December 2003

M9999-122303

MIC2588/MIC2594

Micrel

MIC2588/MIC2594

Single-Channel, Negative High-Voltage Hot

Swap Power Controllers

General Description

The MIC2588 and the MIC2594 are single-channel, nega-
tive-voltage hot swap controllers designed to address the
need for safe insertion and removal of circuit boards into "live"
high-voltage system backplanes, while using very few exter-
nal components. The MIC2588 and the MIC2594 are each
available in an 8-pin SOIC package and work in conjunction
with an external N-Channel MOSFET for which the gate drive
is controlled to provide inrush current limiting and output
voltage slew-rate control. Overcurrent fault protection is also
provided for which the overcurrent threshold is program-
mable. During an output overload condition, a constant-
current regulation loop is engaged to ensure that the system
power supply maintains regulation. If a fault condition ex-
ceeds a built-in 400

s nuisance-trip delay, the MIC2588 and

the MIC2594 will latch the circuit breaker's output off and will
remain in the off state until reset by cycling either the UV/OFF
pin or the power to the IC. A master Power-Good signal is
provided to indicate that the output voltage of the soft-start
circuit is within its valid output range. This signal can be used
to enable one or more DC-DC converter modules.

All support documentation can be found on Micrel's web
site at www.micrel.com.

Typical Application

VDD

VEE

SENSE

GATE

DRAIN

/PWRGD

MIC2588-2BM

FDBK

GATE

FDBK

12.4k

Input Overvoltage = 71.2V
Input Undervoltage = 36.5V
(See "Functional Description" for more detail)

11.8k

698k

0.1

DC-DC Converter

100

/ON/OFF

IN+

+5V

OUT

5V
RETURN

48V

INPUT

(Long Pin)

48V

RETURN

(Short Pin)

48V

RETURN

(Long Pin)

OUT+

OUT

SENSE

Features

· MIC2588:

Pin-for-pin functional equivalent to the
LT1640/LT1640A/LT4250

· Provides safe insertion and removal from live 48V

(nominal) backplanes

· Operates from 19V to 80V
· Electronic circuit breaker function
· Built-in 400

s "nuisance-trip" delay (t

FLT

)

· Regulated maximum output current into faults
· Programmable inrush current limiting
· Fast response to short circuit conditions (< 1

· Programmable undervoltage and overvoltage lockouts

(MIC2588-xBM)

· Programmable UVLO hysteresis (MIC2594-xBM)
· Fault reporting:

Active-HIGH (-1BM) and Active-LOW
(-2BM) Power-Good signal output

Applications

· Central office switching
· 48V power distribution
· Distributed power systems

Micrel, Inc. · 1849 Fortune Drive · San Jose, CA 95131 · USA · tel + 1 (408) 944-0800 · fax + 1 (408) 944-0970 · http://www.micrel.com

December 2003

M9999-122303

MIC2588/MIC2594

Micrel

Pin Description

Pin Number

Pin Name

Pin Function

PWRGD

Power-Good Output: Open-drain. Asserted when the voltage on the DRAIN

/PWRGD

pin (V

DRAIN

) is within V

PGTH

of VEE, indicating that the output voltage is

within proper specifications.

MIC25XX-1

MIC2588-1 and MIC2594-1: PWRGD will be high-impedance when

PWRGD

DRAIN

is less than V

PGTH

, and will pull-down to V

DRAIN

when V

DRAIN

Active-High

greater than V

PGTH

. Asserted State: Open-Drain.

MIC25XX-2

MIC2588-2 and MIC2594-2: /PWRGD will pull-down to V

DRAIN

when

/PWRGD

DRAIN

is less than V

PGTH,

and will be high impedance when V

DRAIN

Active-Low

greater than V

PGTH

. Asserted State: Active-Low.

MIC2588: Overvoltage Threshold Input. When the voltage at the OV pin is

Threshold

greater than the V

OVH

threshold, the GATE pin is immediately pulled low by an

internal 100

A current pull-down.

MIC2594: Turn-On Threshold. At initial system power-up or after the device

Turn-On Threshold

has been shut off by the OFF pin, the voltage on the ON pin must exceed
the V

ONH

threshold in order for the MIC2594 to be enabled.

MIC2588: Undervoltage Threshold Input. When the voltage at the UV pin is

Threshold

less than the V

UVL

threshold, the GATE pin is immediately pulled low by an

internal 100

A current pull-down. The UV pin is also used to cycle the device

off and on to reset the circuit breaker. Taken together, the OV and UV pins
form a window comparator which defines the limits of V

within which the

load may safely be powered.

OFF

MIC2594: Turn-Off Threshold. When the voltage at the OFF pin is less than

Turn-Off Threshold

the V

OFFL

threshold, the GATE pin is immediately pulled low by an internal

100

A current pull-down. The OFF pin is also used to cycle the device off and

on to reset the circuit breaker. Taken together, the ON and OFF pins provide
programmable hysteresis for the turn-on command voltage.

VEE

Negative Supply Voltage Input.

SENSE

Circuit Breaker Sense Input: The current-limit threshold is set by connecting
a resistor between this pin and V

. When the current-limit threshold of

IR = 50mV is exceeded for an internal delay t

FLT

(400

s), the circuit breaker

is tripped and the GATE pin is immediately pulled low. Toggling UV or OV
will reset the circuit breaker. To disable the circuit breaker, externally
connect SENSE and VEE together.

GATE

Gate Drive Output: Connect to the gate of an external N-Channel MOSFET.

DRAIN

Drain Sense Input: Connect to the drain of an external N-Channel MOSFET.

VDD

Positive Supply Input.

MIC2588/MIC2594

Micrel

M9999-122303

December 2003

Absolute Maximum Ratings

(1)

(All voltages are referred to V

)

Supply Voltage (V

) ......................... 0.3V to 100V

DRAIN, PWRGD pins ................................... 0.3V to 100V
GATE pin ..................................................... 0.3V to 12.5V
SENSE, OV, UV, ON, OFF pins ....................... 0.3V to 6V
ESD Ratings

(3)

Human Body Model ................................................... 2kV

Soldering

Vapor Phase .......................... (60 sec.) +220

C +5

Infrared ................................... (15 sec.) +235

C +5

Operating Ratings

(2)

Supply Voltage (V

) .......................... +19V to +80V

Ambient Temperature Range

(

) ............... 40

C to 85

Junction Temperature

(

) ........................................ 125

Package Thermal Resistance

SOIC

(

) ......................................................... 152

C/W

DC Electrical Characteristics

(4)

= 48V, V

= 0V, T

= 25

C, unless otherwise noted. Bold indicates specifications apply over the full operating temperature range of

C to +85

Symbol

Parameter

Condition

Min

Typ

Max

Units

Supply Voltage

Supply Current

TRIP

Circuit Breaker Trip Voltage

TRIP

= V

SENSE

GATEON

GATE Pin Pull-up Current

GATE

= V

to 8V

19V

)

80V

GATEOFF

GATE Pin Sink Current

SENSE

) = 100mV

100

230

GATE

= 2V

GATE

GATE Drive Voltage, (V

GATE

)

15V

)

80V

SENSE

SENSE Pin Current

SENSE

= 50mV

0.2

UVH

UV Pin High Threshold Voltage

Low-to-High Transition

1.213

1.243

1.272

UVL

UV Pin Low Threshold Voltage

High-to-Low Transition

1.198

1.223

1.247

UVHYS

UV Pin Hysteresis

OVH

OV Pin High Threshold Voltage

Low-to-High Transition

1.198

1.223

1.247

OVL

OV Pin Low Threshold Voltage

High-to-Low Transition

1.165

1.203

1.232

OVHYS

OV Pin Hysteresis

ONH

ANSI ON Pin High Threshold

Low-to-High Transition

1.198

1.223

1.247

Voltage

OFFH

ANSI OFF Pin Low Threshold

High-to-Low Transition

1.198

1.223

1.247

Voltage

CNTRL

Input Bias Current

= 1.25V

0.5

(OV, UV, ON, OFF Pins)

PGTH

Power-Good Threshold

High-to-Low Transition

1.1

1.26

1.40

DRAIN

)

OLPG

PWRGD Output Voltage

OLPG

DRAIN

(relative to voltage at the DRAIN pin) 0mA

PG(LOW)

1mA

MIC25XX-1

DRAIN

) < V

PGTH

0.25

0.8

MIC25XX-2

DRAIN

) > V

PGTH

0.25

0.8

LKG(PG)

PWRGD Output Leakage Current

PWRGD

= V

= 80V

Notes:

1. Exceeding the

"Absolute Maximum Ratings"

may damage the devices.

2. The devices are not guaranteed to function outside the specified operating conditions.

3. Devices are ESD sensitive. Handling precautions recommended. Human body model: 1.5k

in series with 100pF. Machine model: 200pF, no series

resistance.

4. Specification for packaged product only.

December 2003

M9999-122303

MIC2588/MIC2594

Micrel

AC Electrical Characteristics

(5)

Symbol

Parameter

Condition

Min

Typ

Max

Units

FLT

Built-in Overcurrent Nuisance Trip

Note 6

400

Time Delay (Figure 1)

OCSENSE

Overcurrent Sense to GATE Low

SENSE

= 100mV

3.5

(Figure 2)

OVPHL

OV to GATE Low (Figure 3)

Note 6

OVPLH

OV to GATE High (Figure 3)

Note 6

UVPHL

UV to GATE Low (Figure 4)

Note 6

UVPLH

UV to GATE High (Figure 4)

Note 6

PGL(1)

DRAIN High to PWRGD Output Low

PULLUP

= 100k

, C

LOAD

on PWRGD = 50pF

(6)

(-1 Version parts only)

PGL(2)

DRAIN Low to /PWRGD Output Low

PULLUP

= 100k

, C

LOAD

on /PWRGD = 50pF

(6)

(-2 Version parts only)

PGH(1)

DRAIN Low to PWRGD Output High

PULLUP

= 100k

, C

LOAD

on PWRGD = 50pF

(6)

(-1 Version parts only)

PGH(2)

DRAIN High to /PWRGD Output High

PULLUP

= 100k

, C

LOAD

on /PWRGD = 50pF

(6)

(-2 Version parts only)

Notes:

5. Specification for packaged product only.

6. Not 100% production tested. Parameters are guaranteed by design.

Test Circuit

[Section under construction]

MIC2588/MIC2594

Micrel

M9999-122303

December 2003

Typical Characteristics

[Section under construction]

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

XXX (X)

MICx xxx

vs. xxx

MIC2588/MIC2594

Micrel

M9999-122303

December 2003

Functional Description

Hot Swap Insertion

When circuit boards are inserted into systems carrying live
supply voltages ("hot swapped"), high inrush currents often
result due to the charging of bulk capacitance that resides
across the circuit board's supply pins. These current spikes
can cause the system's supply voltages to temporarily go out
of regulation, causing data loss or system lock-up. In more
extreme cases, the transients occurring during a hot swap
event may cause permanent damage to connectors or on-
board components.

The MIC2588 and the MIC2594 are designed to address
these issues by limiting the magnitude of the transient current
during hot swap events. This is achieved by controlling the
rate at which power is applied to the circuit board (di/dt and
dv/dt management). In addition, to inrush current control, the
MIC2588 and the MIC2594 incorporate input voltage super-
visory functions and current limiting, thereby providing robust
protection for both the system and the circuit board.

Start-Up Cycle

When the input voltage to the IC is between the overvoltage
and undervoltage thresholds (MIC2588) or is greater than
V

(MIC2594), a start cycle is initiated. At this time, the

GATE pin of the IC applies a constant charging current
(I

GATEON

) to the gate of the external MOSFET (M1). C

FDBK

creates a Miller integrator out of the MOSFET circuit, which
limits the slew-rate of the voltage at the drain of M1. The drain
voltage rate-of-change (dv/dt) of M1 is:

dv M1

DRAIN

GATE()

FDBK

GATEON

FDBK

(

)

where I

GATE(+)

= Gate Charging Current = I

GATEON

;

GATE()

GATE(+)

, due to the extremely high

transconductance values of power MOSFETs; and

dv M1

GATE()

FDBK

DRAIN

(

)

Relating the above to the maximum transient current into the
load capacitance to be charged upon hot swap or power-up
involves a simple extension of the same formula:

dv M1

| I

CHARGE

LOAD

DRAIN

CHARGE

LOAD

GATEON

FDBK

CHARGE

LOAD

GATEON

FDBK

(

)

Transposing:

| I

FDBK

LOAD

GATEON

CHARGE

(1)

GATE

and R

FDBK

prevent turn-on and hot swap current

surges which would otherwise be caused by (C

FDBK

D-G(M1)

) coupling turn-on transients from the drain to the

gate of M1. An appropriate value for C

GATE

may be deter-

mined using the formula for a capacitive voltage divider:

Maximum voltage on C

GATE

at turn-on must be less than

THRESHOLD

of M1:

1. For a standard 10V enhancement N-Channel

MOSFET, V

THRESHOLD

is about 4.25V.

2. Choose 3.5V as a safe maximum voltage to safely

avoid turn-on transients.

G-S(M1)

GATE

+ (C

FDBK

+ C

D-G(M1)

)]

= [(V

(min))

FDBK

+ C

D-G(M1)

)]

G-S(M1)

GATE

= [(V

(min)) V

G-S(M1)

]

FDBK

+ C

D-G(M1)

)

V (min) V

GATE

FDBK

D G(Q1)

G-S(M1)

(

)

(

)

(2)

While the value for R

FDBK

is not critical, it should be chosen

to allow a maximum of several milliamperes to flow in the
gate-drain circuit of M1 during turn-on. While the final value
for R

FDBK

is determined empirically, initial values between

FDBK

= 15k

to 27k

for systems with a maximum value of

75V for (V

(min)) are appropriate.

Resistor R4, in series with the MOSFETs gate, minimizes the
potential for parasitic high frequency oscillations from occur-
ring in M1. While the exact value of R4 is not critical,
commonly used values for R4 range from 10

to 33

For example, let us assume a hot swap controller is required
to maintain the inrush current into a 150

F load capacitance

at 1.7A maximum, and that this circuit may operate from
supply voltages as high as (V

) = 75V. The MOSFET

to be used with the MIC2588/94 is an IRF540NS 100V
D

PAK device which has a typical (C

D-G

) of 250pF.

Calculating a value for C

FBDK

using Equation 1 yields:

150 F

45 A

1.7A

3.97nF

FDBK

µ ×

µ =

Good engineering practice suggests the use of the worst-
case parameter values for I

GATEON

from the

"DC Electrical

Characteristics"

section:

150 F

60 A

1.7A

5.3nF

FDBK

µ × µ =

where the nearest standard 5% value is 5.6nF. Substituting
5.6nF into Equation 2 from above yields:

5.6nF

250pF

75V 3.5V

3.5V

0.12 F

GATE

(

)

(

)

Finally, choosing R4 = 10

and R

FDBK

= 20k

will yield a

suitable, initial design for prototyping.

December 2003

M9999-122303

MIC2588/MIC2594

Micrel

Power-Good (PWRGD or /PWRGD) Output

For the MIC2588-1 and the MIC2594-1, the Power-Good
output signal (PWRGD) will be high impedance when V

DRAIN

drops below V

PGTH

, and will pull down to V

DRAIN

when

DRAIN

is above V

PGTH

. For the MIC2588-2 and the

MIC2594-2, /PWRGD will pull down to the potential of the
V

DRAIN

pin when V

DRAIN

drops below V

PGTH

, and will be high

impedance when V

DRAIN

is above V

PGTH

. Hence, the -1 parts

have an active-high PWRGD signal and the -2 parts have an
active-low /PWRGD output. Either PWRGD or /PWRGD may
be used as an enable signal for one or more subsequent
DC/DC converter modules or for other system uses as
desired. When used as an enable signal, the time necessary
for the PWRGD (or /PWRGD) signal to pull-up (when in high
impedance state) will depend upon the load (RC) that is
present on this output.

Circuit Breaker Function

The MIC2588 and the MIC2594 employ an electronic circuit
breaker that protects the MOSFET and other system compo-
nents against faults such as short circuits. The current limit
threshold is set via an external resistor, R

SENSE

, connected

between the V

and SENSE pins. An internal 400

s timer

limits the length of time (t

FLT

) for which the circuit can draw

current in excess of its programmed threshold before the
circuit breaker is tripped. This short delay prevents nuisance
tripping of the circuit breaker due to system transients while
providing rapid protection against large-scale transient faults.
Whenever the voltage across R

SENSE

exceeds 50mV, two

things happen:

1. A constant-current regulation loop is engaged de-

signed to hold the voltage across R

SENSE

equal to

50mV. This protects both the load and the MIC2588
circuit from excessively high currents. This loop will
engage in less than 1

s from the time at which the

overvoltage condition on R

SENSE

occurs.

2. The internal 400

s timer is started. If the 400

timeout period is exceeded, the circuit breaker trips
and the GATE pin is immediately pulled low by an
internal current pull-down. This operation turns off
the MOSFET quickly and disconnects the input from
the load.

Current Sensing

As mentioned before, the MIC2588 and the MIC2594 employ
an external low-value resistor in series with the source of the
external MOSFET to measure the current flowing into the
load. The V

connection to the IC from the negative supply

is also one input to the part's internal current sensing circuits
and the SENSE input is the other input.

Sense Resistor Selection

The sense resistor is nominally valued at:

(nom)

(typ)

(nom)

SENSE

TRIP

HOT_SWAP

where V

TRIP

(typ) is the nominal circuit breaker threshold

voltage (= 50mV) and I

HOT_SWAP

(nom) is the nominal hot

swap load current level to trip the internal circuit breaker in the
application.

To accommodate worst-case tolerances in the sense resistor
(for a

1% initial tolerance, allow

3% tolerance for variations

over time and temperature) and circuit breaker threshold
voltages, a slightly more detailed calculation must be used to
determine the minimum and maximum hot swap load
currents.

As the MIC2588/94's minimum current limit threshold voltage
is 40mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:

(min)

40mV

1.03 R

(nom)

38.8mV

(nom)

HOT_SWAP

SENSE

(

)

Keep in mind that the minimum hot swap load current should
be greater than the application circuit's upper steady-state
load current boundary. Once the lower value of R

SENSE

has

been calculated, it is good practice to check the maximum hot
swap load current (I

HOT_SWAP

(max)) which the circuit may let

pass in the case of tolerance build-up in the opposite direc-
tion. Here, the worst-case maximum is found using a
V

TRIP

(max) of 60mV and a sense resistor, 3% low in value:

(max)

60mV

0.97 R

(nom)

61.9mV

(nom)

HOT_SWAP

SENSE

(

)

In this case, the application circuit must be sturdy enough to
operate over a ~1.6-to-1 range in hot swap load currents. For
example, if an MIC2594 circuit must pass a minimum hot
swap load current of 4A without nuisance trips, R

SENSE

should be set to

38.8mV

9.7m

, and the nearest 1%

standard value is 9.76m

. At the other tolerance extremes,

HOT_SWAP

(max) for the circuit in question is then simply

(max)

61.9mV

9.76m

6.3A

HOT_SWAP

With a knowledge of the application circuit's maximum hot
swap load current, the power dissipation rating of the sense
resistor can be determined using P = I

R. Here, the I is

HOT_SWAP

(max) = 6.3A and the R is R

SENSE

(min)

(0.97)(R

SENSE

(nom)) = 9.47m

. Thus, the sense resistor's

maximum power dissipation is:

MAX

= (6.3A)

(9.47m

) = 0.376W

A 0.5

sense resistor is a good choice in this application.

Undervoltage/Overvoltage Detection--MIC2588

The MIC2588 has "UV" and "OV" input pins. These pins can be
used to detect input supply rail undervoltage and overvoltage
conditions. Undervoltage lockout prevents energizing the load
until the supply input is stable and within tolerance. In a similar
fashion, overvoltage turn-off prevents damage to sensitive
circuit components should the input voltage exceed normal
operational limits. Each of these pins is internally connected to
an analog comparator with 20mV of hysteresis. When the UV
pin falls below its V

UVL

threshold or the OV pin is above its V

OVH

threshold, the GATE pin is immediately pulled low. The GATE
pin will be held low until UV exceeds its V

UVH

threshold or OV

drops below its V

OVL

threshold. The UV and OV circuit's

threshold trip points are programmed using the resistor divider

MIC2588/MIC2594

Micrel

M9999-122303

December 2003

R1, R2, and R3 as shown in the

"Typical Application."

The

equations to set the trip points are shown below. For the
following example, the circuit's UV threshold is set to V

= 37V

and the OV threshold is placed at V

= 72V, values commonly

used in Central Office power distribution applications.

(typ)

R1+ R2 + R3

R2 + R3

(typ)

R1+ R2 + R3

UVL

OVH

(

)

(

)

(

)

Given V

, V

, and any one resistor value, the remaining

two resistor values can be found. A suggested value for R3
is that which will provide approximately 100

A of current

through the voltage divider chain at V

= V

. This yields the

following as a starting point:

(typ)

100 A

12.23k

OVH

The closest standard 1% value for R3 = 12.4k

. Solving for

R2 and R1 yields:

12.4k

72V

37V

11.729k

The closest standard 1% value for R2 = 11.8k

. Next, the

value for R1 is calculated:

R1 R3

1.223V

R1 12.4k

72V 1.223V

1.223V

R1 705.808k

The closest standard 1% value for R1 = 698k

Using standard 1% resistor values, the circuit's nominal
UV and OV thresholds are:

= 36.5V

= 71.2V

Programmable UVLO Hysteresis--MIC2594

The MIC2594 has user-programmable hysteresis by means of
the ON and OFF pins. This allows setting the part to turn on at
a voltage V1, and not turn off until a second voltage V2, where
V2 < V1. This can significantly simplify dealing with source
impedances in the supply bus while at the same time increasing
the amount of available operating time from a loosely regulated
power supply (for example, a battery supply). Similarly to the
MIC2588, each of these pins is internally connected to an

analog comparator with 20mV of hysteresis. The MIC2594
holds the output off until the voltage at the ON pin exceeds its
V

ONH

threshold value given in the

"Electrical Characteristics"

table. Once the output has been enabled by the ON pin, it will

remain on until the voltage at the OFF pin falls below its V

OFFL

threshold value, or the part turns off due to a fault. Should either
event occur, the GATE pin is immediately pulled low and will
remain low until the ON pin once again exceeds its V

ONH

threshold. The circuit's turn-on and turn-off points are set using
the resistor divider R1, R2, and R3 as shown in the

"Typical

Application."

The equations to establish the trip points are

shown below. In the following example, the circuit's ON thresh-
old is set to V

40V and the circuit's OFF threshold is V

OFF

= 35V.

= V

(typ)

R1 R2 R3

= V

(typ)

R1 R2 R3

R2 R3

ONH

OFF

OFFL

(

)

(

)

(

)

Given V

OFF

, V

, and any one resistor value, the remaining

two resistor values can be readily found. A suggested value
for R3 is that which will provide approximately 100

A of

current through the voltage divider chain at V

= V

OFF

. This

yields the following as a starting point:

R3 =

(typ)

100 A

12.23k

OFFL

The closest standard 1% value for R3 = 12.4k

Then, solving for R2 and R1 yields:

R2 = R3

R2 = 12.4k

40V

35V

R2 = 1.771k

OFF

The closest standard 1% value for R2 = 1.78k

R1= R3

1.223V

R1= 12.4k

40V 1.223V

1.223V

R1= 391.380k

(

)

(

)

The closest standard 1% value for R1 = 392k

Using standard 1% resistor values, the circuit's nominal
ON and OFF thresholds are:

= 40.1V

OFF

= 35V

December 2003

M9999-122303

MIC2588/MIC2594

Micrel

Applications Information

4-Wire Kelvin Sensing

Because of the low value typically required for the sense
resistor, special care must be used to measure accurately the
voltage drop across it. Specifically, the measurement tech-
nique across each R

SENSE

must employ 4-wire Kelvin sens-

ing. This is simply a means of making sure that any voltage
drops in the power traces connecting to the resistors are not
picked up by the signal conductors measuring the voltages
across the sense resistors.

Figure 6 illustrates how to implement 4-wire Kelvin sensing.
As the figure shows, all the high current in the circuit (from V

through R

SENSE

, and then to the source of the output MOSFET)

flows directly through the power PCB traces and R

SENSE

. The

voltage drop resulting across R

SENSE

is sampled in such a

way that the high currents through the power traces will not
introduce any parasitic voltage drops in the sense leads. It is
recommended to connect the hot swap controller's sense
leads directly to the sense resistor's metalized contact pads.

SENSE

Power Trace
From V

PCB Track Width:

0.03" per Ampere

using 1oz Cu

Power Trace

To MOSFET Source

Signal Trace

to MIC2588/94 V

Signal Trace
to MIC2588/94 SENSE Pin

Note: Each SENSE lead trace shall be
balanced for best performance -- equal
length/equal aspect ratio.

SENSE

metalized

contact pads

Figure 6. 4-Wire Kelvin Sense Connections for R

SENSE

Protection Against Voltage Transients

In many telecom applications, it is very common for circuit
boards to encounter large-scale supply-voltage transients in
backplane environments. Because backplanes present a
complex impedance environment, these transients can be as
high as 2.5 times steady-state levels, or 120V in worst-case
situations. In addition, a sudden load dump anywhere on the
circuit card can generate a very high voltage spike at the drain
of the output MOSFET which, in turn, will appear at the
DRAIN pin of the MIC2588/MIC2594. In both cases, it is good
engineering practice to include protective measures to avoid
damaging sensitive ICs or the hot swap controller from these
large-scale transients. Two typical scenarios in which large-
scale transients occur are described below:

1. An output current load dump with no bypass (charge

bucket or bulk) capacitance to V

. For example,

if L

LOAD

= 5

H, V

= 56V and t

OFF

= 0.7

s, the

resulting peak short-circuit current prior to the
MOSFET turning off would reach:

55V

0.7 s

5 H

7.7A

(

)

If there is no other path for this current to take when
the MOSFET turns off, it will avalanche the drain-
source junction of the MOSFET. Since the total
energy represented is small relative to the sturdi-
ness of modern power MOSFETs, it's unlikely that

this will damage the transistor. However, the actual
avalanche voltage is unknown; all that can be
guaranteed is that it will be greater than the V

BD(D-

of the MOSFET. The drain of the transistor is

connected to the DRAIN pin of the MIC2588/94,
and the resulting transient does have enough
voltage and energy and can damage this, or any,
high-voltage hot swap controller.

2. If the load's bypass capacitance (for example, the

input filter capacitors for a set of DC-DC converter
modules) are on a board from which the board with
the MIC2589/MIC2595 and the MOSFET can be
unplugged, the same type of inductive transient
damage can occur to the MIC2588/MIC2594.

Protecting the controller and the power MOSFET from dam-
age against these large-scale transients can take the forms
shown in Figure 7. It is not mandatory that these techniques
are used--the application environment will dictate suitability.
As protection against sudden on-card load dumps at the
DRAIN pin of the controller, a 2.2

F or larger capacitor

directly from DRAIN to V

of the controller can be used to

serve as a charge reservoir. Alternatively, a 68V, 1W, 5%
Zener diode clamp can be installed in a similar fashion. Note
that the clamp diode's cathode is connected to the DRAIN pin
as shown in Figure 7. To protect the hot swap controller from
large-scale transients at the card input, a 100V clamp diode
(an SMAT70A or equivalent) can be used. In either case, the
lead lengths should be short and the layout compact to
prevent unwanted transients in the protection circuit.

[Circuit drawing under construction]

Figure 7. Using Large-Scale Transient Protection

Devices Around the MIC2588/94

Power buss inductance could easily result in localized high-
voltage transients during a turn-off event. The potential for
overstressing the part in such a case should be kept in check
with a suitable input capacitor and/or transient clamping
diode.

Power MOSFET Selection

[Section under construction]

Power MOSFET Operating Voltage Requirements

[Section under construction]

Power MOSFET Steady-State Thermal Issues

[Section under construction]

Power MOSFET Transient Thermal Issues

[Section under construction]

PCB Layout Considerations

[Section under construction]

Power MOSFET and Sense Resistor Vendors

[Section under construction]

MIC2588/MIC2594

Micrel

M9999-122303

December 2003

Package Information

0.244 (6.20)
0.228 (5.79)

0.197 (5.0)
0.189 (4.8)

SEATING

PLANE

0.026 (0.65)

MAX

)

0.010 (0.25)
0.007 (0.18)

0.064 (1.63)
0.045 (1.14)

0.0098 (0.249)
0.0040 (0.102)

0.020 (0.51)
0.013 (0.33)

0.157 (3.99)
0.150 (3.81)

0.050 (1.27)

TYP

PIN 1

DIMENSIONS:

INCHES (MM)

0.050 (1.27)
0.016 (0.40)

8-Pin SOIC (M)

MICREL, INC.

1849 FORTUNE DRIVE

SAN JOSE, CA 95131

USA

TEL

+ 1 (408) 944-0800

FAX

+ 1 (408) 944-0970

WEB

http://www.micrel.com

The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.

Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can

reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into
the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's

use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser's own risk and Purchaser agrees to fully indemnify

Micrel for any damages resulting from such use or sale.

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