Datasheet

t-Time-4ms/div
V (1V/div)
O
I (2 A/div)
O
V (1V/div)
IN
ASIC
+
+
V
o
=2.5V
3.3V
V
I
=5V
R
2.37kW
SET
VCORE
VCCIO
I
o
PTH08T220W
Track
V
I
V
O
GNDInhibit Vadj
+Sense
C
I
+
C
O
-Sense
PTH08T250W
www.ti.com SLTS278G –NOVEMBER 2010–REVISED MARCH 2009
The prebias start-up feature is not compatible with Auto-Track. When the module is under Auto-Track control, it
sinks current if the output voltage is below that of a back-feeding source. To ensure a pre-bias hold-off one of
two approaches must be followed when input power is applied to the module. The Auto-Track function must
either be disabled
(3)
, or the module’s output held off (for at least 50 ms) using the Inhibit pin. Either approach
ensures that the Track pin voltage is above the set-point voltage at start up.
1. Startup includes the short delay (approximately 10 ms) prior to the output voltage rising, followed by the rise
of the output voltage under the module’s internal soft-start control. Startup is complete when the output
voltage has risen to either the set-point voltage or the voltage at the Track pin, whichever is lowest.
2. To ensure that the regulator does not sink current when power is first applied (even with a ground signal
applied to the Inhibit control pin), the input voltage must always be greater than the output voltage throughout
the power-up and power-down sequence.
3. The Auto-Track function can be disabled at power up by immediately applying a voltage to the module’s
Track pin that is greater than its set-point voltage. This can be easily accomplished by connecting the Track
pin to V
I
.
Figure 34. Prebias Startup Waveforms
Figure 35. Application Circuit Demonstrating Pre-Bias Startup
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