RT9718 Просмотр технического описания (PDF) - Richtek Technology
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RT9718 Datasheet PDF : 10 Pages
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RT9718
The OCP threshold can be set by the resistor connected
between the ILIM pin and GND. The OCP threshold can be
calculated by the following equation :
IOCP
=
25000
RILIM
Selecting Capacitors
To get the better performance of the RT9718, it is very
important to select peripherally appropriate capacitors.
These capacitors determine some parameters such as
input inrush current and input over shoot voltage. Generally,
it is necessary to increase the input capacitance CIN for
reducing the input over shoot voltage. However, this will
increase the inrush current of input. There are two
scenarios that can cause the input over shoot voltage.
The first one is that when the AC adapter is hot-plugged
and the second one is when the RT9718 has a step-down
change. The cable between the AC adapter output and the
handheld system input has a parasitic inductance causing
the input over shoot voltage. Generally, the input over shoot
voltage range is 1.5 to 2 times of the input voltage. It is
recommended to use 1μF capacitance for CIN and COUT
and the rated voltage should be higher than at 1.5 to 2
times of the operation voltage.
Thermal Considerations
Thermal protection limits power dissipation in RT9718.
When the operation junction temperature exceeds 140°C,
the OTP circuit starts the thermal shutdown function and
turns the pass element off. The pass elements turn on
again after the junction temperature cools by 20°C.
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 θJA is
the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT9718, the maximum operating junction temperature is
125°C. The junction to ambient thermal resistance θJA for
WDFN-8L 2x2 package is 165°C/W on the standard JEDEC
DS9718-02 May 2011
51-3 single-layer thermal test board. The maximum power
dissipation at TA =25°C can be calculated by following
formula :
PD(MAX) = (125°C − 25°C) / (165°C/W) = 0.606W for WDFN-
8L 2x2 packages
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal resistance
θJA. For RT9718 packages, the Figure 1 of derating curves
allows the designer to see the effect of rising ambient
temperature on the maximum power allowed.
0.8
Single Layers PCB
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 1. Derating Curves for RT9718 Packages
Layout Consideration
For best performance of the RT9718 series, the following
guidelines must be strictly followed.
` Input and output capacitors should be placed close to
the IC and connected to ground plane to reduce noise
coupling.
` The GND and exposed pad should be connected to a
strong ground plane for heat sink.
` Keep the main current traces as possible as short and
wide.
The input and output capacitors should
be placed as close as possible to the IC.
The main current
trace should be
as short and wide
as possible.
CIN
VIN 1
GND 2
NC 3
WRN 4
8 VOUT
COUT
VOUT
7 ILIM
6 VB
RVB
9 5 EN
RILIM
GND
The exposed pad and GND should be connected
to a strong ground plane for heat sinking and
noise prevention.
Figure 2. PCB Layout Guide
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