Datasheet

TPS40055-EP
SGLS310D –JULY 2005–REVISED FEBRUARY 2012 www.ti.com
There is a direct correlation between t
START
and the input current required during start-up. The faster t
START
, the
higher the input current required during start-up. This relationship is describe in more detail in the section titled,
Programming the Current Limit which follows. The soft-start capacitance, C
SS
, is described in Equation 14.
(14)
For applications in which the V
IN
supply ramps up slowly, (typically between 50 ms and 100 ms) it may be
necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO
tripping. The soft-start time should be longer than the time that the V
IN
supply transitions between 6 V and 7 V.
PROGRAMMING CURRENT LIMIT
The TPS40055 uses a two-tier approach for overcurrent protection. The first tier is a pulse-by-pulse protection
scheme. Current limit is implemented on the high-side MOSFET by sensing the voltage drop across the
MOSFET when the gate is driven high. The MOSFET voltage is compared to the voltage dropped across a
resistor connected from VIN pin to the ILIM pin when driven by a constant current sink. If the voltage drop across
the MOSFET exceeds the voltage drop across the ILIM resistor, the switching pulse is immediately terminated.
The MOSFET remains off until the next switching cycle is initiated.
The second tier consists of a fault counter. The fault counter is incremented on an overcurrent pulse and
decremented on a clock cycle without an overcurrent pulse. When the counter reaches seven, a restart is issued
and seven soft-start cycles are initiated. Both the upper and lower MOSFETs are turned off during this period.
The counter is decremented on each soft-start cycle. When the counter is decremented to zero, the PWM is re-
enabled. If the fault has been removed the output starts up normally. If the output is still present, the counter
counts seven overcurrent pulses and re-enters the second-tier fault mode. See Figure 11 for typical overcurrent
protection waveforms.
The minimum current limit setpoint (I
LIM
) depends on t
START
, C
O
, V
O
, and the load current at turn-on (I
L
).
(15)
Figure 11. Typical Current Limit Protection Waveforms
The current limit programming resistor (R
ILIM
) is calculated using Equation 16. Care must be taken in choosing
the values used for V
OS
and I
SINK
in the equation. In order to assure the output current at the overcurrent level,
the minimum value of I
SINK
and the maximum value of V
OS
must be used.
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