TPS2390
TPS2391
SLUS471D JUNE 2002 REVISED JANUARY 2008
5
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DETAILED PIN DESCRIPTIONS (continued)
GATE: Gate drive for external N-channel FET. When enabled, and the input supply is above the UVLO
threshold, the gate drive is enabled and the device begins charging an external capacitor connected to the
IRAMP pin. This pin voltage is used to develop the reference voltage at the non-inverting input of the internal
LCA. The inverting input is connected to the current sense node, ISENS. The LCA acts to slew the pass FET
gate to force the ISENS voltage to track the reference. The reference is internally clamped at 40 mV, so the
maximum current that can be sourced to the load is determined by the sense resistor value as IMAX
≤ 40
mV/RSENSE. Once the load voltage has ramped up to the input dc potential, and current demand drops off, the
LCA drives the GATE output to about 14 V to fully enhance the pass FET, completing the low-impedance supply
return path for the load.
IRAMP: Programming input for setting the inrush current slew rate. An external capacitor connected between
this pin and VIN establishes the load current slew rate whenever power to the load is enabled. The device
charges the external capacitor to establish the reference input to the LCA. The closed-loop control of the LCA
and pass FET acts to maintain the current sense voltage at ISENS at the reference potential. Since the sense
voltage is developed as the drop across a resistor, the charging current ramp rate is set by the voltage ramp
rate at the IRAMP pin. When the output is disabled via the EN input or due to a load fault, the capacitor is
discharged and held low to initialize for the next turn-on.
ISENS: Current sense input. An external low value resistor connected between this pin and VIN is used to feed
back current magnitude information to the TPS2390/91. There are two internal device thresholds associated
with the voltage at the ISENS pin. During ramp-up of the load’s input capacitance, or during other periods of
excessive demand, the HSPM acts to limit this voltage to 40 mV. Whenever the LCA is in current regulation
mode, the capacitor at FLTTIME is charged to activate the timer. If, when the LCA is driving to its supply rail,
a fast-acting fault such as a short-circuit, causes the ISENS voltage to exceed 100 mV (the overload threshold),
the GATE pin is pulled low rapidly, bypassing the fault timer.
RTN: Positive supply input for the TPS2390/91. For negative voltage systems, the supply pin connects directly
to the return node of the input power bus. Internal regulators step down the input voltage to generate the various
supply levels used by the TPS2390 and TPS2391.
VIN: Negative supply input and reference pin for the TPS2390/91. This pin connects directly to the input supply
negative rail. The input and output pins and all internal circuitry are referenced to this pin, so it is essentially the
GND or VSS pin of the device.