LTC3406AB Datasheet, PDF (7/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
OPERATION (Refer to Functional Diagram)
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will
then be determined by the input voltage minus the voltage
drop across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R_DS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore,
the user should calculate the power dissipation when the
LTC3406AB is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current signal
at duty cycles in excess of 40%. Normally, this results in
a reduction of maximum inductor peak current for duty
cycles >40%. However, the LTC3406AB uses a patented
scheme that counteracts this compensating ramp, which
allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
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