RT9971 datasheet(18/22 Pages) RICHTEK

18 / 22 page

DS9971-01 April 2011

www.richtek.com

RT9971

Step-Down :

The converter operates at fixed frequency PWM mode

and continuous current mode (CCM) with internal

MOSFETs, compensation network and synchronous

rectifier for up to 95% efficiency. The CH2 step-down

converter can be operated at 100% maximum duty cycle

to extend the input operating voltage range. While the

input voltage is close to the output voltage, the converter

enters low dropout mode.

The output voltage can be set by the following equation :

VOUT_CH2 = (1+R4/R5) x VFB2

Where VFB2 is 0.8V typically.

CH3 : Synchronous Step-Down DC/DC Converter

The converter operates at fixed frequency PWM mode,

CCM, integrated internal MOSFETs and compensation

network. The CH3 step-down converter can be operated

at 100% maximum duty cycle to extend the battery

operating voltage range. When the input voltage is close

to the output voltage, the converter could enter low dropout

mode with low output ripple.

The output voltage can be set by the following equation :

VOUT_CH3 = (1+R7/R8) x VFB3

Where VFB3 is 0.8V typically.

CH4 : Synchronous Step-Down DC/DC Converter

The converter operates at fixed frequency PWM mode,

CCM, integrated internal MOSFETs and compensation

network. The CH4 step-down converter can be operated

at 100% maximum duty cycle to extend battery operating

voltage range. When the input voltage is close to the output

voltage, the converter could enter low dropout mode with

low output ripple.

The output voltage can be set by the following equation :

VOUT_CH4 = (1+R9/R10) x VFB4

Where VFB4 is 0.8V typically.

CH5 : Step-Up DC/DC Converter

It integrates asynchronous step-up converter with an

internal N-MOSFET, internal compensation and an external

schottky diode to provide CCD positive power supply. The

converter is inactive until the SW5 soft-start procedure is

finished. This feature provides load disconnect function

and effectively limits inrush current at start up.

The output voltage can be set by the following equation :

VOUT_CH5 = (1+R11/R12) x VFB5

Where VFB5 is 1.25V typically.

SW5

SW5 is an internal switch enabled by EN5 and functions

as a load disconnection for CH5. This switch features soft-

start, Powe On Sequence, over voltage (for SW5I) and

under voltage (for SW5O) protection functions.

CH6 : INV DC/DC Converter

This converter integrates an internal P-MOSFET and an

external schottky diode to provide CCD negative power

supply.

The output voltage can be set by the following equation :

VOUT_CH6 = (R13/R14) x (-VREF)

Where R13 and R14 are the feedback resisters connected

to FB6, VREF equals to 1.25V in typical.

Charge Pumps

The charge pump will be enabled while the PVDD6 voltage

is lower than 3.6V. This CH provides pump voltage to

enhance P-MOSFET gate driving capability. This function

is not necessary while battery is Li-ion type.

Reference Voltage

The RT9971 provides a precise 1.25V reference voltage

with souring capability of 100

µA. Connect a 0.1µF ceramic

capacitor from the VREF pin to GND. Reference voltage

is enabled by connecting EN6 to logic high. Furthermore,

this reference voltage is internally pulled to GND at

shutdown.

CH7 : WLED Driver

It is an asynchronous step-up converter with an internal

MOSFET, internal compensation and an external schottky

diode to drive up to 3 WLED. This CH also features PWM

dimming control from EN7 pin and open diode protection.

In addition, CH7 will be turned on until the CH4 soft-start

is finished.

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