MIC2587/MIC2587R
Single-Channel, Positive High-Voltage
Hot Swap Controller
Revision 2.0
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 ensure
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 activeHIGH (−1YM) or an active-LOW (−2YM) “Power-is-Good”
signal to indicate that the output load voltage is within
tolerance of the application circuit’s design objective.
• MIC2587: Pin-for-pin functional equivalent to the
LT1641
• Operates from +10V to +80V with 100V ABS MAX
operation
• Programmable current limit with analog foldback
• Active current regulation minimizes inrush current
• Electronic circuit breaker for overcurrent fault protection:
− Output latch off (MIC2587)
− Output auto-retry (MIC2587R)
• Fast responding circuit breaker (< 1
s
o
short
) t circuit
loads
• Programmable input undervoltage lockout
• Open-drain “Power-is-Good” output for enabling DC/DC
converter(s):
− Active-HIGH: MIC2587-1/MIC2587R-1
− Active-LOW: MIC2587-2/MIC2587R-2
• Fault Reporting
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
•
•
•
•
•
•
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
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
January 24, 2013
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
Typical Application
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MIC2587/MIC2587R
Ordering Information
Part Number
PWRGD Polarity
Circuit Breaker Function
Package
Finish
MIC2587-1YM
Active-HIGH
Latched
8-Pin SOIC
Pb-Free
MIC2587-2YM
Active-LOW
Latched
8-Pin SOIC
Pb-Free
MIC2587R-1YM
Active- HIGH
Auto-retry
8-Pin SOIC
Pb-Free
MIC2587R-2YM
Active-LOW
Auto-retry
8-Pin SOIC
Pb-Free
Pin Configuration
8-pin SOIC (M)
MIC2587-1YM
MIC2587R-1YM
8-pin SOIC (M)
MIC2587-2YM
MIC2587R-2YM
(Top View)
(Top View)
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 IGATEOFF. After a load current fault,
toggling the ON pin LOW then back HIGH will reset the circuit breaker and 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).
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MIC2587/MIC2587R
Pin Description (Continued)
Pin Number
3
Pin Name
PWRGD
(MIC2587-1)
(MIC2587R-1)
Active-HIGH
/PWRGD
(MIC2587-2)
(MIC2587R-2)
Active-LOW
Pin Function
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).
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 the DC/DC
module or other load is equipped with a internal pull-up impedance.
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 sets the time (tFLT)
for which the controller is allowed to remain in current limit before “tripping” the circuit breaker.
The overcurrent filter is designed to prevent nuisance “tripping” of the circuit breaker that can be
caused by transient current spikes or other undesired “noise”. 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 GATE output attempts another start cycle to re-establish the output
voltage when the circuit breaker is tripped upon an overcurrent fault. The capacitor connected to
the TIMER pin sets the period of auto-retry with a fixed, nominal duty cycle of 5%.
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, an 80mA pull-down current
source.
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MIC2587/MIC2587R
Pin Description (Continued)
Pin Number
7
Pin Name
SENSE
Pin Function
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
limit0020timer), 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 circuit breaker trip voltage
decreases linearly to 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.
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VCC
Positive Supply Voltage Input: This pin is the positive supply input to the controller and 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
ON, PWRGD, /PWRGD ........................................ 0V to VCC
FB ...................................................................... 0V to VOUT
GATE .................................................. 0V to (VCC + ∆VGATE)
Ambient Temperature Range (TA) .............. −40°C to +85°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
Junction Temperature (TJ) ....................................... +125°C
Lead Temperature (Soldering, 10s) ......................... +260°C
Storage Temperature .........................–65°C ≤ TA ≤ +150°C
(3)
ESD Rating
Human Body Model ................................................. 2kV
Machine Model ............................................................. 200V
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 TA = −40°C to +85°C.
Symbol
Parameter
VCC
Supply Voltage
ICC
Supply Current
VUVH
VUVL
Supply Voltage Undervoltage Lockout
Condition
Min.
Typ.
Max.
Units
80
V
2
5
mA
10
VON = VFB = 1.5V
VCC rising
7.5
8.0
8.5
VCC falling
7.0
7.5
8.0
VHYSLO
VCC Undervoltage Lockout Hysteresis
VFBH
Feedback Pin Voltage High Threshold
FB Low-to-High transition
1.280
1.313
1.345
V
VFBL
Feedback Pin Voltage Low Threshold
FB High-to-Low transition
1.208
1.233
1.258
V
VHYSFB
Feedback Voltage Hysteresis
∆VFB
FB Pin Threshold Line Regulation
10V ≤ VCC ≤ 80V
IFB
FB Pin Input Current
Circuit Breaker Trip Voltage,
VTRIP
VCC -
(5)
VSENSE
500
V
80
mV
−0.05
0.05
mV/V
0V ≤ VFB ≤ 3V
−1
1
µA
VFB = 0V (see Figure 1)
5
12
17
VFB = 1V (see Figure 1)
39
47
55
∆VGATE
MOSFET Gate Drive, VGATE - VCC
+10V ≤ VCC ≤ +80V
7.5
IGATEON
GATE Pin Pull-Up Current
Start cycle, VGATE = 7V
−10
GATE Pin rapid pull-down current
during a fault condition, until
(VCC − VSENSE) = (VTRIP + 10mV)
30
VGATE = VGATE[TH]
VGATE = 5V
IGATEFLT
mV
mV
18
V
−16
-22
µA
80
200
mA
(VGATE[TH] is the MOSFET threshold)
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, 1.5kΩ in series with 100pF.
4.
Specification for packaged product only.
5.
Circuit breaker trip threshold is verified by monitoring the TIMER pin as it switches from discharging to charging (current), and by the GATE output
voltage going low.
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MIC2587/MIC2587R
DC Electrical Characteristics(4) (Continued)
VCC = +24V and +48V, TA = +25°C, unless otherwise noted. Bold indicates specifications apply over the full operating temperature
range of TA = −40°C to +85°C.
Symbol
Parameter
Condition
Min.
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
ITIMERDN
Timer Pin Pull-Down Current
VTIMERH
(VCC – VSENSE) > VTRIP
Typ.
Max.
1.8
Units
mA
−24
-65
−120
µA
1.5
3.5
5
µA
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
VTIMER = 0V
(VCC – VSENSE) < VTRIP
VTIMER = 0.6V
80
0V ≤ VON ≤ 80V
mV
2
µA
IOL = 1.6mA
0.4
V
IOL = 4mA
0.8
PWRGD or /PWRGD = LOW
VOL
IOFF
Power-Good Output Voltage
Power-Good Leakage Current
PWRGD or /PWRGD = Open-Drain
10
VPG = VCC, VON = 1.5V
µA
AC Electrical Characteristics
VCC = +24V and +48V, TA = 25°C, unless otherwise noted. Bold indicates specifications apply over the full operating temperature range
of TA = −40°C to +85°C.
Symbol
Parameter
Condition
tPONLH
ON High to GATE High
CGATE = 10nF
3
ms
tPONHL
ON Low to GATE Low
VIN = 48V, CGATE = 10nF
1
ms
tPFBLH
FB Valid to PWRGD High
(MIC2587/MIC2587R-1)
RPG = 50kΩ pull-up to 48V
2
µs
tPFBHL
FB Invalid to PWRGD Low
(MIC2587/MIC2587R-1)
RPG = 50kΩ pull-up to 48V
4
µs
tPFBHL
FB Valid to /PWRGD Low
(MIC2587/MIC2587R-2)
RPG = 50kΩ pull-up to 48V
4
µs
tPFBLH
FB Invalid to /PWRGD High
(MIC2587/MIC2587R-2)
RPG = 50kΩ pull-up to 48V
2
µs
tOCSENSE
Overcurrent Sense to GATE Low
(VCC - VSENSE) = (VTRIP + 10mV)
Trip Time
Figure 7
January 24, 2013
Min.
CL=100pF
CL=100pF
CL=100pF
CL=100pF
7
Typ.
1
Max.
2
Units
µs
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
Timing Diagrams
Figure 1. Foldback Current-Limit Transfer Characteristic
Figure 2. ON-to-GATE Timing
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
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MIC2587/MIC2587R
Typical Characteristics
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MIC2587/MIC2587R
Typical Characteristics (Continued)
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MIC2587/MIC2587R
Functional Characteristics
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MIC2587/MIC2587R
Functional Diagram
MIC2587/MIC2587R Block Diagram
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MIC2587/MIC2587R
where CGATE is the total capacitance seen at the GATE
output of the controller (external capacitor from GATE to
ground plus CGS of the external MOSFET). 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 with a maximum gatesource voltage of 20V can be used for +48V and +24V
applications, respectively. However, due to the harsh
electrical environments of most backplanes and other
“live” power supplies, the use of 100V and 60V power
MOSFETs, respectively, is recommended to withstand
transient spikes caused by stray inductances.
Additionally, an external Zener diode (18-V) connected
from the source to the gate as shown in the typical
applications circuit is also recommended. A good choice
for an 18-V Zener diode in this application is the
MMSZ5248B, available in a small SOD123 package.
C4 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 other on-board components.
The MIC2587/MIC2587R was 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.
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, heavier trace plating will
help minimize inductive spikes that may arise during
events that cause a large di/dt to occur (e.g., short circuit
loads). External surge protection, such as a clamping
diode, is also recommended as an added safeguard for
device, and system protection. And 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.
IINRUSH = CLOAD ×
January 24, 2013
Eq. 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 the output to latch-off. The
MIC2587/MIC2587R controller is most often utilized in
applications with large capacitive loads, 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.
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
Equation 1:
dVGATE IGATEON
=
dt
CGATE
IGATEON
CGATE
Eq. 1
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MIC2587/MIC2587R
Programming the response time of the overcurrent
detector helps to prevent nuisance tripping of the circuit
breaker attributed to transient current surges (e.g., inrush
current charging bulk load capacitance). The nominal
overcurrent response time (tFLT) is approximated by
Figure 4:
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 section for further details.
The nominal current limit is determined by Equation 3:
ILIM =
VTRIP(TYP)
R SENSE
t FLT =
t FLT (ms) ≅ 20 × C TIMER (µF)
Eq. 3
Eq. 4
The typical overcurrent filter delay time for several
standard value capacitors is listed in Table 1.
where VTRIP(TYP) is the typical current-limit threshold
specified in the datasheet and RSENSE is the value of the
selected sense resistor. The controllers employ a
constant-current regulation scheme while in current limit.
The internal charge pump’s output voltage, seen 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 current-limit condition goes away before the TIMER
pin voltage rises above the VTIMERH threshold, then
steady-state operation resumes. To prevent excessive
power dissipation in the external MOSFET under load
current fault conditions, the FB pin voltage is used as an
input to a circuit that lowers the current limit as a function
of the FB pin 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.
The transfer characteristic of this foldback current limit
subcircuit is shown in Figure 1. When VOUT = VFB = 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.
Table 1. Overcurrent Filter Delay Time for Several Standard
Capacitor Values
CTIMER (µF)
tFLT (ms)
0.1
2
0.22
4.4
0.33
6.6
0.47
9.4
1.0
20
2.2
44
3.3
66
Whenever the voltage across RSENSE exceeds the
MIC2587/MIC2587R’s circuit-breaker threshold voltage
(47mV typical) during steady-state operation, the
following two events occur:
A constant-current regulation loop engages within 1µs
after an overcurrent condition is detected by the input
sense pins monitoring the voltage across RSENSE.
An internal 65µA current source (ITIMERUP) begins to
charge CTIMER. If the excessive current persists such that
the voltage across CTIMER crosses the VTIMERH threshold
(1.313V, typically), the circuit breaker trips and the GATE
pin is immediately pulled low by a 80mA (typical) internal
current sink while the TIMER pin is discharged to ground
by a 3.5µA current sink (ITIMERDN).
Circuit Breaker Operation
The MIC2587/MIC2587R is equipped with 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 controller’s VCC (Pin 8) and
SENSE (Pin 7) pins. For the MIC2587/MIC2587R, a fault
current timing circuit is set via an external capacitor
CTIMER that determines the length of the time delay for
which the controller remains in current limit before the
circuit breaker is tripped.
January 24, 2013
C TIMER × VTIMERH
I TIMERUP
An initial value for CTIMER is found by calculating the time
it will take for the MIC2587/MIC2587R to completely
charge the output capacitive load during startup. The
turn-on delay time is derived from the expression, I = C ×
(dV/dt):
t TURN-ON =
14
CLOAD × VCC(MAX)
Eq. 5
ILIMIT
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
Using parametric values for the MIC2587/MIC2587R, an
expression relating a worst-case design value for CTIMER,
using the MIC2587/MIC2587R specification limits, to the
circuit's turn-on delay time is:
t
C
TIMER(MIN)
=
TURN − ON
×I
A standard 3.3µF ±5% tolerance capacitor would be a
good initial starting value for prototyping since this value
would allow the controller to start up without nuisance
tripping over the entire voltage range given in the
example application.
Auto-Retry Period
For the MIC2587R, once the overcurrent timer “times out”
and the circuit breaker “trips”, the TIMER pin begins to
discharge. Once the timer pin voltage discharges below
the VTIMERL threshold, the circuit breaker resets to initiate
another start-up cycle. If the overcurrent fault condition is
still present, then the auto-retry cycle 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 nominal period of the autorestart cycle is given by:
TIMERH(MAX)
V
TIMERH(MIN)
C
C
TIMER(MIN)
TIMER(MIN)
=t
=t
120μ2
1.280V
TURN − ON
×
TURN − ON
× 94 × 10 − 6
Eq. 6
μF
sec.
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 initial steps for the
circuit design are:
t AUTO-RETRY = 20 ×
• Choose ILIMIT = IHOT_SWAP(nom) = 2A (1.65A + 20%)
• Select an RSENSE (Closest 1% standard value is
19.6mΩ)
• Using ICHARGE = ILIMIT = 2A, the application circuit turnon time is calculated using Equation 5:
t TURN-ON =
tAUTO-RETRY
(ms)
(C TIMER ) × (VTIMERH − VTIMERL )
ITIMERH
= 250 × CTIMER (µF)
Eq. 7
The auto-restart period for the example above where the
worst-case CTIMER was calculated to be 3.3µF is:
(1000µF × 72V ) = 36ms
2A
tAUTO-RETRY = 825ms
• Allowing for capacitor tolerances and a maximum
36ms turn-on time, an initial worst-case value for
CTIMER is:
C
TIMER(MIN)
= 0.036s × 94 × 10 − 6
The typical auto-restart period for several standard value
capacitors is listed in Table 2.
μF
Table 2. Auto-Restart Period for Several Standard
Capacitor Values
CTIMER (µF)
tAUTO-RETRY (ms)
= 3.3µF
sec.
0.1
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15
25
0.22
55
0.33
82.5
0.47
117.5
1.0
250
2.2
550
3.3
825
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
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), 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).
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.
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Micrel, Inc.
MIC2587/MIC2587R
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 a similar approach as setting the circuit’s ON/OFF
input voltage. The equations to set the trip points are
shown below. For the +48V telecom application shown in
Equation 9, power-is-good output signal PWRGD (or
/PWRGD) is to be de-asserted when the output supply
voltage is lower than +48V-10% (+43.2V):
Application 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 example illustrated in Equation
8, the supply voltage needed to turn on the controller is
set to +37V, a value commonly used in +48V Central
Office power distribution applications.
R1 + R2
VIN(ON) = 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:
Eq. 8
R6 =
Given VONH and R2, a value for R1 can be determined. A
suggested value for R2 is that which will provide at least
100µA of current through the voltage divider chain at VCC
= VONH. This yields the following as a starting point:
R2 =
VONH(TYP)
100μA
=
Eq. 9
VFBL(TYP)
100μ0
=
1.233V
100μ0
= 12.33 kΩ
Eq. 10
The closest standard 1% value for R6 is 12.4kΩ. Now,
solving for R5 yields:
1.313V
= 13.13k Ω
100μA
VOUT(NOT GOOD)
− 1 = 12.4kΩ × 43.2V − 1 = 422 kΩ
R5 = R6 ×
V
1.233V
FBL(TYP)
The closest standard 1% value for R2 is 13kΩ. Now,
solving for R1 yields:
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 and VOUT(NOT GOOD) =
+43.2V. In solving for VOUT(GOOD), substitute VFBH for VFBL
in Equation 9.
VIN(ON)
− 1 = 13kΩ × 37V − 1 = 353.3 kΩ
R1 = R2 ×
V
ONH(TYP)
1.313V
The closest standard 1% value for R1 is 357kΩ.
Using standard 1% resistor values, the external circuit's
nominal ON and OFF thresholds are VIN(ON) = +36V and
VIN(OFF) = +34V. In solving for VIN(OFF), replace VONH with
VONL in Equation 8.
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Micrel, Inc.
MIC2587/MIC2587R
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:
Sense Resistor Selection
The sense resistor is nominally valued at:
R SENSE(NOM) =
VTRIP(TYP)
Eq. 11
IHOT_SWAP(N OM)
R SENSE(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 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 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:
39mV
(1.03 × R SENSE(NOM) )
IHOT_SWAP(MIN) =
=
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(M AX) =
37.9mV
PMAX = (5.8A) × (9.47mΩ) = 0.319W
2
R SENSE(NOM)
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.
55mV
56.7mV
=
0.97 × R SENSE(NOM)
R SENSE(NOM)
(
January 24, 2013
56.7mV
= 5.8A
9.76m Ω
With a knowledge of the application circuit's maximum
hot swap load current, the power dissipation rating of the
2
sense resistor can be determined using P = I × 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:
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
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 worst-case maximum
is found using a VTRIP(MAX) threshold of 55mV and a sense
resistor 3% low in value:
IHOT_SWAP(M AX) =
39mV
= 9.75m Ω
4A
)
18
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Micrel, Inc.
MIC2587/MIC2587R
Other Layout Considerations
Figure 8 is a recommended PCB layout diagram for the
MIC2587-2YM. 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 highfrequency 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.
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 4wire 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.
Figure 6. 4-Wire Kelvin Sense Connections for RSENSE
The voltage drop 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.
In most applications, the use of a capacitor from the
TIMER pin to ground will effectively eliminate nuisance
tripping due to noise and/or transient overcurrent spikes.
If the circuit breaker trips regularly due to a system
environment that is vulnerable to noise being injected
onto the Kelvin sense connections, the example circuit
shown in Figure 7 can be implemented to combat such
noisy environments. This circuit implements a 1.6 MHz
low-pass 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.
January 24, 2013
Figure 7. Current-Limit Sense Filter for Noisy Systems
19
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
Figure 8. Recommended PCB Layout for Sense Resistor,
Power MOSFET, Timer and Feedback Network
January 24, 2013
20
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
MOSFET and Sense Resistor Vendors
Device types, part numbers, and manufacturer contacts
for power MOSFETs and sense resistors are provided in
Tables 3 and 4.
Table 3. MOSFET Vendors
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
Table 4. Resistor Vendors
Resistor Vendors
Vishay - Dale
January 24, 2013
Sense Resistors
Contact Information
“WSL” and “WSR” Series
www.vishay.com/docswsl_30100.pdf
21
Revision 2.0
Micrel, Inc.
MIC2587/MIC2587R
Package Information(1)
8-Pin SOIC (M)
Note:
1. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com.
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
Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This
information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry,
specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual
property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability
whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties
relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right.
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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