RT8011/A
5
DS8011/A-02 March 2011 www.richtek.com
Operation
Main Control Loop
The RT8011/A is a monolithic, constant-frequency, current
mode step-down DC/DC converter. During normal
operation, the internal top power switch (P-Channel
MOSFET) is turned on at the beginning of each clock
cycle. Current in the inductor increases until the peak
inductor current reach the value defined by the voltage on
the COMP pin. The error amplifier adjusts the voltage on
the COMP pin by comparing the feedback signal from a
resistor divider on the FB pin with an internal 0.8V
reference. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier raises the COMP voltage until the
average inductor current matches the new load current.
When the top power MOSFET shuts off, the synchronous
power switch (N-Channel MOSFET) turns on until either
the bottom current limit is reached or the beginning of the
next clock cycle.
The operating frequency is set by an external resistor
connected between the RT pin and ground. The practical
switching frequency can range from 300kHz to 4MHz.
Power Good comparators will pull the PGOOD output low
if the output voltage comes out of regulation by 12.5%. In
an over-voltage condition, the top power MOSFET is turned
off and the bottom power MOSFET is switched on until
either the over-voltage condition clears or the bottom
MOSFET's current limit is reached.
Frequency Synchronization
The internal oscillator of the RT8011 can be synchronized
to an external clock connected to the SYNC pin. The
frequency of the external clock can be in the range of
300kHz to 4MHz. For this application, the oscillator timing
resistor should be chosen to correspond to a frequency
that is about 20% lower than the synchronization
frequency.
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 RT8011/A is designed to operate down to an input
supply voltage of 2.6V. 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
RT8011/A 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 sub-harmonic
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 RT8011/A, however, separated inductor
current signals are used to monitor over current condition.
This keeps the maximum output 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.
