Typical Sequencing Application

Notes on Use of Auto-Track™

PTH08T240F

When under Auto-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 follow

a common signal during power up and power down. The control signal can be an externally generated master

incorporates an internal RC-charge circuit. This operates off the module input voltage to produce a suitable rising

waveform at power up.

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 IC. See U3 in Figure 31.

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

that includes a built-in time delay, is an ideal component for automatically controlling the Track inputs at power

up.

Figure 31 shows how the TL7712A supply voltage supervisor IC (U3) can be used to coordinate the sequenced

power up of PTH08T240F 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 4.3 V. The 28-ms time period is

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.

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 0 V, forcing

the output of each module to follow, as shown in Figure 33. 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.

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.

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.

disabled, the output voltage rises according to its soft-start rate after input power has been applied.

6. The Auto-Track pin should never be used to regulate the module's output voltage for long-term, steady-state

operation.

Copyright © 2006–2008, Texas Instruments Incorporated

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