Datasheet

PTH08T230W, PTH08T231W
SLTS265L –NOVEMBER 2005– REVISED AUGUST 2011 www.ti.com
volt-for-volt basis. By connecting the Track pin of a number of these modules together, the output voltages follow
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 input
incorporates an internal RC-charge circuit. This operates off the module input voltage to produce a suitable rising
waveform at power up.
Typical Auto-Track™ Application
The basic implementation of Auto-Track allows for simultaneous voltage sequencing of a number of Auto-Track
compliant modules. Connecting the Track inputs of two or more modules forces their track input to follow the
same collective RC-ramp waveform, and allows their power-up sequence to be coordinated from a common
Track control signal. This can be an open-collector (or open-drain) device, such as a power-up reset voltage
supervisor device. See U3 in Figure 27.
To coordinate a power-up sequence, the Track control must first be pulled to ground potential. This should be
done at or before input power is applied to the modules. The ground signal should be maintained for at least
20 ms after input power has been applied. This brief period gives the modules time to complete their internal
soft-start initialization
(4)
, enabling them to produce an output voltage. A low-cost supply voltage supervisor
device, that includes a built-in time delay, is an ideal component for automatically controlling the Track inputs at
power up.
Figure 27 shows how the TL7712A supply voltage supervisor device (U3) can be used to coordinate the
sequenced power up of PTH08T230/231W modules. The output of the TL7712A supervisor becomes active
above an input voltage of 3.6 V, enabling it to assert a ground signal to the common track control well before the
input voltage has reached the module's undervoltage lockout threshold. The ground signal is maintained until
approximately 28 ms after the input voltage has risen above U3's voltage threshold, which is 10.95 V. The 28-ms
time period is controlled by the capacitor C3. The value of 2.2 µF provides sufficient time delay for the modules
to complete their internal soft-start initialization. The output voltage of each module remains at zero until the track
control voltage is allowed to rise. When U3 removes the ground signal, the track control voltage automatically
rises. This causes the output voltage of each module to rise simultaneously with the other modules, until each
reaches its respective set-point voltage.
Figure 28 shows the output voltage waveforms after input voltage is applied to the circuit. The waveforms, V
O
1
and V
O
2, represent the output voltages from the two power modules, U1 (3.3 V) and U2 (1.8 V), respectively.
V
TRK
, V
O
1, and V
O
2 are shown rising together to produce the desired simultaneous power-up characteristic.
The same circuit also provides a power-down sequence. When the input voltage falls below U3's voltage
threshold, the ground signal is re-applied to the common track control. This pulls the track inputs to zero volts,
forcing the output of each module to follow, as shown in Figure 29. Power down is normally complete before the
input voltage has fallen below the modules' undervoltage lockout. This is an important constraint. Once the
modules recognize that an input voltage is no longer present, their outputs can no longer follow the voltage
applied at their track input. During a power-down sequence, the fall in the output voltage from the modules is
limited by the Auto-Track slew rate capability.
Notes on Use of Auto-Track™
1. The Track pin voltage must be allowed to rise above the module set-point voltage before the module
regulates at its adjusted set-point voltage.
2. The Auto-Track function tracks 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 the input voltage V
I
.
4. The module cannot follow a voltage at its track control input until it has completed its soft-start initialization.
This takes about 20 ms from the time that a valid voltage has been applied to its input. During this period, it
is recommended that the Track pin be held at ground potential.
5. The Auto-Track function is disabled by connecting the Track pin to the input voltage (V
I
). When Auto-Track is
disabled, the output voltage rises at a quicker and more linear rate after input power has been applied.
28 Copyright © 2005–2011, Texas Instruments Incorporated