PTB48520W Datasheet, PDF (7/18 Pages) Texas Instruments – 25-A, 48-V INPUT ISOLATED DC/DC CONVERTER WITH AUTO-TRACK™ SEQUENCING

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PTB48520W
SLTS233A – NOVEMBER 2004 – REVISED APRIL 2005
APPLICATION INFORMATION (continued)
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 set-point voltage. In addition, the Smart-Sense feature (incorporated into the converter)
only engages sense compensation 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 does 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 V_O COM.
Note: The remote sense feature is not designed to compensate for the forward drop of nonlinear 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.
Overtemperature Protection
Overtemperature protection is provided by an internal temperature sensor, which monitors the temperature of the
converter PCB (close to pin 1). If the PCB temperature exceeds a nominal 115°C, the converter shuts down. The
converter then automatically restarts when the sensed temperature falls to approximately 105°C. When operated
outside its recommended thermal derating envelope (see data sheet derating curves), the converter typcially
cycles 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 damaged from excessive thermal stress.
Undervoltage Lockout
The undervoltage lockout (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 during low input
voltage conditions. The converter meets full specifications when the minimum specified input voltage is reached.
The UVLO circuitry also overrides the operation of the V_O Enable control. Only when the input voltage is above
the UVLO threshold is the V_O Enable control 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 voltage of 1500 VDC. This complies with UL/cUL 60950 and
EN 60950 and the requirements for operational isolation. It allows the converter to be configured for either a
positive or negative input voltage source. The data sheet Terminal Functions table provides guidance as to the
correct reference that must be used for the external control signals.
Output Voltage Adjustment
The V_O Adjust control sets the output voltages to a value higher than 1.8 V. For output voltages other than 1.8 V
a single external resistor, R_(set), must be connected directly between V_O Adjust (pin 7) and (–)Sense (pin 6) pins.
A 0.05-W rated resistor can be used. The tolerance should be 1%, with a temperature stability of 100 ppm/°C (or
better). Place the resistor close to the converter and connect it directly between pins 7 and 6 using dedicated
PCB traces (see typical application). Table 1 gives the preferred value of the external resistor for a number of
standard voltages, along with the actual output voltage that this resistance value provides.
For other output voltages the value of the required adjust resistor may be calculated using Equation 1.
Rset + 6.49 kW
1.225 V
Vset * 1.805 V
* 4.42 kW
(1)
7