RT9986 RICHTEK [Richtek Technology Corporation], RT9986 Datasheet - Page 20

• Manufacturer Part Number: RT9986
• Description: 7-CH DC/DC Converter for DSC
• Manufacturer: RICHTEK [Richtek Technology Corporation]

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 A) 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 F 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:
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20
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 :
P
where T
the ambient temperature, and
thermal resistance.
For recommended operating condition specifications of
the RT9986, the maximum junction temperature is 125 C
and T
thermal resistance,
32L 4x4 packages, the thermal resistance,
W on a standard JEDEC 51-7 four-layer thermal test board.
The maximum power dissipation at T
calculated by the following formula :
P
WQFN-32L 4x4 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T
resistance,
in Figure 1 allows the designer to see the effect of rising
ambient temperature on the maximum power dissipation.
D(MAX)
D(MAX)
A
is the ambient temperature. The junction to ambient
= (T
J(MAX)
= (125 C
JA
J(MAX)
. For the RT9986 package, the derating curve
is the maximum junction temperature, T
T
A
JA
25 C) / (27.8 C/W) = 3.59W for
) /
, is layout dependent. For WQFN-
JA
JA
is the junction to ambient
DS9986-00 May 2011
J(MAX)
A
= 25 C can be
and thermal
JA
, is 27.8 C/
A
is