Application Notes

ATH Series of Wide-Output Adjust Power

Modules (3.3/5-V Input)

North America (USA): 1-888-41-ASTEC

Europe (UK): 44(1384)842-211

Asia (HK): 852-2437-9662

Pre-Bias Startup Capability

Only selected products in the ATH 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 ATH family of power modules incorporate synchro-

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

satisfactory operation of this function, certain conditions

demonstrating the pre-bias startup capability. The start-

up waveforms are shown in Figure 3-9. Note that the

ligible current until its output voltage rises above that

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

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

is fully active and will sink current if the output voltage is

below that of a back-feeding source. Therefore 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

the Inhibit pin. The latter allows Auto-Track’s internal (RC)

voltage ramp to rise above the set-point voltage.

Notes

1. Startup is the relatively short period (approx. 10 ms)

prior to the output voltage rising. The startup period

immediately follows either the application of a valid

input source voltage, or the release of a ground signal

at the Inhibit pin.

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

V

V

R

2

2k21

R

1

1k0

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

+

D

MBR3100

C

OUT

330 µF

+

Figure 3.8; Application Circuit Demonstrating Pre-Bias Startup

Vin (1 V/Div)

Vo (1 V/Div)

Io (5 A/Div)

HORIZ SCALE: 5 ms/Div

Figure 3.9; Pre-Bias Startup Waveforms

ATH15T033-9S