Datasheet

t − Time − 20 ms/div
V
TRK
(1 V/div)
V
0
1 (1 V/div)
V
0
2 (1 V/div)
t − Time − 200 µs/div
V
TRK
(1 V/div)
V
0
1 (1 V/div)
V
0
2 (1 V/div)
PTH04040W
www.ti.com SLTS238C –SEPTEMBER 2005–REVISED JUNE 2010
Figure 15 shows the output voltage waveforms from the circuit of Figure 17 after input voltage is applied to the
circuit. The waveforms, V
O
1 and V
O
2, represent the output voltages from the two power modules, U1 (3.3 V) and
U2 (1.8 V), respectively. V
TRK
, V
O
1, and V
O
2 are shown rising together to produce the desired simultaneous
power-up characteristic.
The same circuit also provides a power-down sequence. When the input voltage falls below U3's voltage
threshold, the ground signal is re-applied to the common Track control. This pulls the Track inputs to zero volts,
forcing the output of each module to follow, as shown in Figure 16. In order for a simultaneous power-down to
occur, the Track inputs must be pulled low before the input voltage has fallen below the modules' undervoltage
lockout. This is an important constraint. Once the modules recognize that a valid input voltage is no longer
present, their outputs can no longer follow the voltage applied at their Track input. During a power-down
sequence, the fall in the output voltage from the modules is limited by the maximum output capacitance and the
Auto-Track slew rate.
NOTES ON USE OF Auto-Track
1. The Track pin voltage must be allowed to rise above the module’s set-point voltage before the module can
regulate at its adjusted set-point voltage.
2. The Auto-Track function tracks almost any voltage ramp during power up, and is compatible with ramp
speeds of up to 1 V/ms.
3. The absolute maximum voltage that may be applied to the Track pin is V
I
.
4. The module does not follow a voltage at its Track input until it has completed its soft-start initialization. This
takes at least 10 ms from the time that the module has sensed that a valid voltage is present. During this
period, the Track input should be held at ground potential.
5. The module is capable of both sinking and sourcing current when following a voltage at its Track input.
Therefore startup into an output prebias is not supported when the module is under Auto-Track control.
Prebias hold off is not necessary when all supply voltages simultaneously under the control of Auto-Track.
6. The Auto-Track function can be disabled by connecting the Track pin to the input voltage (V
I
). With
Auto-Track disabled, the output voltage rises at a quicker and more linear rate after input power is applied.
Figure 15. Auto-Track Simultaneous Power Up Figure 16. Auto-Track Simultaneous Power Down
Waveforms Waveforms
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