8
Introduction
The ISL6111, is an IC device designed to provide control
and protection of the four legacy PCI power supplies (+12V,
-12V, +5V and +3.3V) for a single PCI or PCI-X slot. Unlike
the widely used HIP1011, this device employs an active
current regulation (CR) method to provide system
protection against load faults.
Figure 1 illustrates the typical implementation of the ISL6111.
Key Feature Description and Operation
The ISL6111, 2 power MOSFETs and a few passive
components as configured in Figure 1, completes a power
control solution for the legacy supplies to a PCI slot. It
provides protection via a programmable maximum current
regulation (CR) level to the load for each supply. For the
3.3V and 5V supplies, current monitoring is provided by
sensing the voltage across external current-sense resistors,
and CR protection is provided by active voltage modulation
of external N-Channel MOSFETs. For the +12V and -12V
supplies, current monitoring and CR protection are provided
internally.
During initial power-up of the main bias supply pins (12VI),
the ENABLE input function is inhibited from turning on the
switches, this latch is held in the reset state until the bias
voltage is greater than 10V (POR rising). Additionally the
power good and fault reporting functions are also disabled at
this time and during the soft start duration.
During turn-on of the supplies onto their capacitive loads the
current limiting fail-safe is engaged, this limited current gives
a voltage ramp-up slew rate centered within the PCI specs.
As the startup is current-limited, the CRTIM timer is engaged
during the entire startup, as it should be. This eliminates the
otherwise destructive case of starting up into a dead short.
Depending on loading, the positive 3 supplies will start up
and exit current limiting in about 6ms -10ms. The -12V
supply will take much longer, as it has a fraction of the
available charging current into a potentially relatively very
large load capacitance, and the voltage has to slew to -12V.
The -12V turn-on duration can thus be several times as long
extending to ~50ms for a very capacitive (147µF) load in
conjunction with a maximum current load. In addition if the
CR level is too low then it’s possible that the load
capacitance cannot fully charge in the allowed for time, this
is the consequence of the current regulation limiting
protection.
Once turned on, any subsequent over current (OC) condition
on any output results in the affected switch (external or
internal) to be put into its linear mode of operation, and the
current is regulated to the level determined by the choice of
external CRSET resistor value. An OC condition is defined
as a current level > the programmed CR level and that
transitions through the CR level with a quick ramp, <0.5µs.
This CR level is maintained until the OC condition passes or
the CR duration expires, whichever comes first. The CR
duration is user defined by the capacitor value on the CRTIM
pin. Once in CR mode, the CRTIM pin charges the capacitor
with a 20µA current until the voltage on CRTIM rises to
~2.8V, at which time a turn-off latch is set on all 4 power FET
switches. Also at this time the open drain fault (FLTN) output
is pulled low signalling a latched off state. After a fault has
been asserted and FLTN is latched low, cycling ENABLE low
will clear the FLTN latch.
On-chip references in the ISL6111 are used to monitor the
+5V, +3.3V and +12V outputs for under voltage (UV)
conditions. Once an UV condition is present the open drain
power good (PGOOD) output will pull low to indicate this.
Customizing Circuit Performance
Setting Current Regulation (CR) Level
The ISL6111 allows for easy and simultaneous custom
programming of the CR levels of all 4 supplies by simply
changing the resistor value between CRSET, (pin 18), and
ground. The R
CRSET
value and the CRSET 100A current
source create a reference voltage that is used in each of four
comparators. The IR voltages developed across the 3.3V
and 5V sense resistors are applied to the inputs of their
respective comparators opposite this reference voltage. The
+12V and -12V currents are sensed internally with pilot
devices. Because of the internal current monitoring of the
+12V and -12V switches, their programming flexibility is
limited to R
CRSET
changes whereas the 3.3V and 5V over
current regulation levels depend on both R
CRSET
, and the
value chosen for each sense resistor.
See Table 1 to determine CR protection levels relative to
choice of R
CRSET
and R
SENSE
values.
Over current design guidelines and recommendations are as
follows:
1. For PCI applications, set R
CRSET
to 4.22k, and use
5m 1% sense resistors (see Figure 20). This R
CRSET
value provides a nominal current trip level 110% to 130%
higher than the maximum specified current, to ensure full
current range use by the PCI load. The ISL6111 will trip
off on a slow increasing current ramp approximately 10%
to 20% lower than set CR level.
2. For non PCI specified applications, the following
precautions and limitations apply:
A. Do not exceed the maximum power of the integrated
NMOS and PMOS. High power dissipation must be
coupled with effective thermal management and prudent
CR durations. The integrated PMOS has an r
DS(ON)
of
0.35. With 2.5A of steady load current on the PMOS
device the power dissipation is 2.2W. The thermal
impedance of the package is 31 degrees Celsius per
watt, resulting in a 68°C die temp rise thus limiting the
average DC current on the 12V supply to about 2.5A
maximum at +85°C ambient and imposing an upper limit
on the R
OCSET
resistor. Do not use an R
CRSET
resistor
greater than 15k.
ISL6111
