Datasheet

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SLUS446B – MAY 2000 – REVISED DECEMBER 2000
13
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
APPLICATION INFORMATION
loop compensation and the TPS6210X series of converters (continued)
Note that f
z1
is a lower frequency than f
z2
. It is suggested that a spice or other simulator be used to verify
feedback loop characteristics. When doing this modeling, be sure to use an amplifier that has a band limited
response. Setting the amplifier up for an 80 dB gain with a single dominant pole at 200 Hz (2 MHz GBWP) will
lead to a good loop design. The gain/phase plots for the example circuits were all done using a 2 MHz GBWP
amplifier. Look at the example schematics and note where the poles and zeros are for an indication of where
to start compensating a new design.
current limiting
The TPS6210x family has built-in over-current protection and thermal shutdown. The over-current protection
is done in two stages. The first stage trips at approximately 1 A and simply causes an immediate pulse
termination. If a hard short is present at the output of the LC filter, propagation delays from the first-level over
current could allow the current to ratchet up in the inductor. To prevent this from happening, a second level of
over-current protection trips at approximately 1.2 A. When this level is tripped, the chip is forced to do a soft start.
If the chip is in an abnormally high ambient temperature, or has an inadequate heatsink for the power levels
demanded, the thermal shutdown circuitry causes the chip to shut down if the die temperature ever reaches
170°C. After the die cools, normal operation resumes with a full soft start.
UDG–00050
V
OUT
V
FB
815mV
785mV
V
SW
I
L
200mA
Figure 4. Typical Pulsed-Variable-Frequency Mode (PFM) Circuit Waveform