MAX5915/MAX5915A/MAX5916/MAX5916A
Detailed Description
The MAX5915/MAX5915A/MAX5916/MAX5916A are cir-
cuit-breaker ICs for hot-swap applications where a PCI
card is inserted into a slot that is connected to a live
backplane. Normally, when a card is plugged into a live
backplane, the card’s discharged capacitors provide a
low-impedance path that can momentarily cause the
main power supply to collapse. Both devices provide
startup current limiting and undervoltage/overcurrent
monitoring of two separate PCI card slots. Current limit-
ing and short-circuit protection are achieved using
external n-channel MOSFETs on the +3.3V and +5V
supply lines and internal MOSFETs on the ±12V and
+3.3V auxiliary supply lines.
External sense resistors monitor the output currents of
the +3.3V and +5V supplies. These external sense
resistors adjust the overcurrent trip threshold. PCI stan-
dards dictate maximum values for the supply power
and total power drawn from the backplane. The maxi-
mum power that any one PCI board can draw is 25W.
Table 1 lists PCI standard maximum voltage, current,
and power for each supply.
Table 1 illustrates that both the +5V and +3.3V supplies
can draw up to 25W. Total combination of output power
should be limited to 25W based on PCI standard.
Startup Mode
The +12V input powers the internal circuitry of these
devices. The main supply outputs (3.3VO_, 5VO_,
+12VO_, and -12VO_) can become active only after
both of the following events have occurred:
•V
+12VIN
is above its undervoltage lockout (UVLO)
threshold.
• ON_ is driven high.
Figure 1 displays typical startup waveforms. The main
supplies can be enabled without using the auxiliary
supply; however, PGOOD_ remains in a low state if the
auxiliary supply is not used.
The auxiliary supply (3.3VAUXO_) is available after both
of these events have occurred:
•V
3.3VAUXIN
is above its UVLO threshold.
• AUXON_ is driven high.
Normal Operation +3.3V, +5V,
±12V Outputs
The internal circuitry for these devices monitors the out-
put voltage on all channels except the -12V supply. All
outputs are monitored for overcurrent. An undervoltage
condition occurs when any supply’s output voltage falls
below the set undervoltage level. An overcurrent fault
occurs when a monitored output current reaches the
set overcurrent threshold. Each supply has its own
overcurrent and undervoltage thresholds. If any of the
monitored voltages fall below their respective under-
voltage level, or if any of the monitored output currents
reach their overcurrent threshold, for a time period,
t
DELAY
, the controller disables the channel with the fault
condition (see the Fault Management section).
External sense resistors monitor current through the
external MOSFETs of the +3.3V and +5V outputs, while
the current for the ±12V supplies are internally monitored.
A fault condition on one of the main outputs causes all
the channel’s main outputs to shut down after t
DELAY
and then either latch off (MAX5915/MAX5915A) or auto-
matically restart after t
RESTART
(MAX5916/MAX5916A).
A fault on any of the channel’s main outputs does not
affect the channel’s auxiliary outputs.
Normal Operation +3.3V Auxiliary Output
Auxiliary output voltage and current are monitored inter-
nally. The +3.3V auxiliary output is independent of the
main outputs but the main outputs are dependent on
the auxiliary outputs. Fault conditions on the main out-
puts do not affect the auxiliary. A fault on the auxiliary
supply causes the controller to disable all of the affected
Dual PCI 2.2 Hot-Swap Controllers
12 ______________________________________________________________________________________
Table 1. PCI Standard Maximum Values
