5

FN6855.0

February 25, 2009

Description and Operation

The ISL6115A is targeted for +12V single power supply

distribution control for generic hot swap switching

applications.

This ICs features a highly accurate programmable current

regulation (CR) level with programmable time delay to

latch-off, and programmable soft-start turn-on ramp all set

with a minimum of external passive components. It also

includes severe OC protection that immediately shuts down

the MOSFET switch should a rapid load current transient such

as with a dead short cause the CR Vth to exceed the

programmed level by 150mV. Additionally, it has an UV

indicator and an OC latch indicator. The functionality of the

PGOOD feature is enabled once the IC is biased, monitoring

and reporting any UV condition on the ISEN pin.

Upon initial power-up, the IC can either isolate the voltage

supply from the load by holding the external N-Channel

MOSFET switch off or apply the supply rail voltage directly to

the load for true hot swap capability. The PWRON pin must

be pulled low for the device to isolate the power supply from

the load by holding the external N-Channel MOSFET off.

With the PWRON pin held high or floating the IC will be in

true hot swap mode. In both cases the IC turns on in a

soft-start mode protecting the supply rail from sudden inrush

current.

At turn-on, the external gate capacitor of the N-Channel

MOSFET is charged with a 11µA current source resulting in

a programmable ramp (soft-start turn-on). The internal

ISL6115A charge pump supplies the gate drive for the 12V

passes through the external current sense resistor. When the

voltage across the sense resistor exceeds the user

programmed CR voltage threshold value, (see Table 1 for

enters its current regulation mode. At this time, the time-out

capacitor, on CTIM pin is charged with a 20µA current source

and the controller enters the current limit time to latch-off

period. The length of the current limit time to latch-off duration

is set by the value of a single external capacitor (see Table 2)

time-out to latch-off duration placed from the CTIM pin (pin 6)

to ground. The programmed current level is held until either

the OC event passes or the time-out period expires. If the

former is the case then the N-Channel MOSFET is fully

charges to ~1.8V signaling that the time-out period has

expired, an internal latch is set whereby the FET gate is

quickly pulled to 0V turning off the N-Channel MOSFET

switch, isolating the faulty load.

This IC responds to a severe overcurrent load (defined as a

voltage across the sense resistor >150mV over the OC Vth set

point) by immediately driving the N-Channel MOSFET gate to

0V in about 10µs. The gate voltage is then slowly ramped up

turning on the N-Channel MOSFET to the programmed current

regulation level; this is the start of the time-out period.

Upon a UV condition, the PGOOD signal will pull low when

connected through a resistor to the logic or VDD supply. This

pin is a UV fault indicator. For an OC latch-off indication,

monitor CTIM, pin 6. This pin will rise rapidly from 1.8V to

VDD once the time-out period expires.

See Figures 2 through 13 for graphs and waveforms related

to text.

The IC is reset after an OC latch-off condition by a low level

on the PWRON pin and is turned on by the PWRON pin

being driven high.

Application Considerations

Design applications where the CR Vth is set extremely low

(25mV or less), there is a two-fold risk to consider.

• There is the susceptibility to noise influencing the absolute

CR Vth value. This can be addressed with a 100pF

• Due to common mode limitations of the overcurrent

comparator, the voltage on the ISET pin must be 20mV

this does not happen, the IC may incorrectly report

overcurrent fault at start-up when there is no fault. Circuits

with high load capacitance and initially low load current

are susceptible to this type of unexpected behavior.

Do not signal nor pull-up the PWRON input to > 5V.

Exceeding 6V on this pin will cause the internal charge pump

to malfunction.

During the soft-start and the time-out delay duration with the

MOSFET is reduced driving the MOSFET switch into a (linear

NOMINAL CR VTH

10k

Ω

200mV

4.99k

Ω

100mV

2.5k

Ω

50mV

1.25k

Ω

25mV

NOMINAL CURRENT LIMITED PERIOD

0.022µF

2ms

0.047µF

4.4ms

0.1µF

9.3ms

ISL6115A