Datasheet

£
SENSE
IMAX
34 mV
R
I
RAMP
SENSE
(max)
11.3
C =
di
100 R
dt
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TPS2394
www.ti.com SLVSAA9 –AUGUST 2010
APPLICATION INFORMATION
Setting the Sense Resistor Value
Due to the current-limiting action of the internal LCA, the maximum allowable load current for an implementation
is easily programmed by selecting the appropriate sense resistor value. The LCA acts to limit the sense voltage
V(SENSE-SOURCE) to its internal reference. Once the voltage at the RAMP pin exceeds approximately 4 V, this
limit is the clamp voltage, V
REF_K
. Therefore; a maximum sense resistor value can be determined from
Equation 1.
(1)
Where:
R
SENSE
is the sensing resistor value
I
IMAX
is the minimum desired current limit
When setting the sense resistor value, it is important to consider two factors, the minimum current limit that may
be imposed by the TPS2394, and the maximum load under normal operation of the module. For the first factor,
the specification minimum clamp value is used, as seen in Equation 1. Second factor is to ensure the peak
operating load current is less than I
IMAX
. One example of this is a switching converter which draws higher input
current, for a given power output, when the output is at the low end of its voltage range. To avoid current limit
operation under normal loading, some margin should be designed in between this maximum anticipated load and
the minimum current limit level, or I
IMAX
> I
LOAD(max)
, for Equation 1.
For example, using a 10-mΩ sense resistor for a nominal 2-A load application provides a minimum of 1.4 A of
overhead for load variance/margin. Typical bulk capacitor charging current during turn-on is 4.2 A
(42 mV/10 mΩ).
Setting the Inrush Slew Rate
The TPS2394 device enables user-programming of the maximum current slew rate during load start-up events. A
capacitor tied to the RAMP pin (C
RAMP
in the typical application diagram) controls the di/dt rate. Once the sense
resistor value has been established, a value for C
RAMP
, in microfarads, can be determined from Equation 2.
(2)
Where:
R
SENSE
is the sense resistor value in Ω
(di/dt)
(max)
is the desired maximum slew rate in A/s
For example, if the desired slew rate for the typical application shown is 1500 mA/ms, the calculated value for
C
RAMP
is about 7500 pF. Selecting the next larger standard value of 8200 pF provides some margin for capacitor
and sense resistor tolerances.
Setting the Fault Timing Capacitor
The fault timeout period is established by the value of the capacitor connected to the FLTTIM pin, C
FLT
. The
timeout period permits riding out spurious current glitches and surges that may occur during operation of the
system, and prevents indefinite sourcing into faulted loads. However, to ensure smooth voltage ramping under all
conditions of load capacitance and input supply potential, the minimum timeout should be set to accommodate
these system variables. To do this, a rough estimate of the maximum voltage ramp time for a completely
discharged plug-in card provides a good basis for setting the minimum timer delay. This section presents a quick
procedure for calculating the timing capacitance requirement. However, for proper operation of the TPS2394,
there is an absolute minimum value of 0.01-µF for C
FLT
. This minimum requirement overrides any smaller results
of Equation 7 and Equation 8.
Copyright © 2010, Texas Instruments Incorporated 11
Product Folder Link(s): TPS2394