PTB78560A_15 Datasheet, PDF (13/27 Pages) Texas Instruments – 30-W, 24-V/48-V INPUT DC/DC CONVERTERS WITH AUTO-TRACK™ SEQUENCING

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PTB78560A, PTB78560B
PTB78560C
SLTS249 – JUNE 2005
APPLICATION INFORMATION (continued)
output that exceeds the converter overcurrent threshold (see applicable specification) causes the output voltage
to momentarily fold back, and then shut down. Following shutdown, the module periodically attempts to
automatically 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.
Input Overvoltage Protection
The converter protects itself against input voltage surges and transients of up to 100 V. This is above the
maximum continuous operating input voltage of 60 V. In order to protect itself, the converter output is disabled at
some voltage above 60 V. This is to ensure that the converter internal components are not exposed to voltages
above their stress ratings. The converter output remains off for some of the period that the input voltage is
above the maximum continuous rating. Once the overvoltage event has passed, the output from the converter
automatically restarts by executing a soft-start power up.
Differential Output Voltage Sense
A differential remote sense allows a converter regulation 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. Connecting the
+Sense and –Sense pins to the respective positive and ground reference of the load terminals improves the load
regulation of the converter 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 regulates 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 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 a
power-up sequencing event, 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 circuit. The converter
includes default circuitry that keeps the output voltage in regulation. However, if the remote sense feature is not
used, the –Sense pin should still be connected to V_OCOM.
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.
Output Voltage Adjustment
An external resistor is required to set the nominal output voltage(s) of the converter to a voltage higher than its
minimum value. The resistor, R_SET, must be connected directly between the V_O Adjust (pin 8) and –Sense (pin
7) terminals. 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 using dedicated PCB traces (see
Figure 22). Table 1 gives the nearest standard value of external resistor for the common voltages within each
model's adjust range. The actual output voltage that the resistor value provides is also provided.
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