Application Notes

For technical support and further information visit http://power.ti.com

Pre-Bias Startup Capability

The capability to start up into an output pre-bias condi-

tion is now a feature of the PTH12000 series of modules.

(Note: This is a feature enhancement for the the W-suffix

version; see note 1).

A pre-bias 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 com-

plex 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 synchro-

nous rectifiers. This is because under most operating

conditions, such modules can sink as well as source output

current. The PTH12000x series of modules incorporate

synchronous rectifiers, but will 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 under the control of the

module’s internal soft-start mechanism; see Figure 3-1.

Conditions for Pre-Bias Holdoff

In order for the module to allow an output pre-bias voltage

to exist (and not sink current), certain conditions must be

maintained. The module holds off a pre-bias 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

signal to the Inhibit pin (with input voltage applied), or

when input power is applied. 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 condi-

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 will sink current if a

voltage higher than the nominal regulation value is applied

to its output.

Note: If a pre-bias condition is not present, the soft-start period

will be complete when the output voltage has risen to either

the set-point voltage.

Demonstration Circuit

The circuit shown in Figure 3-4 is a demonstrates the

pre-bias startup feature. Figure 3-5 shows the startup

age, which is passed from the VCCIO to the VCORE

voltage rail through the ASIC. Note that the output cur-

until its output voltage rises above the applied pre-bias.

Figure 3–4; Application Circuit Demonstrating Pre-Bias Startup

130

ASIC

VCORE

VCCIO

+

2k

330 µF

+

330 µF

+

PTH12020W

7

10

4

5

6

2

3

98

Track

GND

Inhibit

1

Up

Dn

Sense

Adjust

330 µF

+

TL7702B

GND

SENSE

RESIN

REF

CT

RESET

RESET

8

7

2

1

3

5

6

4

0.68 µF

330 µF

+

11k0

100k

0.1 µF

10k0

PTH12000L

14

5

2

3

GND

Inh

Adj

PTH12000 Series