V =3.3V

I

V

=2.5V

O

R2

2k21

ASIC

VCORE

VCCIO

Io

PTH03010W

1

10

4

5

6

2

3

9

Track

VI

VO

GND

Inhibit

7

VAdj

Sense

+

C

330 F

I

m

C

330 F

O

m

+

+

8

PTH04040W

SLTS238C – SEPTEMBER 2005 – REVISED JUNE 2010

www.ti.com

PREBIAS STARTUP CAPABILITY

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 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/PTV family of power modules incorporate synchronous rectifiers, but does 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 19 shows an application demonstrating the prebias startup

capability. The start-up waveforms are shown in Figure 20. Note that the output current from the PTH03010W (Io)

shows negligible current until its output voltage rises above that backfed through diodes D1 and D2.

Note: The prebias start-up feature is not compatible with Auto-Track. When the module is under Auto-Track

control, it sinks current if the output voltage is below that of a back-feeding source. To ensure a prebias

hold-off, one of two approaches must be followed when input power is applied to the module. The Auto-Track

function must 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 voltage is above the set-point voltage at start up.

NOTES

1. Startup includes the short delay (approximately 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 powerup 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.

Figure 19. Application Circuit Demonstrating Prebias Startup

20

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