RT8092BCGWSC datasheet(5/27 Pages) RICHTEK

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RT8092

DS8092-03 January 2017

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Operation

The RT8092 is a low voltage synchronous step-down

converter that can support the input voltage range from

2.5V to 5.5V and can deliver up to 4 A at an I2C selectable

voltage ranging from 0.3V to 5.5V, distributed into 4 banks

of output voltages. The converter can operate in Auto mode

or forced PWM mode. Operating modes and output voltage

can be selected via I2C.

By adapting current mode architecture, converter decides

its switching duty by inductor current sense information,

compensation ramp and error amplifier output. The

converter turns on the high-side P-MOSFET whenever the

raising edge of the switching clock. After the decided duty

time, the high-side P-MOSFET would be turned off and

the low-side N-MOSFET would be turned on until the next

frequency clock raising in forced PWM mode or turn off

by ZC (Zero Current Detection) in auto mode. The error

amplifier may adjust its output, COMP, with selected

voltage reference and output feed-back voltage information.

Different selection of voltage reference and different loading

at output node regulate the required COMP voltage, which

regulating the output voltage.

VSEL Function for Immediately Voltage Change

To address different performance operating points and

startup conditions, the device offers two output voltage /

mode presets, which can be chosen via a dedicated VSEL

pin; this allows simple and zero latency output voltage

transition.

Operating Mode Selection

The converter can operate in Auto mode or forced PWM

mode. It can be selected by programming register

0x14.PWM0 for VSEL = Low or 0x14. PWM1 for VSEL =

High. If Auto mode is selected, the converter automatically

switches the operation mode between PWM and PSM

according to the load conditions. If forced PWM mode is

selected, the converter works only in PWM mode.

PWM (Pulse Width Modulation) Operating Mode

The converter operates in PWM (Pulse Width Modulation)

mode from medium to heavy load. In PWM mode, the

converter operates with its nominal switching frequency

of 3MHz, and adapts its duty cycle to regulate the desired

output voltage. In this mode, the inductor current is in

CCM (Continuous Current Mode) and the voltage is

regulated by PWM.

Forced PWM (FPWM) Operating Mode

If forced PWM mode is selected, the converter works only

in PWM mode, and disables the transition from PWM to

PSM as the load decreases. The advantage of forced PWM

is that the switching frequency is fixed to be 3MHz, and

thus it is easy for EMI immunity design. The disadvantage

is that the efficiency at light load is poor due to the negative

inductor current.

Pulse Skipping Modulation (PSM) Operating Mode

When in Forced PWM operating mode without output

current loading, half of FPWM inductor current is negative

to balance total average zero output current. If that negative

inductor current could be saved, the efficiency will be

improved strongly. PSM is one Buck operation mode with

zero inductor current detection. Whenever zero current

occur, the low-side N-MOSFET is turned off immediately

and save the resident power into output capacitor (store

energy in higher VOUT). Then, the converter skips internal

synchronous clock and keeps sleep-state until output

voltage is discharged to below its target value. Once skips

occurs, the power MOSFETs of converter is turned off

and lots of sub-block circuits is in sleep state to save

quiescent consumption. If the output loading is increasing,

the discharge time is shortened, i.e. the switching

frequency depends on the output loading. The switch

frequency is decayed from 3MHz; the lighter output

loading, the lower switch frequency. As result, PSM VOUT

ripple would be slightly larger than FPWM but PSM gains

significant efficiency improvement.

Auto-Zero Current Detector

The Auto-Zero Current (AZC) detector circuit senses the

LX waveform to adjust the inductor zero current threshold

voltage automatically. In traditional trimmed zero current

detectors, the zero current threshold changes due to

VIN / VOUT variation. This would degrade efficiency due to

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