RT9971 datasheet(19/22 Pages) RICHTEK

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DS9971-01 April 2011

www.richtek.com

RT9971

The current flows through WLED can be set by the following

equation :

I (mA) = [250mV/R(

Ω)] x Duty (%)

R : Current sense resistor from CFB7 to GND.

Duty: PWM dimming by EN7 pin. Dimming frequency

range is from 30kHz to 100kHz.

Hold EN7 low for more than 64

µs will turn off CH7.

RTC_Reset

The RT9971 provides an accurate voltage detector for

RTC_LDO voltage detection. It is used to detect whether

RTC_LDO output voltage is ready or not. Its power pin is

RTC_PWR and output pin is RTC_R. The output pin is an

open drain N-MOSFET and the sink capability is above

4mA. Once the RTC_PWR pin reaches 1.6V, it will count

for about 55ms, then the RTC_R will go high.

RTC_LDO

The RT9971 provides a LDO for real time clock. The LDO

function has features of low quiescent current (5

µA) and

high output voltage accuracy since this LDO is running all

the time, even when the system is shutdown. In addition,

LDO share

“OK” and “VOUT1” pin with SW1 and the

function is decided by

“CN” pin. Following table is used

to select LDO or SW1.

Table 3. RTC_LDO and SW1 Setting

Function

RTC_LDO

Logic-High

SW1

Logic-Low

Power On/Off Sequence

The Power On Sequence is :

While EN134 goes high, CH1 will be turned on to wait for

the completion of CH1's soft-start. After that, CH3 will be

turned on to wait for the completion of CH3's soft-start.

And then, CH4 will be turned on to wait for the completion

of CH4's soft-start. Then,SW1 will be turn on and CH2 is

allowed to be turn on by EN2 at any time. Finally, SW1

soft-start will be completed.

The Power-Off Sequence is :

At first, while EN134 goes low, (SW1 is shutdown and

internally pull low, CH2 must be turned off by EN2) SW1

and CH2 (Note A) will be shutdown. After that, CH4 will be

turned off and internally pulled low to wait for the completion

of CH4's shutdown. And then, CH3 will be turned off and

internally pulled low to wait for CH3's shutdown completion.

Then, CH1 will be turned off and internally pulled low

(Note B) to wait for CH1's shutdown completion. Finally,

the whole IC will be shutdown (if EN2, EN5, EN6 and EN7

already go low).

Note A : If CH2 is configured as a step -up, then the CH2

will not be internally pulled low and the completion of

shutdown will not be checked.

Note B : CH1 is configured as a step -up, so the CH1 will

not be internally pulled low and the completion of shutdown

will not be checked.

Table 4. Power On/Off Sequence

Power On

Sequence

CH1 -> CH3 -> CH4 -> (SW1 and CH2)

Power Off

Sequence

(SW1 and CH2) -> CH4 -> CH3 -> CH1

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. The maximum

power dissipation depends on the thermal resistance of

IC package, PCB layout, the rate of surroundings airflow

and temperature difference between junction to ambient.

The maximum power dissipation can be calculated by

following formula :

PD(MAX) = (TJ(MAX)

− TA ) / θJA

Where TJ(MAX) is the maximum operation junction

temperature, TA is the ambient temperature and the

θJAis

the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT9971, The maximum junction temperature is 125

°C.

The junction to ambient thermal resistance

θJA is layout

dependent. For WQFN-40L 5x5 packages, the thermal

resistance

θJA is 36°C/W on the standard JEDEC 51-7

four layers thermal test board. The maximum power

dissipation at TA = 25

°C can be calculated by following

formula :

PD(MAX = (125

°C −25°C) / (36°C/W) = 2.778W for

WQFN-40L 5x5 packages

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