TPS2392

TPS2393

SLUS536C − AUGUST 2002 − REVISED AUGUST 2004

www.ti.com

20

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

R

33 mV

IMAX

where:

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

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,

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

For example, using a 20-m

Ω 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

Ω).

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

determined from equation (2).

C

11

100

R

SENSE

di

where:

For example, if the desired slew rate for the typical application shown is 1500 mA/mS, the calculated value for

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-

µA charging source.

(1)

(2)