RT9986 datasheet(20/24 Pages) RICHTEK

20 / 24 page

DS9986-00 May 2011

www.richtek.com

RT9986

R is the current sense resistor from FB7 to GND and Duty

is the duty of the PWM dimming signal into EN7 pin.

Dimming frequency range is from 1kHz to 100kHz but

2kHz to 20kHz should be avoided to prevent distraction

from audio noise.

VDDM Bootstrap

To support bootstrap function, the RT9986 includes a

power selection circuit which selects between BAT and

PVDD1 to create the internal node voltage VDDI and VDDM.

VDDM is the power of all the RT9986 control circuits and

must be connected to an external decoupling capacitor

by way of the VDDM pin. The VDDI is the power input of

the RTC LDO. The output PVDD1 of CH1 can bootstrap

VDDM and VDDI. The RT9986 includes UVLO circuits to

monitor VDDM and BAT voltage status.

RTC LDO

The RT9986 provides a 3.1V output LDO for real time clock.

The LDO features low quiescent current (5

µA) and high

output voltage accuracy. This LDO is always on, even when

the system is shut down. For better stability, is it

recommended to connect a 0.1

µF to the RTCPWR pin.

The RTC LDO includes pass transistor body diode control

to avoid the RTCPWR node from back-charging into the

input node VDDI.

System Reset

The RT9986 also provides a system voltage detector to

monitor system power status via FB2. If FB2 level is lower

than 90% setting, the open drain output pin SYSR will

pull down. When FB2 level is higher than 95% setting,

the SYSR pin will go high after 10ms.

Voltage Detector

The RT9986 provides a voltage detector to detect the

voltage status at the VCHK pin. The input power of the

voltage detector is RTCPWR and the detector is always

on. 55ms after VCHK voltage > 1.616V, the open drain

output /RST will be pulled high. If VCHK < 1.6V, the /RST

pin will be pulled down to GND immediately.

Power On/Off Sequence for CH1 to CH4

EN1234 will turn on/off CH1 to CH4 in preset sequence.

CH1 to CH4 Power On Sequence is:

When EN1234 goes high, CH1 will turn on first. 3.5ms

after CH1 is turned on, CH3 will turn on. 3.5ms after CH3

is turned on, CH4 will turn on. 3.5ms after CH4 is turned

on, CH2 will turn on.

CH1 to CH4 Power Off Sequence is :

When EN1234 goes low, CH2 will turn off first and internally

discharge output.

When FB2 < 0.1V, CH4 will turn off and also internally

discharge output via the LX4 pin. When FB4 < 0.1V, CH3

will turn off and internally discharge output via the LX3

pin. Likewise, when FB3 < 0.1V, CH1 will turn off and

discharge output. After FB1 < 0.1V, CH1 to 4 shutdown

sequence will be completed.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications of

the RT9986, the maximum junction temperature is 125

°C

and TA is the ambient temperature. The junction to ambient

thermal resistance,

θJA, is layout dependent. For WQFN-

32L 4x4 packages, the thermal resistance,

θJA, is 27.8°C/

W on a standard JEDEC 51-7 four-layer thermal test board.

The maximum power dissipation at TA = 25

°C can be

calculated by the following formula :

PD(MAX) = (125

°C − 25°C) / (27.8°C/W) = 3.59W for

WQFN-32L 4x4 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. For the RT9986 package, the derating curve

in Figure 1 allows the designer to see the effect of rising

ambient temperature on the maximum power dissipation.

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