RT2702 datasheet(13/20 Pages) RICHTEK

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RT2702

DS2702-00 October 2016

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is a registered trademark of Richtek Technology Corporation.

the PWM for operation. Once the input voltage decreases

by approximately 500mV, the VIN under voltage-lockout

(UVLO) circuitry will disable the external driver by setting

DISB low.

Enable and Disable

EN controls the power-up sequencing of the linear regulator

(VDRV) and buck controller. The RT2702 remains in

shutdown if the EN pin is lower than 0.4V. When EN pin

rises above the 1.2V, the RT2702 will begin a new

initialization and soft-start procedure.

Linear Regulator (VDRV)

The RT2702 contains a 5V (VDRV) linear regulator, which

can afford at least 100mA loading. This regulator will be

turned on only when VIN rises above POR threshold 4.5V

and EN is higher than 1.2V.

External Driver Enable (DISB)

The RT2702 provides an output pin DISB to turn on/off

external DrMOS. When the linear regulator VDRV soft-

start is finished, DISB will be pulled high to 5V. Then the

RT2702 begins the soft-start procedure. In addition, if UVP

or OTP occurs, DISB will be pulled low to disable DrMOS

until the protections are clean.

Audio Skipping Mode (ASM)

The RT2702 activates a unique type of diode emulation

mode with switching frequency of 30kHz, called audio

skipping mode. This mode eliminates audio-frequency

modulation that would otherwise be present when a lightly

loaded controller automatically skips pulses. In audio

skipping mode, if the PWM switching frequency is lower

than 30kHz, controller will force PWM go low to turn on

low-side switch of DrMOS. When FB is discharged to

VREF, then a regular PWM pair is generated. Thus, the

sequence of PWM signal would be HiZ-state, low-state,

high-state, low-state, and back to HiZ-state.

Forced-CCM Mode (FCCM)

The low noise, forced-CCM mode disables the zero-

crossing comparator, which means PWM signal won’t

have HiZ-state anymore unless negative over current

protection is toggled. This causes the inductor current to

reverse at light loads as the PWM loop needs to maintain

a duty ratio VOUT/VIN. The benefit of forced-CCM mode is

to keep the switching frequency fairly constant.

Power-On Reset (POR) and UVLO

Power-on reset (POR) occurs when VIN rises above to

approximately 4.5V (maximum), the RT2702 will prepare

Figure 2. Soft-Start Sequence

VIN

VFB

Int. SS

Ext. SS

Int. SSOK

Ext. SSOK

PWM

POK

1.2V

0.6V

1.2V

0.6V

1.2V

0.6V

POR

tSS_VDRV

Soft-Start_Ext

tSS_VDRV

Region 1

Region 2

Region 3

Vext.< VInt.

Vext.> VInt.

Soft-Start_Int

Soft-Start_Int+Ext

……

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