![](http://datasheet.mmic.net.cn/370000/TPS2393DBTRG4_datasheet_16739734/TPS2393DBTRG4_20.png)
SLUS536C AUGUST 2002 REVISED AUGUST 2004
www.ti.com
20
APPLICATION INFORMATION
setting the sense resistor value
Due to the currentlimiting 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
I(ISENS)
to its internal reference. Once the voltage at the IRAMP 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).
33 mV
IMAX
R
SENSE
where:
R
SENSE
is the resistor value
IMAX is the desired current limit
When setting the sense resistor value, it is important to consider two factors, the minimum current that may
be imposed by the TPS2392 or TPS2393, 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-m
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 ia 2 A
(40 mV/20 m
).
setting the inrush slew rate
The TPS2392/93 devices enable user-programming of the maximum current slew rate during load start-up
events. A capacitor tied to the IRAMP pin (C1 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
di
dt
MAX
where:
R
SENSE
is the sense resistor value in
(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
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 in the Detailed Description section of this datasheet, the TPS2392 and TPS2393 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-
μ
A charging source.
(1)
(2)