Datasheet

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SLUS446B – MAY 2000 – REVISED DECEMBER 2000
8
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
APPLICATION INFORMATION
the automatic modes, or the high power and low power modes (MODE = floating)
Leaving the MODE pin unconnected or floating lets the chip automatically select one of two operating modes.
In these two modes, operation is the same as for the forced modes with the exception that the chip is free to
switch between the constant- and variable-frequency modes based upon the operating conditions of the
converter. When the chip is initially powered, it is in the constant frequency (high-power) mode, and stays in
this mode until it senses that the converter is on the verge of breaking into discontinuous operation. When this
condition is sensed, the converter enters the variable-frequency mode of operation. The chip remains in the
variable-frequency mode of operation until the voltage at the FB pin falls to approximately 0.770 V or 3.75%
below nominal. When this happens, the chip enters the constant-frequency mode of operation and remains in
that mode until it senses the converter is about to go discontinuous.
This has some design implications. In order for the transition to occur smoothly, the discontinuous current level
of the converter must be less than 100 mA. This in turn implies a minimum inductor size for a given set of
operating conditions. The minimum inductor size for smooth transitions is approximately:
L
V
O
1
V
O
V
I
0.16 F
Where:
V
O
is the output voltage
V
I
is the input voltage and
F is the frequency of operation
0.16 is the minimum peak-inductor-current in low-power mode
It is recommended that something more than the minimum inductance be used to give a little hysteresis to the
mode transition. A 10% increase in inductance over the minimum value should be sufficient.
Note that in all modes, on initial power-up, after a shutdown, and when there is a second stage overcurrent,
the chip transitions into a constant frequency mode of operation and goes through a soft-start cycle. The chip
remains in the constant frequency mode until the voltage presented to the FB pin exceeds 0.770 V. At this point
the chip may go into a variable frequency mode if MODE is held low, or the load is insufficient to cause
continuous inductor current and the MODE pin is left floating.
soft start
The TPS6210x family has a built in soft-start time of approximately 5 ms. The soft start is a
closed-loop
soft start,
meaning that the reference input to the error amplifier is ramped up over the soft start interval and the converter
control loop is allowed to track the ramping reference signal. This method generally allows for faster soft-start
times with minimal output voltage overshoot at startup.
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