Application Notes
PTH Series of Wide-Output Adjust
Power Modules (3.3/5-V Input)
Auto-Track™ Function
The Auto-Track function is unique to the PTH family,
and is available with the all “Point-of-Load Alliance”
(POLA) products. Auto-Track was designed to simplify
the amount of circuitry required to make the output
voltage from each module power up and power down in
sequence. The sequencing of two or more supply voltages
during power up is a common requirement for complex
mixed-signal applications, that use dual-voltage VLSI ICs
such as DSPs, micro-processors, and ASICs.
other modules, to its respective set-point voltage.
Figure 3-6 shows the output voltage waveforms from the
circuit of Figure 3-5 after the On/Off Control is set from a
high to a low-level voltage. The waveforms, Vo1 and Vo2
represent the output voltages from the two power mod-
ules, U1 (3.3 V) and U2 (1.8 V) respectively. Vo1 and Vo2
are shown rising together to produce the desired simul-
taneous power-up characteristic.
The same circuit also provides a power-down sequence.
Power down is the reverse of power up, and is accom-
plished by lowering the track control voltage back to zero
volts. The important constraint is that a valid input voltage
must be maintained until the power down is complete. It
also requires that Q1 be turned off relatively slowly. This
is so that the Track control voltage does not fall faster than
Auto-Track's slew rate capability, which is 1 V/ms. The
components R1 and C1 in Figure 3-5 limit the rate at
which Q1 can pull down the Track control voltage. The
values of 100 k-ohm and 0.1 µF correlate to a decay rate
of about 0.17 V/ms.
How Auto-Track Works
Auto-Track works by forcing the module’s output voltage
to follow a voltage presented at the Track control pin. This
control range is limited to between 0 V and the module’s
set-point voltage. Once the track-pin voltage is raised
above the set-point voltage, the module’s output remains
1
at its set-point . As an example, if the Track pin of a 2.5-V
regulator is at 1 V, the regulated output will be 1 V. But
if the voltage at the Track pin rises to 3 V, the regulated
output will not go higher than 2.5 V.
When under track control, the regulated output from
the module follows the voltage at its Track pin on a volt-
for-volt basis. By connecting the Track pin of a number
of these modules together, the output voltages will fol-
low a common signal during power-up and power-down.
The control signal can be an externally generated master
ramp waveform, or the output voltage from another power
supply circuit 3. For convenience the Track control incor-
porates an internal RC charge circuit. This operates off
the module’s input voltage to provide a suitable rising
voltage ramp waveform.
The power-down sequence is initiated with a low-to-high
transition at the On/Off Control input to the circuit.
Figure 3-7 shows the power-down waveforms. As the
Track control voltage falls below the nominal set-point
voltage of each power module, then its output voltage
decays with all the other modules under Auto-Track
control.
Notes on Use of Auto-Track™
1.The Track pin voltage must be allowed to rise above
the module’s set-point voltage before the module can
regulate at its adjusted set-point voltage.
Typical Application
The basic implementation of Auto-Track allows for
simultaneous voltage sequencing of a number of Auto-
Track compliant modules. Connecting the Track control
pins of two or more modules forces the Track control of
all modules to follow the same collective RC ramp wave-
form, and allows them to be controlled through a single
transistor or switch; Q1 in Figure 3-5.
2.The Auto-Track function will track almost any voltage
ramp during power up, and is compatible with ramp
speeds of up to 1 V/ms.
3. The absolute maximum voltage that may be applied to the
Track pin is Vin.
4.The module will not follow
a voltage at its Track control
To initiate a power-up sequence the Track control must
first pulled to ground potential. This should be done at
or before input power is applied to the modules, and then
held for at least 10 ms thereafter. This brief period gives
the modules time to complete their internal soft-start
initialization, which enables them to produce an output
voltage.
input until it has completed its soft-start initialization.
This takes about 10 ms from the time that the module
has sensed that a valid voltage has been applied its input.
During this period, it is recommended that the Track
pin be held at ground potential.
5.The module is capable of both sinking and sourcing
current when following a voltage at its Track pin.
Therefore startup into an output prebias is not supported
during Auto-Track control. Note: A pre-bias holdoff is
not necessary when all supply voltages rise simultaneously
under the control of Auto-Track.
Applying a logic-level high signal to the circuit’s On/Off
Control turns Q1 on and applies a ground signal to the
Track control. After completing their internal soft-start
intialization, the output of all modules will remain at zero
volts while Q1 is on. 10 ms after a valid input voltage has
been applied to all modules, Q1 can be turned off. This
allows the track control voltage to automatically rise
toward to the modules' input voltage. During this period
the output voltage of each module will rise in unison with
6.The Auto-Track functioncan be disabled by connecting
the Track pin to the input voltage (Vin). With Auto-Track
disabled, the output voltage will rise at a quicker and
more linear rate after input power is applied.
For technical support and further information visit http://power.ti.com