Datasheet

P =(V V ) I- ´
D IN OUT OUT
R =
qJA
(+125 C T )° -
A
P
D
TPS74701
SBVS099F –NOVEMBER 2007–REVISED NOVEMBER 2010 www.ti.com
device is disabled. The recommended operating LAYOUT RECOMMENDATIONS AND POWER
condition of the PG pin sink current is up to 1mA, so DISSIPATION
the pull-up resistor for PG should be in the range of
An optimal layout can greatly improve transient
10kΩ to 1MΩ. If output voltage monitoring is not
performance, PSRR, and noise. To minimize the
needed, the PG pin can be left floating.
voltage drop on the input of the device during load
transients, the capacitance on IN and BIAS should be
INTERNAL CURRENT LIMIT
connected as close as possible to the device. This
capacitance also minimizes the effects of parasitic
The TPS74701 features a factory-trimmed, accurate
inductance and resistance of the input source and
current limit that is flat over temperature and supply
can therefore improve stability. To achieve optimal
voltage. The current limit allows the device to supply
transient performance and accuracy, the top side of
surges of up to 1A and maintain regulation. The
R
1
in Figure 24 should be connected as close as
current limit responds in about 10ms to reduce the
possible to the load. If BIAS is connected to IN, it is
current during a short-circuit fault.
recommended to connect BIAS as close to the sense
The internal current limit protection circuitry of the
point of the input supply as possible. This connection
TPS74701 is designed to protect against overload
minimizes the voltage drop on BIAS during transient
conditions. It is not intended to allow operation above
conditions and can improve the turn-on response.
the rated current of the device. Continuously running
Knowing the device power dissipation and proper
the TPS74701 above the rated current degrades
sizing of the thermal plane that is connected to the
device reliability.
thermal pad is critical to avoiding thermal shutdown
and ensuring reliable operation. Power dissipation of
THERMAL PROTECTION
the device depends on input voltage and load
Thermal protection disables the output when the
conditions and can be calculated using Equation 4:
junction temperature rises to approximately +160°C,
(4)
allowing the device to cool. When the junction
temperature cools to approximately +140°C, the
Power dissipation can be minimized and greater
output circuitry is enabled. Depending on power
efficiency can be achieved by using the lowest
dissipation, thermal resistance, and ambient
possible input voltage necessary to achieve the
temperature the thermal protection circuit may cycle
required output voltage regulation.
on and off. This cycling limits the dissipation of the
On the SON (DRC) package, the primary conduction
regulator, protecting it from damage as a result of
path for heat is through the exposed pad to the
overheating.
printed circuit board (PCB). The pad can be
Activation of the thermal protection circuit indicates
connected to ground or be left floating; however, it
excessive power dissipation or inadequate
should be attached to an appropriate amount of
heatsinking. For reliable operation, junction
copper PCB area to ensure the device does not
temperature should be limited to +125°C maximum.
overheat. The maximum junction-to-ambient thermal
To estimate the margin of safety in a complete design
resistance depends on the maximum ambient
(including heatsink), increase the ambient
temperature, maximum device junction temperature,
temperature until thermal protection is triggered; use
and power dissipation of the device and can be
worst-case loads and signal conditions. For good
calculated using Equation 5:
reliability, thermal protection should trigger at least
+40°C above the maximum expected ambient
condition of the application. This condition produces a
(5)
worst-case junction temperature of +125°C at the
Knowing the maximum R
qJA
, the minimum amount of
highest expected ambient temperature and
PCB copper area needed for appropriate heatsinking
worst-case load.
can be estimated using Figure 28.
The internal protection circuitry of the TPS74701 is
designed to protect against overload conditions. It is
not intended to replace proper heatsinking.
Continuously running the TPS74701 into thermal
shutdown degrades device reliability.
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