PTB78520W Datasheet, PDF (6/15 Pages) Texas Instruments – 20-A, 18–60-V Input Auto-Track Compatible Isolated DC/DC Converter

Application Notes
PTB78520W
Not Recommended for New Designs
Operating Features and System Considerations
for the PTB78520W DC/DC Converter
Over-Current Protection
To protect against load faults these converters incorporate
output over-current protection. Applying a load to the
output that exceeds the converter’s over-current threshold
(see applicable specification) will cause the output voltage
to momentarily fold back, and then shut down. Following
shutdown the module will periodically attempt to auto-
matically recover by initiating a soft-start power-up.
This is often described as a “hiccup” mode of operation,
whereby the module continues in the cycle of successive
shutdown and power up until the load fault is removed.
Once the fault is removed, the converter automatically
recovers and returns to normal operation.
Output Over-Voltage Protection
The converter continually monitors for an output over-
voltage (OV) condition, directly across the ‘+V_O Bus’
output. The OV threshold automatically tracks the out-
put voltage setpoint to a level that is 25% higher than
that set by the external R_SET voltage adjust resistor. If
the output voltage exceeds this threshold, the converter
is immediately shut down and remains in a latched-off
state. To resume normal operation the converter must be
actively reset. This can only be done by momentarily
removing the input power to the converter. For failsafe
operation and redundancy, the OV protection uses cir-
cuitry that is independent of the converter’s internal
feedback loop.
Differential Output Voltage Sense
A differential remote sense allows a converter’s regula-
tion circuitry to compensate for limited amounts of IR
drop, that may be incurred between the converter and
load, in either the positive or return PCB traces. Con-
necting the (+)Sense and (–)Sense pins to the respective
positive and ground reference of the load terminals will
improve the load regulation of the converter’s output
voltage at that connection point. The (–)Sense pin should
always be connected to the ‘V_O COM’. The (+)Sense pin
may be connected to either the ‘+V_O Bus’ or ‘+V_O Seq’
outputs.
When the (+)Sense pin is connected to the ‘V_O Seq’
output, the voltage at ‘V_o Bus’ voltage will regulate
slightly higher. Depending on the load conditions on the
‘V_O Seq’ output, the voltage at ‘V_O Bus’ may be up to
100 mV higher than the converter’s set-point voltage.
In addition, the Smart-Sense feature (incorporated into
the PTB78520 converter) will only engage sense compen-
sation to the ‘V_O Seq’ output when that output voltage is
close to the set-point. During other conditions, such as
power-up and power-down sequencing events, the sense
circuit automatically defaults to sensing the ‘V_O Bus’
voltage, internal to the converter.
Leaving the (+)Sense and (–)Sense pins open will not
damage the converter or load circuitry. The converter
includes default circuitry that keeps the output voltage in
regulation. If the remote sense feature is not used, the
(–)Sense pin should always be connected to ‘Vo COM’.
Note: The remote sense feature is not designed to compensate
for the forward drop of non-linear or frequency dependent
components that may be placed in series with the converter
output. Examples include OR-ing diodes, filter inductors,
ferrite beads, and fuses. When these components are enclosed
by the sense pin connections they are effectively placed inside
the regulation control loop, which can adversely affect the
stability of the converter.
Over-Temperature Protection
Over-temperature protection is provided by an internal
temperature sensor, which monitors the temperature of
the converter’s PCB (close to pin 1). If the PCB tem-
perature exceeds a nominal 115 °C, the converter will
shut down. The converter will then automatically restart
when the sensed temperature drops back to approximately
105 °C. When operated outside its recommended ther-
mal derating envelope (see data sheet SOA curves), the
converter will typcially cycle on and off at intervals from
a few seconds to one or two minutes. This is to ensure
that the internal components are not permanently dam-
aged from excessive thermal stress.
Under-Voltage Lockout
The Under-Voltage Lock-Out (UVLO) is designed to
prevent the operation of the converter until the input
voltage is close to the minimum operating voltage. The
converter is held off when the input voltage is below the
UVLO threshold, and turns on when the input voltage
rises above the threshold. This prevents high start-up
current during normal power-up of the converter, and
minimizes the current drain from the input source dur-
ing low input voltage conditions. The converter will
meet full specifications when the minimum specified
input voltage is reached. The UVLO circuitry also over-
rides the operation of the Vo Enable control. Only when
the input voltage is above the UVLO threshold will the
Vo Enable control be functional.
Primary-Secondary Isolation
These converters incorporate electrical isolation between
the input terminals (primary) and the output terminals
(secondary). All converters are tested to a withstand volt-
age of 1500 VDC. This complies with UL/cUL 60950
and EN 60950 and the requirements for operational isola-
tion. It allows the converter to be configured for either a
positive or negative input voltage source. The data sheet
‘Pin Descriptions’ section provides guidance as to the
correct reference that must be used for the external con-
trol signals.
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