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PTH05050W

SLTS213D – MAY 2003 – REVISED JUNE 2007

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 IC. See U3 in Figure 10.

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 10 shows how the TPS3808G50 supply voltage supervisor IC (U3) can be used to coordinate the

sequenced power-up of two 5-V input Auto-Track modules. The output of the TPS3808G50 supervisor becomes

active above an input voltage of 0.8 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 27 ms after the input voltage has risen above U3's voltage threshold, which is

4.65 V. The 27-ms time period is controlled by the capacitor C3. The value of 4700 pF 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 11 shows the output voltage waveforms from the circuit of Figure 10 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 12. 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. If the Track pin is pulled low at a slew rate greater than 1 V/ms, the discharge of the

output capacitors will induce large currents which could exceed the peak current rating of the module. This will

result in a reduction in the maximum allowable output capacitance as listed in the Electrical Characteristics

table. When controlling the Track pin of the PTH05050W using a voltage supervisor IC, the slew rate is

Notes on Use of Auto-Track™

1. The Auto-Track function tracks almost any voltage ramp during power up, and is compatible with ramp

speeds of up to 1 V/ms.

2. 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.

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.

Auto-Track is disabled, the output voltage rises at a quicker and more linear rate after input power has

been applied.

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