Datasheet

PREBIAS STARTUP CAPABILITY
V
I
(5 V/div)
t − Time − 5 ms/div
Startup
Period
UVLO Threshold
V
O
(1 V/div)
PTH12030W/L
www.ti.com .................................................................................................................................................... SLTS211H – MAY 2003 – REVISED DECEMBER 2008
Notes on Use of Auto-Track
TM
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 V
I
.
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 (V
I
). When Auto-Track is
disabled, the output voltage rises at a quicker and more linear rate after input power has been applied.
The capability to start up into an output prebias condition is now available to all the 12-V input, PTH series of
power modules. (Note that this is a feature enhancement for the many of the W-suffix products).
[1]
A prebias startup condition occurs as a result of an external voltage being present at the output of a power
module prior to its output becoming active. This often occurs in complex digital systems when current from
another power source is backfed through a dual-supply logic component, such as an FPGA or ASIC. Another
path might be via clamp diodes, sometimes used as part of a dual-supply power-up sequencing arrangement. A
prebias can cause problems with power modules that incorporate synchronous rectifiers. This is because under
most operating conditions, such modules can sink as well as source output current. The 12-V input PTH modules
all incorporate synchronous rectifiers, but will not sink current during startup, or whenever the Inhibit pin is held
low. Startup includes an initial delay (approximately 8 ms – 15 ms), followed by the rise of the output voltage
under the control of the module ’ s internal soft-start mechanism; see Figure 18 .
Figure 18. Startup Waveforms
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