PTB78560A_15 Datasheet, PDF (12/27 Pages) Texas Instruments – 30-W, 24-V/48-V INPUT DC/DC CONVERTERS WITH AUTO-TRACK™ SEQUENCING

PTB78560A, PTB78560B
PTB78560C
SLTS249 – JUNE 2005
APPLICATION INFORMATION
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Operating Features and System Considerations for the PTB78560x DC/DC Converters
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 functional 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.
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 PEN and NEN enable controls. Only when the input
voltage is above the UVLO threshold do these inputs become functional.
Soft-Start Power Up
When the converter is first powered, the internal soft-start circuit limits how fast the output voltage can rise. The
soft-start circuit functions whenever the converter output is enabled from the PEN and NEN inputs, or when a
valid input source is first applied with the output enabled. It also functions on a recovery from a load fault,
overtemperature, or input overvoltage condition. The purpose of the soft-start feature is to limit the surge of
current drawn from the input source when the converter begins to operate. By limiting the rate at which the
output voltage rises, the magnitude of current required to charge up the load circuit capacitance is significantly
reduced.
Figure 21 shows the power-up characteristic of a PTB78560C converter. The output voltage is set to 12 V. The
soft-start circuit introduces a short time delay (typically 10-15 ms) before allowing the output to rise. The output
then progressively rises to the voltage set-point. The waveforms were recorded with a resistive load of 2.5 A.
V_I (10 V/ div)
UVLO Threshold
V_O (5 V/ div)
I_I (1 A/ div)
t_DLY
t − Time − 5 ms/div
Figure 21. Soft-Start Waveform
Overcurrent Protection
To protect against load faults, these converters incorporate output overcurrent protection. Applying a load to the
12
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