L
V V V
V
´
´ ´
- ´
=
TPS51125
www.ti.com SLUS786G –OCTOBER 2007–REVISED JUNE 2012
Light Load Condition in Auto-Skip Operation
The TPS51125 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)
given above. For example, it will be 60 kHz
at I
OUT(LL)
/5 if the frequency setting is 300 kHz.
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