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Auto-Track™ Function

PTH12030W/L

SLTS211F – MAY 2003 – REVISED FEBRUARY 2007

The Auto-Track function is unique to the PTH/PTV family, and is available with all 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 the TMS320™ DSP

family, microprocessors, and ASICs.

How Auto-Track™ Works

This control range is limited to between 0 V and the module set-point voltage. Once the track-pin voltage is

of a 2.5-V regulator is at 1 V, the regulated output is 1 V. If the voltage at the Track pin rises to 3 V, the

regulated output does not go higher than 2.5 V.

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.

Typical 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 IC. See U3 in Figure 14.

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

40 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 14 shows how the TL7712A supply voltage supervisor IC (U3) can be used to coordinate the sequenced

power up of two 12-V input Auto-Track 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 43 ms after the input voltage has risen above U3's voltage threshold, which is 10.95 V. The

43-ms time period is controlled by the capacitor C3. The value of 3.3

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 16 shows the output voltage waveforms from the circuit of Figure 14 after input voltage is applied to the

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 16. In order for a simultaneous power-down to

occur, the track inputs must be pulled low before the input voltage has fallen below the modules' undervoltage

lockout. This is an important constraint. Once the modules recognize that a valid 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 maximum output capacitance and the

Auto-Track slew rate.

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