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V

(1V/div)

TRK

V 2(1V/div)

O

t-Time=20ms/div

V 1(1V/div)

O

V

(1V/div)

TRK

V 2(1V/div)

O

t-Time=400 s/div

m

V 1(1V/div)

O

Prebias Startup Capability

PTV08040W

SLTS257A – SEPTEMBER 2005 – REVISED JANUARY 2007

Figure 16. Simultaneous Power Up With Auto-Track

Figure 17. Simultaneous Power Down With Auto-Track

Control

Control

A prebias startup 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. PTH modules all incorporate

synchronous rectifiers. Those that incorporate the prebias feature do not sink current during startup, 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’s internal soft-start mechanism; see Figure 18.

Conditions for PreBias Holdoff

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 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 (see

Figure 18). 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.

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’s Track control pin, whichever is lowest.

Demonstration Circuit

the prebias voltage, which is passed from the VCCIO to the VCORE voltage rail through the ASIC. Note that the

pre-bias.

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