3
Operation
3.1
Operation
3.2
Fixed Output TPS798xx
4
Thermal Guidelines
4.1
Thermal Considerations
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Operation
Other output voltages can be configured by changing the feedback resistors on the board.
The pins of J6 connect directly to the feedback network of the TPS79801. The feedback network is high
impedance and sensitive to noise or resistance value changes. Do not touch the pins of J6 while the
device is powered because the impedance of a human is enough to alter the output voltage set point. The
output voltage may increase or decrease if J6 is touched which may damage any load connected to the
EVM.
This section provides information about the operation of the TPS79801EVM.
Connect the positive input power supply to J1. Connect the input power return (ground) to J3. The
TPS79801EVM has an absolute maximum input voltage of 82 V. The recommended maximum operating
voltage is 80 V. The actual highest input voltage may be less than 80 V due to thermal conditions. See the
Thermal Considerations section of this manual to determine if the highest input voltage.
Connect the desired load between J2 (positive lead) and J4 (negative or return lead). Configure jumper J6
for the desired output voltage. The function of J6 is described in the Setup section of this manual.
The TPS79801EVM can be used to evaluate fixed output voltage versions of the TPS798xx. The board
layout and part footprints are the same between the adjustable version and fixed output voltage versions
of the TPS798. To evaluate a fixed output voltage version, the integrated circuit (IC) on the board needs to
be changed to the desired fixed output voltage TPS798xx. R1 must be shorted or removed from the EVM
board and replaced with a 0-
Ω
resistor. C5 must be removed and left open. Jumpers must not be installed
on J6 for fixed output voltage version parts.
This section provides guidelines for the thermal management of the TPS79801DCQEM-434 board.
Thermal management is a key component of design of any power converter and is especially important
when the power dissipation in the LDO is high. To better help you design the TPS79801 family into your
application, use
to approximate the maximum power dissipation at a particular ambient
temperature:
T
J
= T
A
+ P
d
× θ
JA
(1)
where T
J
is the junction temperature, T
A
is the ambient temperature, P
d
is the power dissipation in the IC,
and
θ
JA
is the thermal resistance from junction to ambient. All temperatures are in degrees Celsius.
The measured thermal resistance from junction to ambient for the TPS79801EVM has a typically value of
42
°
C/W. The recommended maximum operating junction temperature specified in the data sheet for the
TPS79801 family is 125
°
C. With these two pieces of information, the maximum power dissipation can be
found by using
Example Calculation:
For example, what is the maximum input voltage that can be applied to a TPS79801 with the output
voltage configured to 3.3 V if the ambient temperature is 85
°
C and the full 50 mA of load current is
required?
Given: T
J
= 125
°
C, T
A
= 85
°
C,
θ
JA
= 42
°
C/W
(2)
Using
, substitute the preceding given values, and find that the maximum power dissipation for
the part is P
d
= 0.952 W.
125
°
C = 85
°
C + P
d
(42
°
C/W)
(3)
SLUU346 – June 2009
TPS79801DCQEVM-434
3
