PTH12030W/L
www.ti.com
SLTS211F–MAY 2003–REVISED FEBRUARY 2007
CONDITIONS FOR PREBIAS HOLDOFF
In order for the module to allow an output prebias voltage to exist (and not sink current), certain conditions must
be maintained. The module holds off a prebias voltage when the Inhibit pin is held low, and whenver the output
is allowed to rise under soft-start control. Power up under soft-start control occurs upon the removal of the
ground signal to the Inhibit pin (with input voltage applied), or when input power is applied with Auto-Track
disabled.[2] To further ensure that the regulator doesn’t sink output current, (even with a ground signal applied to
its Inhibit), the input voltage must always be greater than the applied prebias source. This condition must exist
throughout the power-up sequence.[3]
The soft-start period is complete when the output begins rising above the prebias voltage. Once it is complete,
the module functions as normal, and sinks current if voltage higher than the nominal regulation value is applied
to its output.
Note:If a prebias condition is not present, the soft-start period is complete when the output voltage has risen
to either the set-point voltage, or the voltage applied at the module’s Track control pin, whichever is lowest.
DEMONSTRATION CIRCUIT
Figure 18 shows the startup waveforms for the demonstration circuit shown in Figure 19. The initial rise in VO2 is
the prebias voltage, which is passed from the VCCIO to the VCORE voltage rail through the ASIC. Note that the
output current from the PTH12010L module (IO2) is negligible until its output voltage rises above the applied
prebias.
V 1 (1 V/div)
O
V 2 (1 V/div)
O
I 2 (5 V/div)
O
t − Time − 10 ms/div
Figure 18. Prebias Startup Waveforms
NOTES
1. Output prebias holdoff is an inherent feature to all PTH120x0L and PTV120x0W/L modules. It has now
been incorporated into all modules (including W-suffix modules with part numbers of the form
PTH120x0W), with a production lot date code of 0423 or later.
2. The prebias start-up feature is not compatible with Auto-Track. If the rise in the output is limited by the
voltage applied to the Track control pin, the output sinks current during the period that the track control
voltage is below that of the back-feeding source. For this reason, it is recommended that Auto-Track be
disabled when not being used. This is accomplished by connecting the Track pin to the input voltage, VI.
This raises the Track pin voltage well above the set-point voltage prior to the module’s start up, thereby
defeating the Auto-Track feature.
3. To further ensure that the regulator’s output does not sink current when power is first applied (even with a
ground signal applied to the Inhibit control pin), the input voltage must always be greater than the applied
prebias source. This condition must exist throughout the power-up sequence of the power system.
18
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