Datasheet

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SLUS446B – MAY 2000 – REVISED DECEMBER 2000
7
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
APPLICATION INFORMATION
forced variable frequency (MODE = low)
In this mode, the chip behaves like a 100-mA current source that is turned on and off as the output voltage falls
below or rises above predetermined thresholds. These thresholds are located approximately 1.9% above and
below the nominal output voltage. For instance, if the nominal output is 3.3 V, then the on and off thresholds
are approximately 3.237 V and 3.363 V, respectively. The operational sequence is as follows:
1. As the output voltage falls below the turn-on threshold, the chip enters a burst period, and the buck switch
is turned on.
2. When the current in the buck switch rises to 200 mA, the switch is turned off and the synchronous rectifier
is turned on to pass the freewheel current.
3. As the current in the synchronous rectifier decays to zero, it is turned off and the buck switch is again turned
on, continuing at step 2.
4. When the output voltage rises above the turn-off threshold, the buck switch is immediately turned off and
the synchronous rectifier is turned on to handle the last cycle of free wheel current. The chip is also taken
out of its burst period and remains dormant until the output voltage falls below the turn-on threshold, at which
point operation continues at step 1.
The reason for limiting the current to 200 mA is to place a limit on the amount of overshoot that can occur from
charging the inductor up with a large current and having a relatively small output capacitance available to absorb
the energy stored.
In this mode, the error amplifier is not used and is essentially turned off, and its output is disconnected from the
COMP pin. The FB pin is connected to two comparators, which generate the internal signals that put the chip
into and out of a burst interval. The threshold levels, referenced to the FB pin, are 0.815 V and 0.785 V (off and
on). Also, the largest load current that can be supplied by the converter operating in this mode is 100 mA.
Sustained load currents greater than 100 mA deplete the energy in the output storage capacitor and cause the
output voltage to fall.