Datasheet

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Sequenced Power Up with POL Modules
Overview
Auto-Track Features
PTB78560A , PTB78560B
PTB78560C
SLTS249 – JUNE 2005
The main output from the PTB78560x converters is V
O
Bus. In power sequencing applications, V
O
Bus is used as
the intermediate supply voltage for powering one or more downstream nonisolated power modules that
incorporate Auto-Track™
1
. The output voltage from Auto-Track compliant modules can be sequenced using a
control input called Track. The Track input directly controls the output of a module from zero to its set-point
voltage. The control is on a volt-for-volt basis, and allows multiple modules to follow a common analog signal
during power-up events.
The Track signal attempts to start rising when the nonisolated modules are first powered from V
O
Bus. However,
for proper sequencing, the voltage must be held at ground potential for at least 20 ms (40 ms for 12-V input
modules) after V
O
Bus is in regulation. This is necessary to allow time for the nonisolated modules to complete
their power-up initialization. The Track pin of each PTB78560x converter has an internal open-drain transistor
that automatically holds the Track signal at ground potential to comply with this requirement.
The PTB78560B (1.8 V to 3.6 V) has a V
O
Seq output. V
O
Seq is internally derived from V
O
Bus and regulated to
the same set-point voltage. It has the added feature of being controlled by the Track input. During power up, this
output can sequence with the outputs of the nonisolated modules powered from V
O
Bus.
Figure 26 shows a block diagram of the converter Auto-Track features. The components shaded are only
present in the PTB78560B. During power up, V
O
Bus rises promptly, after the converter is connected to a valid
input source and its output is enabled. V
O
Seq (PTB78560B) is the Auto-Track compatible output that is
controlled by the voltage presented at the Track terminal. The control is active from 0 V up to a voltage just
below the V
O
Bus output. Between these limits, the voltage at V
O
Seq follows that at the Track terminal. Once the
Track voltage is at the V
O
Bus voltage, raising it higher has no further effect. The voltage at V
O
Seq cannot go
higher than V
O
Bus, and if connected to +Sense, it regulates at the set-point voltage.
2
The Track input to the PTB78560x series of converters include a pullup resistor (R
TRK
) to V
O
Bus, and a 1-µF
capacitor (C
TRK
) to -Sense. These components are standard on all Auto-Track compatible modules. They form
an R–C time constant that cause the Track voltage to rise when the internal MOSFET is turned off. The
unity-gain relationship between V
O
Seq and the Track input is the same as all other Auto-Track compliant
outputs.
3
The V
O
Seq output also follows a compatible external ramp waveform applied to the Track pin.
4, 5
The
internal MOSFET is designed to hold the Track voltage at ground potential for the required period after the
V
O
Bus output is in regulation.
17
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