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Prebias Start-Up Capability
t-Time=10ms/div
V (1V/div)
I
V (1V/div)
O
Start-up
Period
t-Time=10ms/div
V (1V/div)
I
V 1(1V/div)
O
I (5 A/div)
O
PTV03010W
SLTS241A – FEBRUARY 2005 – REVISED JULY 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, to its
output.
Demonstration Circuit
Figure 15 shows the start-up waveforms for the demonstration circuit shown in Figure 16 . The initial rise in V
O
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 (I
O
) is negligible until its output voltage rises above the applied prebias.
Figure 14. PTV03010W Start-Up Figure 15. Prebias Start-Up Waveforms
15
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