RT8030 datasheet(3/11 Pages) RICHTEK

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RT8030

DS8030-02

March 2011

www.richtek.com

Operation

Main Control Loop

The RT8030 is a monolithic, constant-frequency, current

mode step-down DC/DC converter. During normal

operation, the internal top power switch (P-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-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 2.5MHz.

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-MOSFET and the inductor.

Low Supply Operation

The RT8030 is designed to operate down to an input supply

voltage of 2.6V. One important consideration at low input

supply voltages is that the RDS(ON) of the P-Channel and

N-Channel power switches increases. The user should

calculate the power dissipation when the RT8030 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 RT8030, 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.

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