RT8004
Dropout Operation
When the input supply voltage decreases toward 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 eventually
reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across
the internal P-Channel MOSFET and the inductor.
Low Supply Operation
The RT8004 is designed to operate down to an input supply voltage of 2.65V. One important consideration
at low input
supply voltages is that the R
DS(ON)
of the P-Channel and N-Channel power switches increases. The user should calculate
the power dissipation when the RT8004 is used at 100% duty cycle with low input voltages to ensure that thermal limits
are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at
duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current
signal. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the RT8004,
however, separated inductor current signals are used to monitor over current condition
and minimum peak current.
This
keeps the maximum output current and minimum peak current relatively constant regardless of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. A current
runaway detector is used to monitor inductor current. As current increasing beyond the control of current loop, switching
cycles will be skipped to prevent current runaway from occurring.
DS8004-03
September 2007
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