dVńdt +VOUT
COUT 80kWøRLOAD
TPS73219
SBVS166 –JUNE 2011
www.ti.com
TRANSIENT RESPONSE Any tendency to activate the thermal protection circuit
indicates excessive power dissipation or an
The low open-loop output impedance provided by the inadequate heatsink. For reliable operation, junction
NMOS pass element in a voltage follower temperature should be limited to +125°C maximum.
configuration allows operation without an output To estimate the margin of safety in a complete design
capacitor for many applications. As with any (including heatsink), increase the ambient
regulator, the addition of a capacitor (nominal value temperature until the thermal protection is triggered;
1μF) from the OUT pin to ground will reduce use worst-case loads and signal conditions. For good
undershoot magnitude but increase its duration. In reliability, thermal protection should trigger at least
the adjustable version, the addition of a capacitor, +35°C above the maximum expected ambient
CFB, from the OUT pin to the FB pin will also improve condition of your application. This produces a
the transient response. worst-case junction temperature of +125°C at the
highest expected ambient temperature and
The TPS73219 does not have active pull-down when worst-case load.
the output is over-voltage. This allows applications
that connect higher voltage sources, such as The internal protection circuitry of the TPS73219 has
alternate power supplies, to the output. This also been designed to protect against overload conditions.
results in an output overshoot of several percent if the It was not intended to replace proper heatsinking.
load current quickly drops to zero when a capacitor is Continuously running the TPS73219 into thermal
connected to the output. The duration of overshoot shutdown will degrade device reliability.
can be reduced by adding a load resistor. The
overshoot decays at a rate determined by output POWER DISSIPATION
capacitor COUT and the internal/external load
resistance. The rate of decay is given by: The ability to remove heat from the die is different for
each package type, presenting different
(Fixed voltage version) considerations in the PCB layout. The PCB area
around the device that is free of other components
moves the heat from the device to the ambient air.
(4) Performance data for JEDEC low- and high-K boards
are shown in the Thermal Information table. Using
REVERSE CURRENT heavier copper will increase the effectiveness in
removing heat from the device. The addition of plated
The NMOS pass element of the TPS73219 provides through-holes to heat-dissipating layers will also
inherent protection against current flow from the improve the heat-sink effectiveness.
output of the regulator to the input when the gate of
the pass device is pulled low. To ensure that all Power dissipation depends on input voltage and load
charge is removed from the gate of the pass element, conditions. Power dissipation (PD) is equal to the
the EN pin must be driven low before the input product of the output current times the voltage drop
voltage is removed. If this is not done, the pass across the output pass element (VIN to VOUT):
element may be left on due to stored charge on the (5)
gate. Power dissipation can be minimized by using the
After the EN pin is driven low, no bias voltage is lowest possible input voltage necessary to assure the
needed on any pin for reverse current blocking. Note required output voltage.
that reverse current is specified as the current flowing
out of the IN pin due to voltage applied on the OUT PACKAGE MOUNTING
pin. There will be additional current flowing into the
OUT pin due to the 80kΩinternal resistor divider to Solder pad footprint recommendations for the
ground (see Figure 2). TPS73219 are presented in Application Bulletin
Solder Pad Recommendations for Surface-Mount
THERMAL PROTECTION Devices (SBFA015), available from the Texas
Instruments web site at www.ti.com.
Thermal protection disables the output when the
junction temperature rises to approximately +160°C,
allowing the device to cool. When the junction
temperature cools to approximately +140°C, the
output circuitry is again enabled. Depending on power
dissipation, thermal resistance, and ambient
temperature, the thermal protection circuit may cycle
on and off. This limits the dissipation of the regulator,
protecting it from damage due to overheating.
14 Copyright ©2011, Texas Instruments Incorporated