Datasheet

P =(V V )xI-
D IN OUTOUT
R =
qJA
(+125 C T )° -
A
P
D
TPS749xx
SBVS082G –JUNE 2007–REVISED NOVEMBER 2010 www.ti.com
condition of PG pin sink current is up to 1mA, so the LAYOUT RECOMMENDATIONS AND POWER
pull-up resistor for PG should be in the range of 10kΩ DISSIPATION
to 1MΩ. PG is only provided on the QFN package. If
An optimal layout can greatly improve transient
output voltage monitoring is not needed, the PG pin
performance, PSRR, and noise. To minimize the
can be left floating.
voltage droop 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 TPS749xx 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 4A and maintain regulation. The
R
1
in Figure 25 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
TPS749xx is designed to protect against overload
minimizes the voltage droop 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 TPS749xx above the rated current degrades
sizing of the thermal plane that is connected to the
device reliability.
tab or 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 QFN (RGW) 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 will not
temperature should be limited to +125°C maximum.
overheat. On the DDPAK (KTW) package, the
To estimate the margin of safety in a complete design
primary conduction path for heat is through the tab to
(including heatsink), increase the ambient
the PCB. That tab should be connected to ground.
temperature until thermal protection is triggered; use
The maximum junction-to-ambient thermal resistance
worst-case loads and signal conditions. For good
depends on the maximum ambient temperature,
reliability, thermal protection should trigger at least
maximum device junction temperature, and power
+40°C above the maximum expected ambient
dissipation of the device and can be calculated using
condition of the application. This condition produces a
Equation 5:
worst-case junction temperature of +125°C at the
highest expected ambient temperature and
worst-case load.
(5)
The internal protection circuitry of the TPS749xx is
Knowing the maximum R
qJA
, the minimum amount of
designed to protect against overload conditions. It is
PCB copper area needed for appropriate heatsinking
not intended to replace proper heatsinking.
can be estimated using Figure 29.
Continuously running the TPS749xx into thermal
shutdown degrades device reliability.
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