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Prebias Startup Capability

ASIC

+

+

3.3V

R

2.37kW

SET

VCORE

VCCIO

PTH08T240W

Track

VI

VO

GND

Inhibit

Vadj

+Sense

+

-Sense

PTH08T240W, PTH08T241W

SLTS264D – NOVEMBER 2005 – REVISED OCTOBER 2006

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 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, these types of modules can sink as well as source output current.

or whenever the Inhibit pin is held low. However, to ensure satisfactory operation of this function, certain

voltage rises above the voltage backfed through the intrinsic diodes.

The prebias start-up feature is not compatible with Auto-Track. When the module is under Auto-Track control, it

sinks current if the output voltage is below that of a back-feeding source. To ensure a pre-bias hold-off one of

two approaches must be followed when input power is applied to the module. The Auto-Track function must

ensures that the Track pin voltage is above the set-point voltage at start up.

1. Startup includes the short delay (approximately 10 ms) prior to the output voltage rising, followed by the rise

of the output voltage under the module’s internal soft-start control. Startup is complete when the output

voltage has risen to either the set-point voltage or the voltage at the Track pin, whichever is lowest.

2. To ensure that the regulator 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 output voltage

throughout the power-up and power-down sequence.

3. The Auto-Track function can be disabled at power up by immediately applying a voltage to the module’s

Track pin that is greater than its set-point voltage. This can be easily accomplished by connecting the Track

Figure 28. PreBias Startup Application Circuit

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