PTB48560A_15 Datasheet, PDF (16/29 Pages) Texas Instruments – 30-W, 48-V INPUT DC/DC CONVERTERS WITH AUTO-TRACK™ SEQUENCING

PTB48560A, PTB48560B
PTB48560C
SLTS236A – MARCH 2005 – REVISED OCTOBER 2005
Sequenced Power Up with POL Modules
www.ti.com
Overview
The main output from the PTB48560x converters is V_OBus. In power sequencing applications, V_OBus 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_OBus. 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_OBus is in regulation. This is necessary to allow time for the nonisolated modules to complete
their power-up initialization. The Track pin of each PTB48560x converter has an internal open-drain transistor
that automatically holds the Track signal at ground potential to comply with this requirement.
The PTB48560B (1.8 V to 3.6 V) has a V_OSeq output. V_OSeq is internally derived from V_OBus 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_OBus.
Auto-Track Features
Figure 18 shows a block diagram of the converter Auto-Track features. The components shaded are only present
in the PTB48560B. During power up, V_OBus rises promptly, after the converter is connected to a valid input
source and its output is enabled. V_OSeq (PTB48560B) 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_OBus
output. Between these limits, the voltage at V_OSeq follows that at the Track terminal. Once the Track voltage is
at the V_OBus voltage, raising it higher has no further effect. The voltage at V_OSeq cannot go higher than V_OBus,
and if connected to +Sense, it regulates at the set-point voltage.²
The Track input to the PTB48560x series of converters include a pullup resistor (R_TRK) to V_OBus, 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_OSeq and the Track input is the same as all other Auto-Track compliant
outputs.³ The V_OSeq 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_OBus output is in regulation.
16