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C

O

+

C

I

V

I

GND

Margin Down

L

O

A

D

Q2

+V

O

Q1

+

Margin Up

0V

+V

O

R

D

R

U

PT H12010W

(Top View)

1

2

10 98

7

6

5

4

3

GND

RSET

0.1 W, 1%

PREBIAS STARTUP CAPABILITY

CONDITIONS FOR PREBIAS HOLDOFF

PTH12010W, PTH12010L

SLTS205E–JUNE 2003–REVISED APRIL 2006

Table 6. Margin Up/Down Resistor Values

% ADJUST

50 kΩ

4

24.9 kΩ

3

66.5 kΩ

2

150 kΩ

1

397 kΩ

Figure 19. Margin Up/Down Application Schematic

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

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 does not sink current during startup, or whenever the Inhibit

pin is held low. Startup includes an initial delay (approx. 8–15 ms), followed by the rise of the output voltage

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 theInhibit 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

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 pre-bias source. This condition must exist throughout

The soft-start period is complete when the output begins rising above the pre-bias voltage. Once it is complete

the module functions as normal, and sinks current if 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.

20

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