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PTH12010W, PTH12010L

SLTS205G – JUNE 2003 – REVISED FEBRUARY 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 14.

To coordinate a power-up sequence, the Track control must first be pulled to ground potential through RTRK as

defined in Figure 14. 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 soft-start initialization (4), enabling them to produce an output voltage. A low-cost

supply voltage supervisor IC, 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 15 shows the output voltage waveforms from the circuit of Figure 14 after input voltage is applied to the

circuit. The waveforms, VO1 and VO2, represent the output voltages from the two power modules, U1 (3.3 V) and

U2 (1.8 V), respectively. VTRK, VO1, and VO2 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 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. 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 PTH12060W using a voltage supervisor IC, the slew rate is

increased, therefore COmax is reduced to 2200 µF.

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 VI.

4. The module cannot follow a voltage at its track control input until it has completed its soft-start

initialization. This takes about 40 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 (VI). When

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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