ISL78310 Datasheet, PDF (10/13 Pages) Intersil Corporation – High Performance 1A LDO

ISL78310
Current Limit Protection
The ISL78310 LDO incorporates protection against overcurrent
due to any short or overload condition applied to the output pin.
The current limit circuit performs as a constant current source
when the output current exceeds the current limit threshold
noted in the “Electrical Specifications” table on page 5. If the
short or overload condition is removed from V_OUT, then the
output returns to normal voltage mode regulation. In the event of
an overload condition on the DFN package, the LDO will begin to
cycle on and off due to the die temperature exceeding thermal
fault condition.
Functional Description
Enable Operation
The Enable turn-on threshold is typically 0.8V with a hysteresis of
80mV. The Enable pin does not have an internal pull-up or
pull-down resistor. As a result, this pin must not be left floating.
This pin must be tied to V_IN if it is not used. A pull-up resistor
(typically 1kΩ to 10kΩ) will be required for applications that use
open collector or open drain outputs to control the Enable pin.
The Enable pin may be connected directly to V_IN for applications
that are always on.
Soft-Start Operation
The soft-start circuit controls the rate at which the output voltage
rises up to regulation at power-up or LDO enable. This start-up
ramp time can be set by adding an external capacitor from the
SS pin to GND. An internal 2µA current source charges up this
C_SS and the feedback reference voltage is clamped to the
voltage across it. The startup time of the regulator output voltage
for a given C_SS value can be calculated using Equation 2.
t_RAMP
=
-^C---^S----^S---^x---⁰----^.-⁵----^V--
2μA
(EQ. 2)
The soft-start function also effectively limits the amount of in-
rush current to less than the programmed current limit during
start-up or an enable sequence, to avoid an over current fault
condition. This can be an issue for applications that require large,
external bulk capacitances on V_OUT where high levels of charging
current can be seen for a significant period of time. High in-rush
currents can cause V_IN to drop below minimum, which could
cause V_OUT to shutdown.
FIGURE 14. IN-RUSH CURRENT WITH NO C_SS,
C_OUT = 1000µF, IN-RUSH CURRENT = 1.8A
FIGURE 15. IN-RUSH CURRENT WITH C_SS = 15nF, C_OUT = 1000µF,
IN-RUSH CURRENT = 0.5A
FIGURE 16. IN-RUSH CURRENT WITH C_SS = 33nF, C_OUT = 1000µF, IN-RUSH CURRENT = 0.2A
10
FN7810.0
February 17, 2011