RT6214A/B
PD(MAX) = (TJ(MAX) TA) / JA
Soft-Start
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and JA is the
junction-to-ambient thermal resistance.
(For RT6214A/BHRGJ8F)
The RT6214A/B provides adjustable soft-start function.
When the EN pin becomes high, the SS charge current
(ISS) begins charging the capacitor which is connected
from the SS pin to GND (CSS).The soft-start function is
used to prevent large inrush current while converter is
being powered-up. The soft-start timing can be
programmed by the external capacitor CSS between SS
and GND. An internal current source ISS (6A) charges
an external capacitor to build a soft-start ramp voltage.
The VFB voltage will track the internal ramp voltage
during soft-start interval. The typical soft-start time is
calculated as follows :
For continuous operation, the maximum operating
junction temperature indicated under Recommended
Operating
Conditions
is
125C.
The
junction-to-ambient thermal resistance,JA, is highly
package dependent. For a TSOT-23-6 (FC) package,
the thermal resistance, JA, is 60C/W on a standard
JEDEC 51-7 high effective-thermal-conductivity
four-layer test board. For a TSOT-23-8 (FC) package,
the thermal resistance, JA, is 60C/W on a standard
JEDEC 51-7 high effective-thermal-conductivity
four-layer test board. The maximum power dissipation
at TA = 25C can be calculated as below :
Soft-Start time tSS = CSS x 0.7 / 11A + CSS x VFB / 6A
Power-Good Output
PD(MAX) = (125C 25C) / (60C/W) = 1.667W for a
TSOT-23-6 (FC) package.
(For RT6214A/BHRGJ8F)
PD(MAX) = (125C 25C) / (60C/W) = 1.667W for a
The PGOOD pin is an open-drain power-good
indication output and is to be connected to an external
voltage source through a pull-up resistor. The
power-good function is activated after soft-start is
TSOT-23-8 (FC) package.
The maximum power dissipation depends on the
operating ambient temperature for the fixed TJ(MAX) and
the thermal resistance, JA. The derating curves in
Figure 8 allows the designer to see the effect of rising
ambient temperature on the maximum power
dissipation.
finished and is controlled by the feedback signal VFB
.
During soft-start, PGOOD is actively held low and only
allowed to transition high after soft-start is over. If VFB
rises above a power-good threshold (VTH_PGLH
)
(typically 90% of the target value), the PGOOD pin will
be in high impedance and VPGOOD will be held high
after a certain delay elapsed. Once being started-up, if
any internal protection is triggered, PGOOD will be
pulled low to GND.
2.0
Four-Layer PCB
1.8
1.6
1.4
TSOT-23-6 (FC)
1.2
TSOT-23-8 (FC)
1.0
Thermal Considerations
0.8
0.6
0.4
0.2
0.0
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding
airflow, and the difference between the junction and
ambient temperatures. The maximum power
dissipation can be calculated using the following
formula :
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum Power
Dissipation
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20
DS6214A/B-04 August 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
