RT8015D datasheet(4/14 Pages) RICHTEK

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RT8015D

DS8015D-02 March 2011

www.richtek.com

Operation

Main Control Loop

The RT8015D 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-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 2MHz.

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 RT8015D 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

RT8015D 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 RT8015D, 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.

RT8015D Datasheet(PDF) 4 Page - Richtek Technology Corporation

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