1. The art of hot-swapping: from “band aid” to
effective hot-swap solution (Part 2 of 2)
Hamed Sanogo, Applications Engineering Manager
Maxim Integrated Products, Inc
“Your future system reliability issues are related to today’s choice of hot-swap circuits”
(Editor's Note: Part 1 of this article examined the overall hot-swap problem, as well as
some marginally effective solutions for it.)
“True” Inrush Peak Current Control
The best way the engineer can impact the long-term reliability of the circuit is to design a
hot-swap circuit implementation that does three key things:
1. Limits the inrush current (to the inserted card)
2. Protects against over current conditions and load transients, and
3. Maintains the long term reliability of the entire system by decreasing the number
of failure points.
There are portfolios of hot-swap controller ICs in the market that have reached higher
levels of integration (e.g.; no sense resistor needed with some controller ICs). There are
also many ICs that make implementing a hot-swap circuit an easy and very effective
solution.
For example, designers can now find the following functions supported in a single device:
under-voltage and over-voltage protection, active current limiting with constant current
source during overload, electronic circuit breaker with faulty loads disconnect before
supply dropout, reverse current protection with an additional drive FET to provide “ideal
diode”, multiple voltage sequencing, digital voltage and current monitoring, and automatic
retry after load fault, to just name a few. The features of a typical hot-swap controller are
discussed in an application note offered by Maxim [3].
Hot-swap circuit designers should seriously consider these highly integrated hot-swap
controllers. A few analog semiconductor suppliers have introduced a wide variety of hot-
swap solutions to meet a large number of system requirements. These include new
generation hot swap ICs with a wide variety of both analog and digital features, such as
the ability to continually monitor the supply current long after the card has been seated
and powered up.
This monitoring feature ensures that the card is continuously protected against short-ircuit
and over-current conditions during normal operation of the embedded card. Continuous
monitoring also allows malfunctioning cards to quickly be identified and removed from

2. the system before they can completely fail and impact the “zero downtime” aspect that
telecom carriers view as a critical system statistic.
Hot-swap solutions from Maxim Integrated Products, Analog Devices, and Linear
Technology come with digital fault and statistical data (or flight) recording features. The
term “digital hot swap”IC is also used to refer to these ADCs used to enabled hot-swap
solutions.
Table 1 shows a side-by-side comparison of some of the key specifications of hot-swap
chips from Maxim, Analog Devices, and Linear Technology. The MAX5967, although not
listed in the table, is pin- and function-compatible with the LTC4215.
LTC4215 ADM1175 MAX5961 MAX5970
ADC Resolution 8-bit 12-bit 10-bit 10-bit
Conversion Rate 10Hz Not Specified 10kHz 10kHz
Automatic or Polled? Auto Polled Auto Auto
History "Depth" 1 sample 1 sample 50 samples 50 samples
INL (LSB) 0.2 LSB, 0.5 LSB Not Specified 0.5 LSB 0.5 LSB
FS Error (volt,
current)
±5.5 LSB, ±5.0
LSB
±60.0 LSB, ±100.0 LSB ±10 LSB, ±30.0 LSB ±10 LSB, ±30.0 LSB
Interface I2
C/SMBus I2
C I2
C/SMBus I2
C/SMBus
H-S Voltage (Min,
Max)
2.9V, 15V 3.15V, 13.2V 0V, 16V 0V, 16V
GATE Pull-Up
Current
20μA 12μA 5μA 5μA
GATE Pull-Down
Current, Normal
1mA 2mA 500mA 500mA
Slow-Trip Circuit
Breaker Threshold
25mV 85mV
12.5mV, 25mV, 50mV
(& 8-bit programmable)
12.5mV, 25mV, 50mV
(& 8-bit programmable)
Fast-Trip Circuit
Breaker Threshold
N/A 115mV
125%, 150%, 175%, 200%
of programmed slow-trip
125%, 150%, 175%, 200%
of programmed slow-trip
Load UV analog 2 ea. 10-bit programmable 2 ea. 10-bit programmable
Load OV 2 ea. 10-bit programmable 2 ea. 10-bit programmable
Table 1: Digital Hot-Swap ICs Side-by-side Comparison (Courtesy of Dwight Larson)
The internal ADCs on the chips primarily give the hot-swap controllers the extended
ability to monitor and report the power-supply states, in addition to the other vital signs at
the instant the fault occurs. For example, the MAX5961 provides the added advantage to
be able to store several milliseconds of past voltage and current measurements (Figure 5).
This data can be used to ease system debugging and failure analysis afterwards.

3. Figure 5: Advanced system-monitoring features on the MAX5961 include buffers to hold
several milliseconds of past voltage and current measurements
The addition of an ADC to these latest-generation hot-swap controller ICs has also created
opportunities for OEMs to become more creative with their products. Companies and add
more “value-added” features to their systems to enhance their advanced board
management control to perform one or more of the following functions:
Information gathering: A designer can use today’s current system vital data to define his
next generation systems in terms of efficiencies as well as how best to optimize the
efficiencies curve for the system.
Constant monitoring: During normal operation of these high-availability systems, there
could be a desire to log certain “vital statistics” of the power levels of the card through a
constant monitoring of the power temperature levels. This can be used later for predicting
certain specific faults.
Power budget: This can also be used to ensure that no embedded card is using more than
its share of the total power budget (by reading past and current fault conditions). This will
facilitate early identification of abnormal operating conditions and to help mitigate or
eliminate its effects on the rest of the system.
The I2
C interface of the controller can be used by card’s microprocessor to collect the
system’s vital statistics. This interface is also the means by which the controller is
configured to behave, latch off or continuous restart, and how a problem card is identified
early-on by the system’s management firmware. This is essentially the chassis’s warning
display to the service technician similar to the service-engine-soon light seen on a
dashboard of a car.
Conclusions
An engineer would generally find tracing an inrush caused malfunction of a PC board
back to any insertion event a very challenging task. The so-called ”band aid” hot-swap
solutions will create more of a negative impact on your system’s long-term performance
than the engineer can ever imagine. There are, though, a variety of highly-integrated hot-
swap solutions that will ensure that a hot-plug event in a system or product does not cause

4. data transmission errors or resetting of the cards already in the system. These solutions
will help keep your system’s long-term reliability intact. Don’t forget, it is all about
meeting and exceeding the 5-NINEs.
References
[1] Application Note 1785, “Flexible Hot-Swap Current Limiter Allows Thermal
Protection”, Maxim Integrated Products, available at http://www.maxim-
ic.com/appnotes.cfm/an_pk/1785
[2] A complete product brief for the MAX5961 can be found at http://www.maxim-
ic.com/quick_view2.cfm/qv_pk/5853
[3] Application Note 2736, “Understanding, Using, and Selecting Hot-Swap
Controllers”, Maxim Integrated Products, available at http://pdfserv.maxim-
ic.com/en/an/AN2736.pdf
Acknowledgment
The author would like to thank Dennis Wommack and Dwight Larsen for their assistance
in capturing the scope images and creating the Hot-Swap IC table, respectively.
About the Author
Hamed M. Sanogo is an applications engineering manager with Maxim Integrated
Products. He graduated from the University of Alabama at Birmingham (UAB), and then
earned an MSEE at the University of Michigan (Dearborn) and an MBA in technology
management at the University of Dallas Graduate School of Management. Before joining
Maxim, Hamed was a senior staff design engineer for Motorola, working on hot-swap
enabled embedded telecomm cards for cellular base-transceiver stations (BTS) in
Motorola’s UMTS, CDMA, and WiMax systems. Hamed can be reached at
hamed.sanogo@maxim-ic.com.