MIC2587/MIC2587R
Single-Channel, Positive High-Voltage
Hot Swap Controller
General Description
Features
The MIC2587 and MIC2587R are single-channel positive
voltage hot swap controllers designed to provide safe
insertion and removal of boards for systems that require
live (always-powered) backplanes. These devices use few
external components and act as controllers for external Nchannel power MOSFET devices to provide inrush current
control and output voltage slew rate control. Overcurrent
fault protection is provided via programmable analog
foldback current limit circuitry equipped with a
programmable overcurrent filter. These protection circuits
combine to limit the power dissipation of the external
MOSFET to insure that the MOSFET is in its SOA during
fault conditions. The MIC2587 provides a circuit breaker
function that latches the output MOSFET off if the load
current exceeds the current limit threshold for the duration
of the programmable timer. The MIC2587R provides a
circuit breaker function that automatically attempts to
restart power after a load current fault at a low duty cycle to
prevent the MOSFET from overheating. Each device
provides either an active-HIGH (-1BM) or an active-LOW
(-2BM) “Power-is-Good” signal to indicate that the output
load voltage is within tolerance.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
• MIC2587: Pin-for-pin functional equivalent to the LT1641
• Operates from +10V to +80V with 100V ABS MAX operation
• Industrial temperature specifications at VCC = +24V and
VCC = +48V
• Programmable current limit with analog foldback
• Active current regulation minimizes inrush current
• Electronic circuit breaker for overcurrent fault protection
- Output latch off (MIC2587) or
- Output auto-retry (MIC2587R)
• Fast responding circuit breaker (< 2µs) to short circuit loads
• Programmable input undervoltage lockout
• Fault Reporting:
Open-drain “Power-is-Good” output for enabling DC/DC
converter(s)
- Active-HIGH:
MIC2587-1/MIC2587R-1
- Active-LOW:
MIC2587-2/MIC2587R-2
Applications
•
•
•
•
•
•
General-purpose hot board insertion
High-voltage, high-side electronic circuit breaker
+12V/+24V/+48V Distributed Power Systems
+24V/+48V Industrial/Alarm Systems
Telecom Systems
Medical Systems
Ordering Information
PWRGD Polarity
Circuit Breaker Function
Package
MIC2587-1YM
Active-HIGH
Latched
8 pin SOIC
MIC2587-2YM
Active-LOW
Latched
8 pin SOIC
MIC2587R-1BM
MIC2587R-1YM
Active- HIGH
Auto-retry
8 pin SOIC
MIC2587R-2BM
MIC2587R-2YM
Active-LOW
Auto-retry
8 pin SOIC
Part Number
Standard
Pb-Free
MIC2587-1BM
MIC2587-2BM
Power, Connect, and Protect is a trademark of Micrel Inc.
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax +1 (408) 474-1000 • http://www.micrel.com
October 2004
M9999-102204
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MIC2587/MIC2587R
Typical Application
MIC2587/87R Typical Application Circuit
Pin Configuration
8-pin SOIC (M)
MIC2587-1BM
MIC2587R-1BM
October 2004
8-pin SOIC (M)
MIC2587-2BM
MIC2587R-2BM
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MIC2587/MIC2587R
Pin Description
Pin Number
1
2
Pin Name
ON
FB
Pin Function
Enable Input: When the voltage at the ON pin is higher than the VONH threshold, a start cycle
is initiated. An internal current source (IGATEON) is activated which charges the GATE pin,
ramping up the voltage at this pin to turn on an external MOSFET. Whenever the voltage at
the ON pin is lower than the VONL threshold, an undervoltage lockout condition is detected
and the IGATEON current source is disabled while the GATE pin is pulled low by another
internal current source (IGATEOFF). After a load current fault, toggling the ON pin LOW will
reset the circuit breaker then back HIGH (ON pin) will initiate another start cycle.
Output Voltage Feedback Input: This pin is connected to an external resistor divider that is
used to sample the output load voltage. The voltage at this pin is measured against an
internal comparator whose output controls the PWRGD (or /PWRGD) signal. PWRGD (or
/PWRGD) asserts when the FB pin voltage crosses the VFBH threshold. When the FB pin
voltage is lower than its VFBL threshold, PWRGD (or /PWRGD) is de-asserted. The FB
comparator exhibits a typical hysteresis of 80mV.
The FB pin voltage also affects the MIC2587/MIC2587R’s foldback current limit operation
(see the “Functional Description” section for further information).
Power-is-Good (PWRGD or /PWRGD), Open-drain Output: This pin remains de-asserted
during start up while the FB pin voltage is below the VFBH threshold. Once the voltage at the
FB pin rises above the VFBH threshold, the Power-is-Good output asserts with minimal delay
(typically ≤ 5µs).
3
PWRGD
(MIC2587-1)
(MIC2587R-1)
Active-HIGH
/PWRGD
(MIC2587-2)
(MIC2587R-2)
Active-LOW
For the (-1) options, the PWRGD output pin will be high-impedance when the FB pin voltage
is higher than VFBH and will pull down to GND when the FB pin voltage is less than VFBL.
For the (-2) options, the /PWRGD output pin will be high-impedance when the FB pin
voltage is lower than VFBL and will pull down to GND when the FB pin voltage is higher than
VFBH.
The Power-is-Good output pin is connected to an open-drain, N-channel transistor
implemented with high-voltage structures. These transistors are capable of operating with
pull-up resistors to supply voltages as high as 100V.
To use this signal as a logic control in low-voltage dc-dc conversion applications, an external
pull-up resistor between this pin and the logic supply voltage is recommended, unless an
internal pull-up impedance is provided by the dc-dc module or other device (load).
4
5
GND
TIMER
Tie this pin directly to the system’s analog GND plane
Current Limit Response Timer: A capacitor connected from this pin to GND provides
overcurrent filtering to prevent nuisance “tripping” of the circuit breaker by setting the time
(tFLT) for which the controller is allowed to remain in current limit. Once the MIC2587 circuit
breaker trips, the output latches off. Under normal (steady state) operation, the TIMER pin
is held to GND by an internal 3.5µA current source (ITIMERDN). When the voltage across the
external sense resistor exceeds the VTRIP threshold, an internal 65µA current source
(ITIMERUP) is activated to charge the capacitor connected to the TIMER pin. When the TIMER
pin voltage reaches the VTIMERH threshold, the circuit breaker is tripped pulling the GATE pin
low, the ITIMERUP current source is disabled, and the TIMER pin capacitor is discharged by
the ITIMERDN current source. When the voltage at the TIMER pin is less than 0.5V, the
MIC2587 can be restarted by toggling the ON pin LOW then HIGH.
For the MIC2587R, the capacitor connected to the TIMER pin sets the period of auto-retry
where the duty cycle is fixed at a nominal 5%.
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MIC2587/MIC2587R
Pin Description
Pin Number
Pin Name
Pin Function
Gate Drive Output: This pin is the output of an internal charge pump connected to the gate
of an external, N-channel power MOSFET. The charge pump has been designed to provide
a minimum gate drive (∆VGATE = VGATE - VCC) of +7.5V over the input supply’s full operating
range. When the ON pin voltage is higher than the VONH threshold, a 16µA current source
(IGATEON) charges the GATE pin.
6
GATE
When in current limit, the output voltage at the GATE pin is adjusted so that the voltage
across the external sense resistor is held equal to VTRIP while the capacitor connected to the
TIMER pin charges. If the current limit condition goes away before the TIMER pin voltage
rises above the VTIMERH threshold, then steady-state operation resumes.
The GATE output pin is shut down whenever: (1) the input supply voltage is lower than the
VUVL threshold, (2) the ON pin voltage is lower than the VONL threshold, (3) the TIMER pin
voltage is higher than the VTIMERH threshold, or (4) the difference between the VCC and
SENSE pins is greater than VTRIP while the TIMER pin is grounded. For cases (3) and (4) –
overcurrent fault conditions – the GATE is immediately pulled to ground by IGATEFLT, a 30mA
(minimum) pull-down current.
7
SENSE
Circuit Breaker Sense Input: This pin is the (-) Kelvin sense connection for the output
supply rail. A low-valued resistor (RSENSE) between this pin and the VCC pin sets the circuit
breaker’s current limit trip point. When the current limit detector circuit is enabled (as well as
the current limit timer), while the FB pin voltage remains higher than 1V, the voltage across
the sense resistor (VCC-VSENSE) will be regulated to VTRIP (47mV, typically) to maintain a
constant current into the load. When the FB pin voltage is less than ≅0.5V, the voltage
across the sense resistor decreases linearly to a minimum of 12mV (typical) when the FB
pin voltage is at 0V.
To disable the circuit breaker (and defeat all current limit protections), the SENSE pin and
the VCC pin can be tied together.
8
October 2004
VCC
Positive Supply Voltage Input: This pin is the (+) Kelvin sense connection for the output
supply rail. The nominal operating voltage range for the MIC2587 and the MIC2587R is
+10V to +80V, and VCC can withstand input transients up to +100V. An undervoltage
lockout circuit holds the GATE pin low whenever the supply voltage to the MIC2587 and the
MIC2587R is less than the VUVH threshold.
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MIC2587/MIC2587R
Absolute Maximum Ratings(1)
Operating Ratings(2)
(All voltages are referred to GND)
Supply Voltage (VCC)...................................... +10V to +80V
Ambient Temperature Range (TA) ............... -40°C to +85°C
Junction Temperature (TJ) ....................................... +125°C
Package Thermal Resistance (θJA)
8-pin SOIC...................................................... 160 °C/W
Supply Voltage (VCC) pin .............................. -0.3V to +100V
GATE pin...................................................... -0.3V to +100V
ON, SENSE pins .......................................... -0.3V to +100V
PWRGD, /PWRGD pins ............................... -0.3V to +100V
FB pin ........................................................... -0.3V to +100V
TIMER pin ........................................................ -0.3V to +6V
ESD Rating
Human Body Model ................................................. 2kV
Machine Model ......................................................200V
Lead Temperature (Soldering)
Standard Package (-xBM)
IR Reflow…………………………240°C + 0°C/-5°C
Lead-Free Package (-xYM)
IR Reflow…………………………260°C + 0°C/-5°C
DC Electrical Characteristics(4)
VCC = +24V and +48V, TA = 25°C, unless otherwise noted. BOLD indicates specifications apply over the full operating temperature
range of -40°C to +85°C.
Symbol
Parameter
VCC
Supply Voltage
ICC
Supply Current
VUVH
Supply Voltage Undervoltage Lockout
VUVL
Condition
Min
Typ
10
Max
Units
80
V
2
5
mA
VCC rising
7.5
8.0
8.5
V
VCC falling
7.0
7.5
8.0
V
VHYSLO
VCC Undervoltage Lockout Hysteresis
500
mV
VFBH
Feedback Pin Voltage High Threshold
FB Low-to-High transition
1.280
1.313
1.345
VFBL
Feedback Pin Voltage Low Threshold
FB High-to-Low transition
1.208
1.233
1.258
VHYSFB
Feedback Voltage Hysteresis
∆VFB
FB Pin Threshold Line Regulation
10V ≤ VCC ≤ 80V
IFB
FB Pin Input Current
VTRIP
Circuit Breaker Trip Voltage, VCC-VSENSE
∆VGATE
MOSFET Gate Drive, VGATE-VCC
IGATEON
IGATEFLT
80
V
V
mV
-0.05
0.05
mV/V
0V ≤ VFB ≤ 3V
-1
1
µA
VFB = 0V (See Fig. 1)
5
12
17
mV
VFB = 1V (See Fig. 1)
39
47
+10V ≤ VCC ≤ +80V
7.5
GATE Pin Pull-up Current
Start cycle, VGATE = 7V
-10
GATE Pin rapid pull-down current (during
a fault condition, until VGATE = VGATE[TH])
(VCC - VSENSE) = (VTRIP + 10mV)
30
55
mV
18
V
-16
-22
µA
80
200
mA
VGATE = 5V
(VGATE[TH] is the MOSFET threshold)
IGATEOFF
GATE Pin Turn-off Current
Normal turn-off, or from VGATE[TH]
(MOSFET) to 0V after a fault condition
ITIMERUP
TIMER Pin Charging Current
(VCC – VSENSE) > VTRIP
1.8
-24
-65
mA
-120
µA
VTIMER = 0V
October 2004
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Micrel
MIC2587/MIC2587R
DC Electrical Characteristics(4)
VCC = +24V and +48V, TA = 25°C, unless otherwise noted. BOLD indicates specifications apply over the full operating temperature
range of -40°C to +85°C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
ITIMERDN
TIMER Pin Pull-down Current
(VCC – VSENSE) < VTRIP
1.5
3.5
5
µA
VTIMERH
TIMER Pin High Threshold Voltage
1.280
1.313
1.345
V
VTIMERL
TIMER Pin Low Threshold Voltage
0.4
0.49
0.6
V
VONH
ON Pin High Threshold Voltage
ON Low-to-High transition
1.280
1.313
1.355
V
VONL
ON Pin Low Threshold Voltage
ON High-to-Low transition
1.208
1.233
1.258
V
VHYSON
ON Pin Hysteresis
ION
ON Pin Input Current
0V ≤ VON ≤ 80V
VOL
Power-Good Output Voltage
PWRGD or /PWRGD = LOW
VTIMER = 0.6V
IOFF
Power-Good Leakage Current
80
mV
2
µA
IOL = 1.6 mA
0.4
V
IOL = 4 mA
0.8
V
PWRGD or /PWRGD = Open-Drain
10
µA
VPG = VCC, VON = 1.5V
AC Electrical Characteristics
VCC = +24V and +48V, TA = 25°C unless otherwise noted. BOLD indicates specifications apply over the full operating temperature
range of -40°C to +85°C.
Symbol
Parameter
Condition
tPONLH
ON High to GATE High
IRF530, CGATE = 10nF
3
ms
tPONHL
ON Low to GATE Low
VIN = 48V, IRF530, CGATE = 10nF
1
ms
tPFBLH
FB Valid to PWRGD High
(MIC2587/87R-1)
RPG = 50kΩ pull-up to 48V
2
µs
tPFBHL
FB Invalid to PWRGD Low
(MIC2587/87R-1)
4
µs
4
µs
2
µs
tPFBHL
tPFBLH
tOCSENSE
FB Valid to /PWRGD Low
(MIC2587/87R-2)
FB Invalid to /PWRGD High
(MIC2587/87R-2)
Min
Typ
Max
Units
CL=100pF
RPG = 50kΩ pull-up to 48V
CL=100pF
RPG = 50kΩ pull-up to 48V
CL=100pF
RPG = 50kΩ pull-up to 48V
CL=100pF
Overcurrent Sense to GATE Low
(VCC - VSENSE) = (VTRIP + 10mV)
Trip Time
Figure 7
1
2
µs
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
4. Specification for packaged product only.
October 2004
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MIC2587/MIC2587R
Timing Diagrams
Figure 2. ON to GATE Timing
Figure 1. Foldback Current Limit Transfer Characteristic
Figure 3. MIC2587/87R-1 FB to PWRGD Timing
Figure 4. MIC2587/87R-2 FB to /PWRGD Timing
Figure 5. Overcurrent Sense to GATE Timing
October 2004
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MIC2587/MIC2587R
Function Diagram
MIC2587/MIC2587R Block Diagram
October 2004
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MIC2587/MIC2587R
Zener diode (18-V) connected from the source to the gate
as shown in the "Typical Applications" circuit is highly
recommended. A good choice for an 18-V Zener diode in
this application is the MMSZ5248B, available in a small
SOD123 package.
CGATE is used to adjust the GATE voltage slew rate while
R3 minimizes the potential for high-frequency parasitic
oscillations from occurring in M1. However, note that
resistance in this part of the circuit has a slight destabilizing
effect upon the MIC2587/MIC2587R's current regulation
loop. Compensation resistor R4 is necessary for
stabilization of the current regulation loop. The current
through the power transistor during initial inrush is given
by:
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 MIC2587/MIC2587R is designed to address these
issues by limiting the maximum current that is allowed to
flow during hot swap events. This is achieved by
implementing a constant-current control loop at turn-on. In
addition to inrush current control, the MIC2587 and
MIC2587R incorporate input voltage supervisory functions
and user-programmable overcurrent protection, thereby
providing robust protection for both the system and the
circuit board.
IINRUSH = CLOAD ×
Overcurrent Protection
The MIC2587 and the MIC2587R use an external, lowvalue resistor in series with the drain of the external
MOSFET to measure the current flowing into the load. The
VCC connection (Pin 8) and the SENSE connection (Pin 7)
are the (+) and (-) inputs, respectively, of the device's
internal current sensing circuits. Kelvin sense connections
are strongly recommended for sensing the voltage across
these pins. See the “Applications Information” for further
details.
The nominal current limit is determined by the following
equation.
Start-Up Cycle
When the power supply voltage to the MIC2587/MIC2587R
is higher than the VUVH and the VONH threshold voltages, a
start cycle is initiated. When the controller is enabled, an
internal 16µA current source (IGATEON) is turned on and the
GATE pin voltage rises from 0V with respect to ground at a
rate equal to:
ILIMIT =
(1)
VTRIP(TYP)
RSENSE
(3)
where VTRIP(TYP) is the typical current limit threshold
specified in the datasheet and RSENSE is the value of the
selected sense resistor.
As the MIC2587 and the
MIC2587R employ a constant-current regulation scheme in
current limit, the charge pump’s output voltage at the
GATE pin is adjusted so that the voltage across the
external sense resistor is held equal to VTRIP while the
capacitor connected to the TIMER pin is being charged. If
The internal charge pump has sufficient output drive to fully
enhance commonly available power MOSFETs for the
lowest possible DC losses. The gate drive is guaranteed
to be between 7.5V and 18V over the entire supply voltage
operating range (10V to 80V), so 60V BVDSS and 30V
BVDSS N-channel power MOSFETs can be used for +48V
and +24V applications, respectively. However, an external
October 2004
(2)
The drain current of the MOSFET is monitored via an
external current sense resistor to ensure that it never
exceeds the programmed threshold, as described in the
"Circuit Breaker Operation" section.
A capacitor connected to the controller’s TIMER pin sets
the value of overcurrent detector delay, tFLT, which is the
time for which an overcurrent event must last to signal a
fault condition and to cause an output latch-off. These
devices will be driving a capacitive load in most
applications, so a properly chosen value of CTIMER prevents
false-, or nuisance-, tripping at turn-on as well as providing
immunity to noise spikes after the start-up cycle is
complete. The procedure for selecting a value for CTIMER is
given in the "Circuit Breaker Operation" section.
Input Supply Transient Suppression and Filtering
The MIC2587/MIC2587R is guaranteed to withstand
transient voltage spikes up to 100V. However, voltage
spikes in excess of 100V may cause damage to the
controller. In order to suppress transients caused by
parasitic inductances, wide (and short) power traces
should be utilized. Alternatively, a heavier trace plating will
help minimize inductive spikes that may arise during
events (e.g., short circuit loads) that can cause a large di/dt
to occur. External surge protection, such as a clamping
diode, is also recommended as an added safeguard for
device (and system) protection. Lastly, a 0.1µF decoupling
capacitor from the VCC pin to ground is recommended to
assist in noise rejection. Place this filter capacitor as close
as possible to the VCC pin of the controller.
dVGATE IGATEON
=
dt
CGATE
IGATEON
CGATE
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MIC2587/MIC2587R
GATE pin is immediately pulled low by a 30mA
(minimum) internal current sink. This operation
turns off the MOSFET quickly and disconnects the
input from the load. The value of CTIMER should be
selected to allow the circuit's minimum regulated
output current (IOUT) to equal ILIMIT for somewhat
longer than the time it takes to charge the total
load capacitance.
An initial value for CTIMER is found by calculating the time it
will take for the MIC2587/MIC2587R to completely charge
up the output capacitive load. Assuming the load is
enabled by the PWRGD (or /PWRGD) signal of the IC, the
turn-on delay time is derived from the following expression,
I = C × (dV/dt):
the current-limit condition goes away before the TIMER pin
voltage rises above the VTIMERH threshold, then steadystate operation resumes. To prevent excessive power
dissipation in the external MOSFET under load current
fault conditions, the FB pin voltage is used as the control
element in a circuit that lowers the current limit as a
function of the output voltage. When the load current
increases to the point where the output voltage at the load
approaches 0V (likewise, the MIC2587/MIC2587R’s FB pin
voltage also approaches 0V), the result is a proportionate
decrease in the maximum current allowed into the load.
This foldback current limit subcircuit’s transfer
characteristic is shown in Figure 1. Under excessive load
conditions (output and FB voltage equals 0V), the foldback
current limiting circuit controls the MIC2587/MIC2587R’s
GATE drive to force a constant 12mV (typical) voltage drop
across the external sense resistor.
t TURN-ON =
CTIMER(MAX) =
VTIMERH(MIN)
µF ⎞
⎛
CTIMER (MAX) = t TURN -ON × ⎜ 94 × 10 - 6
⎟
sec ⎠
⎝
(6)
For example, in a system with a CLOAD = 1000µF, a
maximum VCC = +72V, and a maximum load current on a
nominal +48V buss of 1.65A, the nominal circuit design
equations steps are:
1. Choose ILIMIT = IHOT_SWAP(nom) = 2A (1.65A + 20%);
2. Select an RSENSE (Closest 1% standard value is
19.6mΩ);
3. Using ICHARGE = ILIMIT = 2A, the application circuit
turn-on time is calculated using Equation 5:
(4)
Whenever the voltage across RSENSE exceeds the
MIC2587/MIC2587R’s nominal circuit breaker threshold
voltage of 47mV during steady-state operation, two things
occur:
1. A constant-current regulation loop will engage
within 1µs after an overcurrent condition is
detected by RSENSE, and the control loop is
designed to hold the voltage across RSENSE equal
to 47mV. This feature protects both the load and
the MIC2587/MIC2587R circuits from excessively
high currents.
2. Capacitor CTIMER is then charged up to the VTIMERH
threshold (1.313V) by an internal 65µA current
source (ITIMERUP). If the excessive current persists
such that the voltage across CTIMER crosses the
VTIMERH threshold, the circuit breaker trips and the
October 2004
t TURN-ON × ITIMERUP(MAX)
⎛ 120 µA ⎞
CTIMER(MAX) = t TURN -ON × ⎜
⎟
⎝ 1.280V ⎠
CFILTER × VTIMERH
ITIMERUP
tFLT (ms) = 20 × CFILTER ( µF)
(5)
ILIMIT
Using parametric values for the MIC2587/MIC2587R, an
expression relating a worse-case design value for CTIMER,
using the MIC2587/MIC2587R specification limits, to the
circuit's turn-on delay time is:
Circuit Breaker Operation
The MIC2587/MIC2587R employ an electronic circuit
breaker that protects the external N-channel power
MOSFET and other system components against largescale output current faults, both during initial card insertion
or during steady-state operation. The current limit threshold
is set via an external resistor, RSENSE, connected between
the circuit’s VCC pin and SENSE pin. For the
MIC2587/MIC2587R, a fault current timing circuit is set via
an external capacitor (CTIMER) that determines the length of
the time delay (tFLT) for which the controller remains in
current limit before the circuit breaker is tripped.
Programming the response time of the overcurrent detector
helps to prevent nuisance tripping of the circuit breaker
because of high inrush currents charging bulk and
distributed capacitive loads. The nominal overcurrent
response time is calculated using the following equation:
tFLT =
CLOAD × VCC(MAX)
t TURN-ON =
(1000 µF × 72V)
2A
= 36ms
Allowing for capacitor tolerances and a nominal 36ms turnon time, an initial worse-case value for CTIMER is:
µF ⎞
⎛
CTIMER(MAX) = 0.036s × ⎜ 94 × 10 - 6
⎟ = 3.38 µF
sec ⎠
⎝
The closest standard ±5% tolerance capacitor value is
3.3µF and would be a good initial starting value for
prototyping.
Whenever the MIC2587 is not in current limit, CTIMER is
discharged to GND by an internal 3.5µA current sink
(ITIMERDN).
For the MIC2587R, the circuit breaker automatically resets
after (20) tFLT_AUTO time constants. If the fault condition still
exists, capacitor CTIMER will begin to charge up to the
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MIC2587/MIC2587R
VTIMERH threshold, and if exceeded, trip the circuit breaker.
Capacitor CTIMER will then be discharged by ITIMERDN until
the voltage across CTIMER drops below the VTIMERL
threshold, at which time another start cycle is initiated. This
will continue until either of the following occurs: a) the fault
condition is removed, b) the input supply voltage power is
removed/recycled, or c) the ON pin is toggled LOW then
HIGH. The duty cycle of the auto-restart function is
therefore fixed at 5% and the period of the auto-restart
cycle is given by:
Power-is-Good Output Signals
For the MIC2587-1 and MIC2587R-1, the power-good
output signal (PWRGD) will be high impedance when the
FB pin voltage is higher than the VFBH threshold and will
pull down to GND when the FB pin voltage is lower than
the VFBL threshold. For the MIC2587-2 and MIC2587R-2,
the power-good output signal (/PWRGD) will pull down to
GND when the FB pin voltage is higher than the VFBH
threshold and will be high impedance when the FB pin
voltage is lower than the VFBL threshold. Hence, the (-1)
parts have an active-HIGH PWRGD signal and the (-2)
parts have an active-LOW /PWRGD output. PWRGD (or
/PWRGD) may be used as an enable signal for one or
more following 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 (RC) load at the Power-is-Good pin.
The Power-is-Good output pin is connected to an opendrain, N-channel transistor implemented with high-voltage
structures. These transistors are capable of operating with
pull-up resistors to supply voltages as high as 100V.
t AUTO −RESTARt = 20 × tFLT_AUTO
t AUTO -RESTART = 20 ×
(CTIMER ) × (VTIMERH − VTIMERL )
ITIMERUP
⎛
ms ⎞
⎟
t AUTO-RESTART = CTIMER × ⎜⎜ 250
µF ⎟⎠
⎝
(7)
The auto-restart period for the example above where the
worse-case CTIMER was calculated to be 3.3µF is:
tAUTO-RESTART = 825ms
Input Undervoltage Lockout
The MIC2587/MIC2587R have an internal undervoltage
lockout circuit that inhibits operation of the controller’s
internal circuitry unless the power supply voltage is stable
and within an acceptable tolerance. If the supply voltage to
the controller, with respect to ground is greater than the
VUVH threshold voltage (8V typical), then the controller’s
internal circuits are enabled and the controller is then ready
for normal operation pending the state of the ON pin
voltage. Once in steady-state operation, the controller’s
internal circuits remain active so long as the supply voltage
with respect to ground is higher than the controller’s
internal VUVL threshold voltage (7.5V typical).
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a similar approach as setting the circuit’s ON/OFF input
voltage. The equations to set the trip points are shown
below. For the following +48V telecom application, poweris-good output signal PWRGD (or /PWRGD) is to be deasserted when the output supply voltage is lower than
+48V-10% (+43.2V).
Applications Information
External ON/OFF Control
The MIC2587/MIC2587R have an ON pin input that is used
to enable the controller to commence a start-up sequence
upon card insertion or to disable controller operation upon
card removal. In addition, the ON pin can be used to reset
the MIC2587/MIC2587R’s internal electronic circuit breaker
in the event of a load current fault. To reset the electronic
circuit breaker, the ON pin is toggled LOW then HIGH. The
ON pin is internally connected to an analog comparator
with 80mV of hysteresis. When the ON pin voltage falls
below its internal VONL threshold, the GATE pin is
immediately pulled low. The GATE pin will be held low until
the ON pin voltage is above its internal VONH threshold. The
external circuit's ON threshold voltage level is programmed
using a resistor divider (R1 & R2) as shown in the "Typical
Application” circuit. The equations to set the trip points are
shown below. For the following example, the external
circuit's ON threshold is set to VONH(EX) = +37V, a value
commonly used in +48V Central Office power distribution
applications.
⎛ R1+ R2 ⎞
VONH(EX) = VONH × ⎜
⎟
⎝ R2 ⎠
⎛ R5 + R6 ⎞
VOUT(NOT GOOD) = VFBL × ⎜
⎟
⎝ R6 ⎠
Given VFBL and R6, a value for R5 can be determined. A
suggested value for R6 is that which will provide
approximately 100µA of current through the voltage divider
chain at VOUT(NOT GOOD) = VFBL. This yields the following
equation as a starting point:
R6 =
VONH(TYP)
100 µA
=
100 µA
=
1.233V
= 12.33 kΩ
100 µA
⎡⎛ V
⎞ ⎤
⎡⎛ 43.2V ⎞ ⎤
OUT(NOT GOOD)
⎟⎟ − 1⎥ = 12.4kΩ × ⎢⎜
R5 = R6 × ⎢⎜⎜
⎟ − 1⎥ = 422 kΩ
⎣⎝ 1.233V ⎠ ⎦
⎢⎣⎝ VFBL(TYP)
⎠ ⎥⎦
The closest standard 1% value for R5 is 422kΩ.
Using standard 1% resistor values, the external circuit's
nominal “power-is-good” and “power-is-not-good” output
voltages are:
VOUT(GOOD) = +46V
VOUT(NOT GOOD) = +43.2V
In solving for VOUT(GOOD), substitute VFBH for VFBL in
Equation 9.
Sense Resistor Selection
The sense resistor is nominally valued at:
(8)
1.313V
= 13.13kΩ
100 µA
The closest standard 1% value for R2 is 13kΩ. Now,
solving for R1 yields:
R SENSE(NOM) =
⎡⎛ V
⎞ ⎤
⎡⎛ 37V ⎞ ⎤
ONH(EX)
⎟⎟ − 1⎥ = 13kΩ × ⎢⎜
R1= R2 × ⎢⎜⎜
⎟ − 1⎥ = 353.3 kΩ
⎣⎝ 1.313V ⎠ ⎦
⎢⎣⎝ VONH(TYP) ⎠ ⎥⎦
VTRIP(TYP)
(10)
IHOT_SWAP(NOM)
where VTRIP(TYP) is the typical (or nominal) circuit breaker
threshold voltage (47mV) and IHOT_SWAP(NOM) is the nominal
inrush load current level to trip the internal circuit breaker.
To accommodate worse-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 MIC2587/MIC2587R's minimum current limit
threshold voltage is 39mV, the minimum hot swap load
current is determined where the sense resistor is 3% high:
The closest standard 1% value for R1 is 357kΩ.
Using standard 1% resistor values, the external circuit's
nominal ON and OFF thresholds are:
VON(EX) = +36V
VOFF(EX) = +34V
In solving for VOFF(EX), replace VONH with VONL in Equation 8.
Output Voltage Power-is-Good Detection
The MIC2587/MIC2587R includes an analog comparator
used to monitor the output voltage of the controller through
an external resistor divider as shown in the “Typical
Application” circuit. The FB input pin is connected to the
non-inverting input and is compared against an internal
reference voltage. The analog comparator exhibits a
hysteresis of 80mV.
Setting the “Power-is-Good” threshold for the circuit follows
October 2004
VFBL(TYP)
The closest standard 1% value for R6 is 12.4kΩ. Now,
solving for R5 yields:
Given VONH and R2, a value for R1 can be determined. A
suggested value for R2 is that which will provide
approximately 100µA of current through the voltage divider
chain at VCC = VONH. This yields the following as a starting
point:
R2 =
(9)
IHOT_SWAP(MIN) =
39mV
(1.03 × R SENSE(NOM) )
=
37.9mV
R SENSE(NOM)
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 RSENSE has been calculated, it is good practice to check
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the maximum hot swap load current (IHOT_SWAP(MAX)) which
the circuit may let pass in the case of tolerance build-up in
the opposite direction. Here, the worse-case maximum is
found using a VTRIP(MAX) threshold of 55mV and a sense
resistor 3% low in value:
IHOT_SWAP(MAX) =
55mV
(0.97 × R SENSE(NOM) )
=
across RSENSE is sampled in such a way that the high
currents through the power traces will not introduce
significant 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. The Kelvin sense signal traces should be
symmetrical with equal length and width, kept as short as
possible and isolated from any noisy signals and planes.
Additionally, for designs that implement Kelvin sense
connections that exceed 1” in length and/or if the Kelvin
(signal) traces are vulnerable to noise possibly being
injected onto these signals, the example circuit shown in
Figure 7 can be implemented to combat noisy
environments. This circuit implements a 1.6 MHz lowpass filter to attenuate higher frequency disturbances on
the current sensing circuitry. However, individual system
analysis should be used to determine if filtering is
necessary and to select the appropriate cutoff frequency
for each specific application.
56.7mV
R SENSE(NOM)
In this case, the application circuit must be sturdy enough
to operate over a ~1.5-to-1 range in hot swap load
currents. For example, if an MIC2587 circuit must pass a
minimum hot swap load current of 4A without nuisance
trips, RSENSE should be set to:
R SENSE(NOM) =
39mV
= 9.75m Ω
4A
where the nearest 1% standard value is 9.76mΩ. At the
other tolerance extremes, IHOT_SWAP(MAX) for the circuit in
question is then simply:
IHOT_SWAP(MAX) =
56.7mV
= 5.8A
9.76mΩ
Other Layout Considerations
Figure 8 is a recommended PCB layout diagram for the
MIC2587-2BM. Many hot swap applications will require
load currents of several amperes. Therefore, the power
(VCC and Return) trace widths (W) need to be wide enough
to allow the current to flow while the rise in temperature for
a given copper plate (e.g., 1oz. or 2oz.) is kept to a
maximum of 10°C to 25°C. Also, these traces should be
as short as possible in order to minimize the IR drops
between the input and the load. The feedback network
resistor values in Figure 8 are selected for a +24V
application. The resistors for the feedback (FB) and ON
pin networks should be placed close to the controller and
the associated traces should be as short as possible to
improve the circuit’s noise immunity. The input “clamping
diode” (D1) is referenced in the “Typical Application Circuit”
on Page 1. If possible, use high-frequency PCB layout
techniques around the GATE circuitry (shown in the
“Typical Application Circuit”) and use a dummy resistor
(e.g., R3 = 0Ω) during the prototype phase.
If R3 is
needed to eliminate high-frequency oscillations, common
values for R3 range between 4.7Ω to 20Ω for various
power MOSFETs. Finally, the use of plated-through vias
will be needed to make circuit connection to the power and
ground planes when utilizing multi-layer PCBs.
MOSFET and Sense Resistor Vendors
Device types, part numbers, and manufacturer contacts for
power MOSFETs and sense resistors are provided in
Table 1.
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 = I2 × R. Here,
The current is IHOT_SWAP(MAX) = 5.8A and the resistance
RSENSE(MIN) = (0.97)(RSENSE(NOM)) = 9.47mΩ. Thus, the sense
resistor's maximum power dissipation is:
PMAX = (5.8A)2 × (9.47mΩ) = 0.319W
A 0.5W sense resistor is a good choice in this application.
When the MIC2587/MIC2587R’s foldback current limiting
circuit is engaged in the above example, the current limit
would nominally fold back to 1.23A when the output is
shorted to ground.
PCB Layout Recommendations
4-Wire Kelvin Sensing
Because of the low value typically required for the sense
resistor, special care must be used to accurately measure
the voltage drop across it. Specifically, the measurement
technique across RSENSE must employ 4-wire Kelvin
sensing. This is simply a means of ensuring that any
voltage drops in the power traces connected 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 VCC through RSENSE and then to the drain of the
N-channel power MOSFET) flows directly through the
power PCB traces and through RSENSE. The voltage drop
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Figure 7. Current Limit Sense Filter for Noisy
Systems
Figure 6. 4-Wire Kelvin Sense Connections for RSENSE
Figure 8. Recommended PCB Layout for Sense Resistor,
Power MOSFET, Timer and Feedback Network.
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MOSFET Vendors
Key MOSFET Type(s)
Breakdown Voltage (VDSS)
Contact Information
SUM75N06-09L (TO-263)
SUM70N06-11 (TO-263)
SUM50N06-16L (TO-263)
60V
60V
60V
www.siliconix.com
(203) 452-5664
SUP85N10-10 (TO-220AB)
SUB85N10-10 (TO-263)
SUM110N10-09 (TO-263)
SUM60N10-17 (TO-263)
100V
100V
100V
100V
www.siliconix.com
(203) 452-5664
International Rectifier
IRF530 (TO-220AB)
IRF540N (TO-220AB)
100V
100V
www.irf.com
(310) 322-3331
Renesas
2SK1298 (TO-3PFM)
2SK1302 (TO-220AB)
2SK1304 (TO-3P)
60V
100V
100V
www.renesas.com
(408) 433-1990
Vishay - Siliconix
Resistor Vendors
Vishay - Dale
Sense Resistors
Contact Information
“WSL” and “WSR” Series
www.vishay.com/docswsl_30100.pdf
(203) 452-5664
IRC
October 2004
www.irctt.com/pdf_files/OARS.pdf
“OARS” Series
www.irctt.com/pdf_files/LRC.pdf
“LR” Series
(828) 264-8861
second source to “WSL”
Table 1. MOSFET and Sense Resistor Vendors
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Package Information
8-Pin SOIC (M)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 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 a Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any
damages resulting from such use or sale.
© 2004 Micrel, Incorporated.
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