TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
Output Overload Protection
In the event of an overcurrent during soft start on either output (such as starting into an output short),
pulse-by-pulse current limiting and PWM frequency division are in effect for that output until the internal soft start
timer ends. At the end of the soft start time, a UV condition is declared and a fault is declared. During this fault
condition, both PWM outputs are disabled and the small pulldown MOSFETs (from SWx to GND) are turned ON.
This process ensures that both outputs discharge to GND in the event that overcurrent is on one output while the
other is not loaded. The converter then enters a hiccup mode timeout before attempting to restart. "Frequency
Division" means if an overcurrent pulse is detected, six clock cycles are skipped before a next PWM pulse is
initiated, effectively dividing the operating frequency by six and preventing excessive current build up in the
inductor.
In the event of an overcurrent on either output after the output reaches regulation, pulse-by-pulse current limit is
in effect for that output. In addition, an output undervoltage (UV) comparator monitors the FBx voltage (that
follows the output voltage) to declare a fault if the output drops below 85% of regulation. During this fault
condition, both PWM outputs are disabled and the small pulldown MOSFETs (from SWx to GND) are turned ON.
This design ensures that both outputs discharge to GND, in the event that overcurrent is on one output while the
other is not loaded. The converter then enters a hiccup mode timeout before attempting to restart.
The overcurrent threshold for Output 1 is set nominally at 4.5 A. The overcurrent level of Output 2 is determined
by the state of the ILIM2 pin. The ILIM setting of Output 2 is not latched in place and may be changed during
operation of the converter.
Table 2. Current Limit Threshold Adjustment for
Output 2
ILIM2 Connection
OCP Threshold for Output 2
4.5 A nominal setting
BP
(floating)
GND
3.0 A nominal setting
1.5 A nominal setting
DESIGN HINT
The OCP threshold refers to the peak current in the internal switch. Be sure to add
one-half of the peak inductor ripple current to the dc load current in determining how
close the actual operating point is to the OCP threshold
Operating Near Maximum Duty Cycle
If the TPS5438x operates at maximum duty cycle, and if the input voltage is insufficient to support the output
voltage (at full load or during a load current transient), then there is a possibility that the output voltage will fall
from regulation and trip the output UV comparator. If this should occur, the TPS5438x protection circuitry will
declare a fault and enter a shut down-and-restart cycle.
DESIGN HINT
Ensure that under ALL conditions of line and load regulation, there is sufficient duty
cycle to maintain output voltage regulation.
To calculate the operating duty cycle, use Equation 15.
VOUT + VDIODE
d =
V
+ VDIODE
IN
(15)
where
•
VDIODE is the voltage drop of the rectifier diode
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