RT8004

5

DS8004-03

September 2007

www.richtek.com

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 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,

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.