RT9706 データシートの表示（PDF） - Richtek Technology
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RT9706 Datasheet PDF : 14 Pages
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RT9706
Once this current limit threshold is exceeded the device
enters constant current mode until the thermal shutdown
occurs or the fault is removed.
Thermal Shutdown
Thermal shutdown is employed to protect the device from
damage if the die temperature exceeds approximately
130°C. If enabled, the switch automatically restarts when
the die temperature falls 20°C. The output and FLG signal
will continue to cycle on and off until the device is disabled
or the fault is removed.
Power Dissipation and Thermal Consideration
The device “S” junction temperature depends on several
factors such as the load, PCB layout, ambient temperature
and package type. The output pin of RT9706 can deliver a
current of up to 500mA, respectively over the full operating
junction temperature range. However, the maximum output
current must be derated at higher ambient temperature to
ensure the junction temperature does not exceed 100°C.
With all possible conditions, the junction temperature must
be within the range specified under operating conditions.
Power dissipation can be calculated based on the output
current and the RDS(ON) of switch as below.
PD = RDS(ON) x (IOUT)2
Although the devices are rated for 500mA of output current,
but the application may limit the amount of output current
based on the total power dissipation and the ambient
temperature. The final operating junction temperature for
any set of conditions can be estimated by the following
thermal equation :
PD(MAX) = ( TJ(MAX) − TA ) / θJA
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
RT9706, where TJ(MAX) is the maximum junction
temperature of the die (125°C) and TA is the maximum
ambient temperature. The junction to ambient thermal
resistance θJA is layout dependent. For SOT-23-5
packages, the thermal resistance θJA is 250°C/W on the
standard JEDEC 51-3 single-layer thermal test board.
DS9706-02 March 2007
The maximum power dissipation at TA = 25°C can be
calculated by following formula :
PD(MAX) = (125°C − 25°C) / 250°C/W = 0.4 W for
SOT-23-5 packages
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. For RT9706 packages, the Figure 2 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
0.6
0.5
SOT-23-5
0.4
Single Layer PCB
0.3
0.2
0.1
0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curves for RT9706 Package
Universal Serial Bus (USB) & Power Distribution
The goal of USB is to be enabled device from different
vendors to interoperate in an open architecture. USB
features include ease of use for the end user, a wide range
of workloads and applications, robustness, synergy with
the PC industry, and low-cost implement- ation. Benefits
include self-identifying peripherals, dynamically attachable
and reconfigurable peripherals, multiple connections
(support for concurrent operation of many devices), support
for as many as 127 physical devices, and compatibility
with PC Plug-and-Play architecture.
The Universal Serial Bus connects USB devices with a
USB host: each USB system has one USB host. USB
devices are classified either as hubs, which provide
additional attachment points to the USB, or as functions,
which provide capabilities to the system (for example, a
digital joystick). Hub devices are then classified as either
Bus-Power Hubs or Self-Powered Hubs.
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