Datasheet

( )
f
IN OUT OUT
OUT(LL)
IN
1
I
2 L
V V V
V
´
´ ´
- ´
=
TPS51123
www.ti.com SLUS890E –DECEMBER 2008–REVISED JANUARY 2013
Light Load Condition in Auto-Skip Operation
The TPS51123 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.
Enable and Soft Start
EN0 is the control pin of VREG5, VREG3 and VREF regulators. Bring this node down to GND disables those
three regulators and minimize the shutdown supply current to 10 μA. Pulling this node up to 3.3 V or 5 V will turn
the three regulators on to standby mode. The two switch mode power supplies (channel-1, channel-2) become
ready to enable at this standby mode. The TPS51123 has an internal, 1.6 ms, voltage servo softstart for each
channel. When the ENC pin becomes higher than the enable threshold voltage, which is typically 1.26 V, an
internal DAC begins ramping up the reference voltage to both of the PWM comparators at the same time.
Smooth control of the output voltage is maintained during start up.
Table 3. Enabling State
EN0 ENC VREF VREG5 VREG3 CH1 CH2
GND Don’t Care Off Off Off Off Off
Open Off On On On Off Off
Open On On On On On On
VREG5/VREG3 Linear Regulators
There are two sets of 100-mA standby linear regulators which outputs 5 V and 3.3 V, respectively. The VREG5
serves as the main power supply for the analog circuitry of the device and provides the current for gate drivers.
The VREG3 is intended mainly for auxiliary 3.3-V supply for the notebook system during standby mode.
Add a ceramic capacitor with a value of at least 33 μF and place it close to the VREG5 pin, and add at most
10 μF to the VREG3 pin. Total capacitance connected to the VREG3 pin should not exceed 10 μF .
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