Datasheet

I
OUT_PFM_enter
+
VIN
DCDC
32 W
I
OUT_PFM_leave
+
VIN
DCDC
24 W
TPS62410-Q1
www.ti.com SLVSAA8 –MARCH 2010
POWER SAVE MODE
The Power Save Mode is enabled with Mode/Data Pin set to 0 for both converters. If the load current of a
converter decreases, this converter will enter Power Save Mode operation automatically. The transition to Power
Save Mode of a converter is independent from the operating condition of the other converter. During Power Save
Mode the converter operates with reduced switching frequency in PFM mode and with a minimum quiescent
current to maintain high efficiency. The converter will position the output voltage in PFM mode to typically
1.01xVOUT. This voltage positioning feature minimizes voltage drops caused by a sudden load step.
In order to optimize the converter efficiency at light load the average inductor current is monitored. The device
changes from PWM Mode to Power Save Mode, if in PWM mode the inductor current falls below a certain
threshold. The typical output current threshold depends on VIN and can be calculated according to Equation 1
for each converter.
Equation 1: Average output current threshold to enter PFM Mode
(1)
Equation 2: Average output current threshold to leave PFM Mode
(2)
In order to keep the output voltage ripple in Power Save Mode low, the output voltage is monitored with a single
threshold comparator (skip comparator). As the output voltage falls below the skip comparator threshold (skip
comp) of 1.01 x V
OUT
nominal, the corresponding converter starts switching for a minimum time period of typically
1µs and provides current to the load and the output capacitor. Therefore the output voltage increases and the
device maintains switching until the output voltage trips the skip comparator threshold (skip comp) again. At this
moment all switching activity is stopped and the quiescent current is reduced to minimum. The load is supplied
by the output capacitor until the output voltage has dropped below the threshold again. Hereupon the device
starts switching again. The Power Save Mode is exited and PWM Mode entered in case the output current
exceeds the current IOUT_PFM_leave, or if the output voltage falls below a second comparator threshold, called
skip comparator low (Skip Comp Low) threshold. This skip comparator low threshold is set to –2% below nominal
Vout, and enables a fast transition from Power Save Mode to PWM Mode during a load step. In Power Save
Mode the quiescent current is reduced typically to 19µA for one converter and 32µA for both converters active.
This single skip comparator threshold method in Power Save Mode results in a very low output voltage ripple.
The ripple depends on the comparator delay and the size of the output capacitor. Increasing output capacitor
values minimizes the output ripple. The Power Save Mode can be disabled through the MODE/DATA pin set to
high. Both converters then operate in fixed PWM mode. Power Save Mode Enable/Disable applies to both
converters.
Dynamic Voltage Positioning
This feature reduces the voltage under/overshoots at load steps from light to heavy load and vice versa. It is
activated in Power Save Mode operation. It provides more headroom for both the voltage drop at a load step,
and the voltage increase at a load throw-off. This improves load transient behavior.
At light loads, in which the converter operate in PFM Mode, the output voltage is regulated typically 1% higher
than the nominal value. In case of a load transient from light load to heavy load, the output voltage drops until it
reaches the skip comparator low threshold set to –2% below the nominal value and enters PWM mode. During a
load throw off from heavy load to light load, the voltage overshoot is also minimized due to active regulation
turning on the N-channel switch.
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