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+

GND

Margin Up

Margin Down

Q2

Q1

+

0V

PTH12010W

(Top View)

1

2

10

9

8

7

6

5

4

3

GND

L

O

A

D

V

I

+V

O

O

R

1%, 0.1 W

SET

C

I

R

D

R

U

C

O

Prebias Startup Capability

PTH12040W

SLTS237A–DECEMBER 2004–REVISED OCTOBER 2005

Table 5. Margin Up/Down Resistor Values

% ADJUST

50 kΩ

4

24.9 kΩ

3

66.5 kΩ

2150 kΩ

1397 kΩ

Figure 16. Margin Up/Down Application Schematic

The capability to start up into an output prebias condition is available to all the 12-V input series of PTH/PTV

power modules.

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. The 12-V input PTH modules

all incorporate synchronous rectifiers, but 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

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 whenver the output is

allowed to rise under soft-start control. Power up under soft-start control occurs upon the removal of the ground

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.

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