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Overcurrent Protection (OCP)
Auto-Track™ Function
PTV03010W
SLTS241A – FEBRUARY 2005 – REVISED JULY 2007
For protection against load faults, the modules incorporate output overcurrent protection. Applying a load that
exceeds the overcurrent threshold causes the regulated output to shut down. Following shutdown, a module
periodically attempts to recover by initiating a soft-start power up. This is described as a hiccup mode of
operation, whereby the module continues in the cycle of successive shutdown and power up until the load fault
is removed. During this period, the average current flowing into the fault is significantly reduced. Once the fault is
removed, the module automatically recovers and returns to normal operation.
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
Auto-Track works by forcing the module output voltage to follow a voltage presented at the Track control pin
(1)
.
This control range is limited to between 0 V and the module set-point voltage. Once the track-pin voltage is
raised above the set-point voltage, the module output remains at its set-point
(2)
. As an example, if the Track pin
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.
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
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 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 11 .
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 IC,
that includes a built-in time delay, is an ideal component for automatically controlling the track inputs at power
up.
Figure 11 shows how the TPS3808G33 supply voltage supervisor IC (U3) can be used to coordinate the
sequenced power-up of two 3.3-V input Auto-Track modules. The output of the TPS3808G33 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 3.07 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 12 shows the output voltage waveforms from the circuit of Figure 11 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 (2.5 V) and
U2 (1.2 V), respectively. V
TRK
, V
O
1, and V
O
2 are shown rising together to produce the desired simultaneous
power-up characteristic.
12
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