Vo1 (1 V/Div)
Vo2 (1 V/Div)
On/Off Control
(5 V/Div)
HORIZ SCALE: 10 ms/Div
Vo1 (1 V/Div)
Vo2 (1 V/Div)
On/Off Control
(5 V/Div)
HORIZ SCALE: 10 ms/Div
Prebias Start-Up Capability
PTV12020W/L
SLTS231A – NOVEMBER 2004 – REVISED FEBRUARY 2005
Figure 17. Simultaneous Power Up With Auto-Track Figure 18. Simultaneous Power Down With Auto-Track
Control Control
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 12-V input 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 19 .
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
2
. To
further ensure that the regulator does not 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 a 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 Track control pin, whichever is lowest, to its
output.
Demonstration Circuit
Figure 20 shows the start-up waveforms for the demonstration circuit shown in Figure 21 . The initial rise in V
O
2 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
2) is negligible until its output voltage rises above the applied prebias.
17
