MIC2596/2597 Micrel
MIC2596/2597 6 April 2001
Functional Description
Hot Swap Insertion
When circuit boards are inserted into systems with live power
supplies, high inrush currents can result due to the charging
of bulk capacitance that resides across the circuit board’s
supply pins. This inrush current, although transient in nature,
can be of significant amplitude. Such current spikes can
cause supply voltages to go out of regulation, resulting in
system crashes. Additionally, the high rates of di/dt may do
permanent damage to electronic components and/or assem-
blies.
The MIC2596 family of parts is designed to address these
issues by limiting the inrush current which a PC board or other
load can draw during a hot-swap event. In addition to this
inrush current control, the MIC2596/MIC2597 provide output
current limiting and supervisory functions to ensure robust
protection for both the host system and the circuit board.
V
DDA
and V
DDL
In some systems, considerable ground differentials can oc-
cur between the physical plant monitor and the actual power
control circuitry. An example of this would be in a system
spanning several racks of equipment, with a centralized CPU
watching over the hot-swap functions. In order to alleviate the
need for optical isolators or similar devices when hot-swap
parts are used in such a system, the MIC2596/MIC2597 have
separate grounds for their on-chip power-related functions
and their digital interface pins (ON1, ON2, FAULT1#,
FAULT2#). The unique architecture of these parts allows
voltage differentials of ±8 volts to exist between V
DDA
(the
Functional Diagram
VDDL
CONTROLLER
GROUND
VDDA
MIC2696
MIC2597
SYSTEM
CONTROLLER
VOUT1
VOUT2ON1/ON2
VCLAMP
Rpullup1
Rpullup2
FAULT#1/FAULT#2
Control Signals
3.3V or 5V Supply
Status Signals
VEE
Figure 1. Use of V
DDA
and V
DDL
to Mitigate Ground Noise Effects
“analog” ground) and V
DDL
(the “logic” ground), without
disturbing device performance.
In distributed systems, V
DDA
should be connected to the local
return of the power which the MIC2596/MIC2597 is control-
ling, while V
DDL
should be separately connected to the
monitor and control logic’s ground. See Figure 1. If the
capability to tolerate voltage differentials between V
DDA
and
V
DDL
is not required, the two grounds should be tied together
at the chip.
Start-Up Cycle
Referring to the Typical Applications Circuit, when a channel’s
ON pin is asserted the respective MIC2596/MIC2597 output
is enabled. To minimize inrush current transients the output
current is regulated to ensure that it does not exceed the
value programmed by the resistor R1(R2) connected to the
I
LIMIT
pin.
Circuit Breaker Function
The MIC2596 and MIC2597 act as electronic circuit breakers
to protect loads, connectors, power supplies, and other
system components against faults such as short circuits. The
circuit breaker function trips upon overcurrent, open-load or
thermal shutdown conditions. The FAULT# output is as-
serted (taken low) when the circuit breaker is tripped. The
timer capacitor C2 (C3) is normally pulled low by a small
current source. However, whenever the current limit thresh-
old is exceeded C2 is charged by a much stronger current
source. If an overcurrent condition exists for a long enough
time to allow the voltage at the C
TIMER
pin to cross the
