Datasheet

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SLUS471D − JUNE 2002 − REVISED JANUARY 2008
13
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APPLICATION INFORMATION
When setting the sense resistor value, it is important to consider two factors, the minimum current that may be
imposed by the TPS2390 or TPS2391, 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). This method accounts for
the tolerance in the sourced current limit below the typical level expected (40 mV/R
SENSE
). (The clamp
measurement includes LCA input offset voltage; therefore, this offset does not have to be factored into the
current limit again.) Second, if the load current varies over a range of values under normal operating conditions,
then the maximum load level must be allowed for by the value of R
SENSE
. One example of this is when the load
is a switching converter, or brick, which draws higher input current, for a given power output, when the
distribution bus is at the low end of its operating range, with decreasing draw at higher supply voltages. 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 IMAX > I
LOAD(max)
, for equation (1).
For example, using a 20-mΩ sense resistor for a nominal 1-A load application provides a minimum of 650 mA
of overhead for load variance/margin. Typical bulk capacitor charging current during turn-on is 2 A
(40 mV/20 mΩ).
setting the inrush slew rate
The TPS2390 and TPS2391 devices enable user-programming of the maximum current slew rate during load
start-up events. A capacitor tied to the IRAMP pin (C2 in the typical application diagram) controls the di/dt rate.
Once the sense resistor value has been established, a value for ramp capacitor C
IRAMP
, in microfarads, can be
determined from equation (2).
C
IRAMP
+
11
100 R
SENSE
ǒ
di
dt
Ǔ
MAX
where:
• R
SENSE
is in ohms, and
• (di/dt)
MAX
is the desired maximum slew rate, in amps/second.
For example, if the desired slew rate for the typical application shown is 1500 mA/ms, the calculated value for
C
IRAMP
is about 3700 pF. Selecting the next larger standard value of 3900 pF (as shown in the diagram) provides
some margin for capacitor and sense resistor tolerances.
As described earlier in this section, the TPS2390 and TPS2391 initiate ramp capacitor charging, and
consequently, load current di/dt at a reduced rate. This reduced rate applies until the voltage on the IRAMP pin
is about 0.5 V. The maximum di/dt rate, as set by equation (2), is effective once the device has switched to the
10-µA charging source.
(2)