RT9986 datasheet(19/24 Pages) RICHTEK

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DS9986-00 May 2011

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RT9986

Table 1. CH7 WLED setting

CH7 Operating Mode

LX7

Current Source

< 0.25V

Synchronous Step-Up

> 1V

CH3 : Synchronous Step-Down DC/DC Converter

CH3 operates in fixed frequency PW M mode with

integrated internal MOSFETs and compensation network.

The CH3 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 enters low dropout mode with low

output ripple.

The output voltage can be set by the following equation :

VOUT_CH3 = (1 + R5 / R6) x VFB3

where VFB3 is 0.8V typically.

CH4 : Synchronous Step-Down DC/DC Converter

CH4 operates at fixed frequency PWM mode with

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 enters low dropout mode with low

output ripple.

The output voltage can be set by the following equation:

VOUT_CH4 = (1 + R7 / R8) x VFB4

where VFB4 is 0.8V typically.

CH5 : Synchronous Step-Up DC/DC Converter

CH5 is a high voltage synchronous step-up converter for

CCD positive power. The converter operates at fixed

frequency PWM mode, CCM, DCM, and PSM (pulse skip

mode) with integrated internal MOSFETs, compensation

network and load disconnect function.

The output voltage can be set by the following equation:

VOUT_CH5 = (1 + R9 / R10) x VFB5

where VFB5 is 1.25V typically.

CH6 : INV DC/DC Converter

This converter integrates an internal P-MOSFET with

internal compensation and needs an external Schottky

diode to provide CCD negative power supply.

The output voltage can be set by the following equation :

VOUT_CH6 =

−(R11 / R12) x (1.2V) + 0.6V

where R11 and R12 are the feedback resistors connected

to FB6, 1.2V equals to (VREF

− VFB6) and 0.6V is VFB6

typical.

Reference Voltage

The RT9986 provides a precise 1.8V reference voltage,

VREF, with souring capability of 100

µA. Connect a 0.1µF

ceramic capacitor from the VREF pin to GND. Reference

v oltage is enabled by pulling EN6 to logic-high.

Furthermore, this reference voltage is internally pulled to

GND at shutdown.

CH7 : WLED Driver

CH7 is a WLED driver that can operate in either current

source mode or synchronous step-up mode, as determined

by LX7

's initial voltage level.

When CH7 works in current source mode, it sinks an

accurate LED current modulated by EN7 high duty such

that it is easily dimmed from 0mA to 30mA. If CH7 works

in synchronous step-up mode, it integrates synchronous

step-up mode with an internal MOSFET and internal

compensation to output a voltage up to 15V. The LED

current is set via an external resistor and controlled via

the PWM duty on the EN7 pin. Regardless of the mode,

holding EN7 low for more than 32ms will turn off CH7.

In addition, CH7 will be turned on until the CH2 soft-start

is finished.

CH7 WLED Current Dimming Control

If CH7 is in synchronous step-up mode, the WLED current

is set by an external resistor. If CH7 is in current source

mode, the sink current into the FB7 pin is 30mA typically

when EN7 is high. Regardless of the mode, dimming is

always controlled by the duty of pulse-width modulated

signal on the EN7 pin. The PWM dimming duty must be

over 10%.

The average current through WLED can be set by the

following equations :

ILED (mA) = [250mV / R (W)] x Duty (%) (for step-up mode)

or ILED (mA) = 30mA x Duty (%) (for current source mode)

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