Datasheet

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2.5
2.0
1.5
1.0
0.5
0
012345
I
PGD
− Powergood Current − mA
V
PGD
− Powergood Voltage − V
V
VDD
= 0 V
Pre-Bias Operation
Remote Sense
TPS40100
SLUS601–MAY 2005
POWERGOOD VOLTAGE
vs
POWERGOOD CURRENT
Figure 17.
Some applications require that the converter not sink current during startup if a pre-existing voltage exists at the
output. Since synchronous buck converters inherently sink current some method of overcoming this characteristic
must be employed. Applications that require this operation are typically power rails for a multiple supply
processor or ASIC. The method used in this controller, is to not allow the low side or rectifier FET to turn on until
there the output voltage commanded by the start up ramp is higher than the pre-existing output voltage. This is
detected by monitoring the internal pulse width modulator (PWM) for its first output pulse. Since this controller
uses a closed loop startup, the first output pulse from the PWM does not occur until the output voltage is
commanded to be higher than the pre-existing voltage. This effectively limits the controller to sourcing current
only during the startup sequence. If the pre-existing voltage is higher that the intended regulation point for the
output of the converter, the converter starts and sinks current when the soft-start time has completed.
The TPS 40100 is capable of remotely sensing the load voltage to improve load regulation. This is accomplished
by connecting the GND pin of the device and the feedback voltage divider as near to the load as possible.
CAUTION:
Long distance runs for the GND pin will cause erratic controller behavior.
This begins to appear as increased pulse width jitter. As a starting point, the GND pin connection should be no
further than six inches from the PGND connection. The actual distance that starts causing erratic behavior is
application and layout dependent and must be evaluated on an individual basis. If the controller exhibits output
pulse jitter in excess of 25 ns and the GND pin is tied to the load ground, connecting the GND pin closer to the
PGND pin (and thereby sacrificing some load regulation) may improve performance. In either case, connecting
the feedback voltage divider at the point of load should not cause any problems. For layout, the voltage divider
components should be close to the device and a trace can be run from there to the load point.
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