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Prebias Start-Up Capability
t-Time=10ms/div
Start-Up
Period
V (1V/div)
O
V (1V/div)
I
t-Time=10ms/div
I (5 A/div)
O
V (1V/div)
O
V (1V/div)
I
SLTS243A – FEBRUARY 2005 – REVISED OCTOBER 2007
A prebias start-up condition occurs as a result of an external voltage being present at the output of a power
module prior to its output becoming active. This often occurs in complex digital systems when current from
another power source is backfed through a dual-supply logic component, such as an FPGA or ASIC. Another
path might be via clamp diodes, sometimes used as part of a dual-supply power-up sequencing arrangement. A
prebias can cause problems with power modules that incorporate synchronous rectifiers. This is because under
most operating conditions, such modules can sink as well as source output current. The PTH/PTV modules
incorporate synchronous rectifiers but do not sink current during start-up, or whenever the Inhibit pin is held low.
Start-up includes an initial delay (approximately 8–15 ms), followed by the rise of the output voltage under the
control of the module internal soft-start mechanism; see
Figure 14.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 whenever the output
is allowed to rise under soft-start control. Power up under soft-start control occurs on the removal of the ground
signal to the Inhibit pin (with input voltage applied), or when input power is applied with Auto-Track disabled(1). To
further ensure that the regulator does not sink output current (even with a ground signal applied to its Inhibit), the
input voltage must also be greater than the applied prebias source, throughout the power-up sequence(2).
The soft-start period is complete when the output begins rising above the prebias voltage. The module then
functions as normal, and sinks current if a voltage higher than its set-point 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 Track control pin, whichever is lowest.
Demonstration Circuit
Figure 15 shows the start-up waveforms for the demonstration circuit shown in Figure 16. The initial rise in VO 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 module (IO) is negligible until its output voltage rises above the applied prebias.
Figure 14. PTV03020W Start-Up
Figure 15. Prebias Start-Up Waveforms
NOTES:
1. 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, Auto-Track should be disabled when not
being used. This is accomplished by connecting the Track pin to the input voltage, VI. This raises the Track
pin well above the set-point voltage prior to start-up, thereby defeating the Auto-Track feature.
2. To further ensure that the regulator output does not sink current when power is first applied (even with a
Copyright 2005–2007, Texas Instruments Incorporated
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