Application Notes

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PTH Series of Wide-Output Adjust

Power Modules (3.3/5-V Input)

Pre-Bias Startup Capability

Only selected products in the PTH family incorporate this

capability. Consult Table 3-1 to identify which products

are compliant.

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 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.

The PTH family of power modules incorporate synchro-

nous rectifiers, but will not sink current during startup 1,

or whenever the Inhibit pin is held low. However, to ensure

satisfactory operation of this function, certain conditions

must be maintained. 2 Figure 3-9 shows an application

demonstrating the pre-bias startup capability. The start-

up waveforms are shown in Figure 3-10. Note that the

output current from the PTH03010W (Io) shows negli-

gible current until its output voltage rises above that

backfed through diodes D1 and D2.

Note: The pre-bias start-up feature is not compatible with

Auto-Track. When the module is under Auto-Track control,

it will sink 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 either be disabled 3,

or the module’s output held off (for at least 50 ms) using the

Inhibit pin. Either approach ensures that the Track pin volt-

age is above the set-point voltage at start up.

Notes

1.Startup includes the short delay (approx.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 pin to

Vin.

V

o = 2.5 V

V

IN = 3.3 V

R

2

2k21

ASIC

VCORE

VCCIO

I

o

PTH03010W

1

10

4

5

6

2

3

98

Track

V

IN

V

O

GND

Inhibit

7

Vadj

Sense

+

C

IN

330 µF

+

C

OUT

330 µF

+

Figure 3–9; Application Circuit Demonstrating Pre-Bias Startup

Vin (1 V/Div)

Vo (1 V/Div)

Io (5 A/Div)

HORIZ SCALE: 5 ms/Div

Figure 3–10; Pre-Bias Startup Waveforms