MIC2587/MIC2587R
Single-Channel, Positive High-Voltage
Hot Sw ap Controller
Revision 2.0
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
General Description
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 (a lwa ys-powered) backplanes. These devices use few
external com ponents and act as controllers f or external N-
channel power MOSFET devices to pr ovide inrush current
control and output voltage slew rate control. Overcurrent
fault protection is provided via programmable analog fold-
back current-limit circuitry equipped with a programmable
overcurren t filter. These protec tion circ uits com bine to lim it
the power dissipation of the external MOSFET to ensure
that the MOSF ET is in its S OA during f ault condit ions. T he
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 (−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.
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Features
• 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 of f (MIC2587 )
− Output auto-retry (MIC2587R)
• Fast responding circuit breaker (< 1 s) to short circuit
loads
• Programmable input undervoltage lockout
• Open-drain “Power-is-Good” output for enabling DC/DC
converter(s):
− Active-HIGH: MIC 2 587-1/MIC2587R-1
− Active-LOW: MIC2587-2/MIC2587R-2
• Fault Reporting
Applications
• General-purpose hot board insertion
• High-volta ge, hig h-side electronic circuit breaker
• +12V/+24V/+48V distributed power systems
• +24V/+48V industrial/alarm systems
• Telecom systems
• Medical systems
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
2 Revision 2.0
Typical Application
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
3 Revision 2.0
Ordering Information
Part Number PWRGD Polarity Circuit Breaker Funct i on 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
(Top View)
8-pin SOIC (M)
MIC2587-2YM
MIC2587R-2YM
(Top View)
Pin Description
Pin Number
Pin Name
Pin Function
1 ON
Enable Input: When the voltage at the ON pin is higher than the VONH threshold, a start cycl e is
initiated. An inter nal curr ent sour ce (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 thre sho ld, 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 wil l reset the circuit breaker and initiate another start
cycle.
2 FB
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 pi n 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).
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
4 Revision 2.0
Pin Description (Continued)
Pin Number Pin Name Pin Function
3
PWRGD
(MIC2587-1)
(MIC2587R-1)
Active-HIGH
/PWRGD
(MIC2587-2)
(MIC2587R-2)
Active-LOW
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 PW RGD output pin will be high-i mpedance when the FB pin voltage is
higher than VFBH and will pul l 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 transi stor implemented
with high-voltage structures. These transistors are c apable of operating with pull-up resistors to
supply voltages as high as 100V.
To use this s ignal as a logic control in low-voltage DC/DC conversion applicati ons, an external
pull-up resistor between this pin and the logic supply voltage is recommended unless the DC/DC
module or other load is equi pped with a internal pull-up impedanc e.
4 GND Tie this pin direc tly to the system ’s analog GND plane.
5 TIMER
Current-Limit Response Timer: A capac itor con nected 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 di s abled, 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 restar ted by togglin g
the ON pin LOW then HIGH.
For the MIC2587R, the GATE output attempts another start cycle to re-establish the output
voltage when the circ uit brea k er is tripped upo n an overcurrent fault. The capaci tor connected to
the TIMER pin sets the period of auto-retry with a fixed, nominal duty cyc le of 5%.
6 GATE
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
minim um 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 t hr es hol d, a 16µA current source (IGATEON)
charges the GATE pin.
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.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
5 Revision 2.0
Pin Description (Continued)
Pin Number Pin Name Pin Function
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 curr ent-li mit 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 cur r ent
into the load. When the FB pin voltage is less than ≅ 0.5V, the circuit breaker trip voltage
decreases li nearly 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.
8 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.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
6 Revision 2.0
Absolute Maximum Ratings(1)
(All voltages are referred to GND)
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 +10 0V
FB pin .......................................................... −0.3V to +100V
TIMER pin ....................................................... −0.3V to +6 V
Junction Temperature (TJ) ....................................... +125°C
Lead Temperature (Soldering, 10s) ......................... +260°C
Storage Temperature .........................–65°C ≤ TA ≤ +150°C
ESD Rating(3)
Human Body Model ................................................. 2kV
Machine Mod el ............................................................. 200V
Operating Ratings(2)
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
DC Electrical Characteristic s(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 Condition Min. Typ. Max. Units
VCC Supply Voltage 10 80 V
ICC Supply Current VON = VFB = 1.5V 2 5 mA
VUVH
VUVL Supply Voltage Undervoltage Lockout VCC rising
VCC falling
7.5
7.0 8.0
7.5
8.5
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 V
VFBL Feedback Pin Voltage Low Threshold FB High-to-Low transition
1.208
1.233
1.258
V
VHYSFB Feedback Voltage Hysteresis 80 mV
∆VFB FB Pin Threshold Line Regulation 10V ≤ VCC ≤ 80V
−
0.05 0.05 mV/V
IFB FB Pin Input Current 0V ≤ VFB ≤ 3V
−
1 1 µA
VTRIP Circuit Breaker Trip Voltage,
VCC - VSENSE(5)
VFB = 0V (see Figure 1)
VFB = 1V (see Figure 1)
5
39 12
47
17
55 mV
∆VGATE MOSFET Gate Drive, VGATE - VCC +10V ≤ VCC ≤ +80V 7.5 18 V
IGATEON GATE Pin Pul l -Up Current Start cycle, VGATE = 7V
−
10 −16 -22 µA
IGATEFLT
GATE Pin rapid pull-down current
during a fault condition, until
VGATE = VGATE[TH]
(VGATE[TH] is the MOSFET threshold)
(VCC − VSENSE) = (VTRIP + 10mV)
VGATE = 5V
30
80
200
mA
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to functi on outside its operati ng r ating.
3. Devices are ESD sensitive. Handling precauti ons recommended. Human body model, 1.5kΩ in series with 100pF.
4. Specific at i on for packaged product only.
5. Circuit break er trip t hreshol d is verified by monit ori ng the TIMER pin as it switches from discharging to charging (current), and by the GATE output
voltage going low.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
7 Revision 2.0
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. Typ. Max. Units
IGATEOFF GATE Pin Turn-off Current Normal turn-off, or from VGATE[TH]
(MOSFET) to 0V after a fault condition 1.8 mA
ITIMERUP Timer Pin Charging Current (VCC – VSENSE) > VTRIP
VTIMER = 0V
−
24 -65
−
120 µA
ITIMERDN Timer Pin Pull-Down Current (VCC – VSENSE) < VTRIP
VTIMER = 0.6V 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 80 mV
ION ON Pin Input Current 0V ≤ VON ≤ 80V 2 µA
VOL Power-Good Output Voltage
PWRGD or / PWRGD = LOW
IOL = 1.6mA
IOL = 4m A
0.4
0.8 V
IOFF Power-Good Leakage Current PWRGD or /PWR GD = Open-Drain
VPG = VCC, VON = 1.5V 10 µA
AC Elect rical Chara cteristics
VCC = +24V and +48V, TA = 25°C, unless otherwise noted. Bold indicates specifications appl y over the full operating temperature range
of TA = −40°C to +85°C.
Symbol Parameter Condition Min. Typ. Max. Units
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
CL=100pF 2 µs
tPFBHL FB Invalid to PWRGD Low
(MIC2587/MIC2587R-1) RPG = 50kΩ pull-up to 48V
CL=100pF 4 µs
tPFBHL FB Valid to /PWRGD Low
(MIC2587/MIC2587R-2) RPG = 50kΩ pull-up to 48V
CL=100pF 4 µs
tPFBLH FB Invalid to /PWRGD High
(MIC2587/MIC2587R-2) RPG = 50kΩ pull-up to 48V
CL=100pF 2 µs
tOCSENSE
Overcurrent Sense to GATE Low
Trip Time
(VCC - VSENSE) = (VTRIP + 10mV)
Figure 7 1 2 µs
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
8 Revision 2.0
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
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
9 Revision 2.0
Typical Characteris tics
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
10 Revision 2.0
Typical Characteristics (Continued)
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
11 Revision 2.0
Functional Characteristics
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
12 Revision 2.0
Functional Diagram
MIC2587/MIC2587R Block Diagram
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
13 Revision 2.0
Functional Description
Hot Swap Insertion
When circuit boards are inserted into systems carrying
live s uppl y voltages ("hot s wapped "), high inrus h currents
often result due to the charging of bulk capacitance that
resides across the circuit board's supply pins. These
current s pikes can ca use the syst em's suppl y voltages to
temporarily go out of regulation causing data loss or
system lock-up. In more extreme cases, the transients
occurr ing during a hot s wap event ma y c ause perm anent
damage to connectors or other on-board components.
The MIC2587/MIC2587R was designed to address these
issues by limiting the m aximum 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 Suppre ssion 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 ca use a large di/dt to occur (e.g., shor t 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
recom mended to assist i n noise reject ion. Plac e this f ilter
capacitor as close as possible to the VCC pin of the
controller.
Start-Up Cycle
When the power supply voltage to the
MIC2587/ MIC2587 R is h igher tha n 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
dt =IGATEON
CGATE
Eq. 1
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 p o wer M OSF ET s with a maxim um gate-
source 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 a djust the G AT E voltag e slew rate whil e 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:
IINRUSH =CLOAD ×IGATEON
CGATE
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 c om plete. The pr ocedure
for selecting a value for CTIMER is given in the "Circuit
Breaker Operation" section.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
14 Revision 2.0
Overcurrent Protection
The MIC2587 and the MIC2587R use an external, low-
value 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 Equat ion 3:
SENSE
TRIP(TYP)
LIM R
V
I=
Eq. 3
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 c o nnec te d to the TIMER pin is b eing c har g e d. 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 c ircuit that lower s the current limit as a f unction
of the FB pi n voltage. W hen the load c urrent incr eases to
the point wh ere the out put voltag e at the l oad a pproaches
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.
Circuit Breaker Operation
The MIC2587/MIC2587R is equipped with an electronic
circuit break er that protect s the ex ter nal N-c hannel power
MOSFET and other system components against large-
scale 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.
Programming the response time of the overcurrent
detector helps to prevent nuisance tripping of the circuit
break er attributed to transi ent curren t surges ( e.g., inr ush
current charging bulk load capacitance). The nominal
overcurrent response time (tFLT) is approximated by
Figure 4:
TIMERUP
TIMERHTIMER
FLT IVC
t×
=
×≅ 20(ms)tFLT
TIMER
C
(µF) Eq. 4
The typical overcurrent filter delay time for several
standard value capacitors is listed in Table 1.
Table 1. Overcurrent Filter Delay Time for Several Standard
Capacitor Values
C
TIMER
(µF)
t
FLT
(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 t wo 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, t yp ically), the circ uit breaker tr ips and the GATE
pin is immediately pulled low by a 80mA (typical) internal
current sink while the T IMER pin is discharged to ground
by a 3.5µA current sink (ITIMERDN).
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 de lay tim e is derive d from the ex pressio n, I = C ×
(dV/dt):
LIMIT
CC(MAX)LOAD
ON-TURN I
VC
t×
=
Eq. 5
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
15 Revision 2.0
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:
−
−
−
−
××
=
×
=
×
=
sec.
μF
1094t
C
1.280V
120μ2
t
C
V
It
C
6
ONTURN
TIMER(MIN)
ON
TURN
TIMER(MIN)
)
TIMERH(MIN
)
TIMERH(MAXONTURN
TIMER(MIN)
Eq. 6
For example, in a system with a CLOAD = 1000µF, a
maximum VCC = + 72V, and a max imum load current o n a
nominal +48V buss of 1.65A, the initial steps for the
circuit design are:
• 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 turn-
on time is calculated using Equation 5:
( )
36ms
2A 72V1000µF
tON-TURN =
×
=
• Allowing for capacitor tolerances and a maximum
36ms turn-on time, an initial worst-case value for
CTIMER is:
−××= sec.
μF
610940.036s
C
TIMER(MIN)
= 3.3µF
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 MIC 2 58 7R, o nc e t h e o ver cur rent 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 s tart-up cycle. If the overcur rent fault cond ition 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 auto-
restart cycle is given by:
( ) ( )
TIMERH
TIMERLTIMERHTIMER
RETRY-AUTO I
VVC
20t −×
×=
tAUTO-RETRY(ms) = 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:
tAUTO-RETRY = 825ms
The typical auto-restart period for several standard value
capacitors is listed in Table 2.
Table 2. Auto-Restart Period for Several Standard
Capacitor Values
C
TIMER
(µF)
t
AUTO-RETRY
(ms)
0.1
25
0.22
55
0.33
82.5
0.47
117.5
1.0
250
2.2
550
3.3
825
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
16 Revision 2.0
Input Undervoltage Lockout
The MIC2587/MIC2587R have an internal undervoltage
lockout circuit that inhibits operation of the controller’s
internal c irc uitry unless th e po wer s u pply voltage is sta ble
and within an acceptable tolerance. If the supply voltage
to the contr oller with respe ct to gr ound 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 sig nal (PW RGD) will be hig h impedanc e 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-goo d output signal (/PW RGD) 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 /PW RGD 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. W hen used as an enab le 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 pi n.
The Power-is-Good output pin is connected to an open-
drain, N-chann el tra ns istor i mplemented wit h h igh-voltage
structures. These transistors are capable of operating
with pu ll-up resistors to supply voltages as high as 100V.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
17 Revision 2.0
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
operatio n upon card rem oval. In addi tion, the ON pi n can
be used to reset the MIC2587/MIC2587R’s internal
electronic circuit breaker in the event of a load current
fault. T o reset the elec tronic circuit break er, the ON pi n 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.
+
×= R2
R2R1
VV
ONHIN(ON)
Eq. 8
Give n VONH and R2, a va lue for R1 can be det ermined. A
suggested value for R2 is that which will provide at least
100µA of current through t he voltage divider chain at V CC
= VONH. This yields the following as a starting point:
Ω==
=13.13k
A100μ
1.313V
A100μ
V
R2
ONH(TYP)
The closest standard 1% value for R2 is 13kΩ. Now,
solving for R1 yields:
Ω=
−
×Ω
=
−
×
=k 353.3
1
1.313V
37V
13k1
VV
R2R1
ONH(TYP)
IN(ON)
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.
Output Voltage Power-is-Good Dete ction
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 s imilar appr oac h as s etting the c ircui t’s ON/ OFF
input voltage. The equations to set the trip points are
shown belo w. 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 lo wer than +48 V-10% (+43.2V):
+
×= R6R6R5
VV
FBLGOOD) OUT(NOT
Eq. 9
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:
kΩ 12.33
100μ0
1.233V
100μ0
V
R6
FBL(TYP)
===
Eq. 10
The closest standard 1% value for R6 is 12.4kΩ. Now,
solving for R5 yields:
Ω=
−
×Ω=
−
×= k 4221
1.233V
43.2V
12.4k1
V
V
R6R5 FBL(TYP)
GOOD) OUT(NOT
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.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
18 Revision 2.0
Sense Resistor Selection
The sense resistor is nominally valued at:
OM)HOT_SWAP(N
TRIP(TYP)
SENSE(NOM) IV
R=
Eq. 11
where:
VTRIP(TYP) is the typical (or nominal) circuit breaker
threshold voltage (47mV) and IHOT_SWAP(NOM) is the
nominal inrus h load cur rent level to tri p the intern al 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:
( )
SENSE(NOM)SENSE(NOM)
IN)HOT_SWAP(M R37.9mV
R1.03 39mV
I=
×
=
Keep in mind that the minimum hot swap load current
should be greater than the application circuit's upper
steady-state load c urrent b oundar y. Onc e the lo wer valu e
of RSENSE has been calculated, it is good practice to check
the max imum hot swap loa d curr ent (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 us ing a VTRIP(MAX) t hreshol d of 55m V and a sense
resistor 3% low in value:
( )
SENSE(NOM)SENSE(NOM)
AX)HOT_SWAP(M
R56.7mV
R0.97 55mV
I=
×
=
In this case, the ap plic ation circuit m ust be s turdy enough
to operate over a ~1.5-to-1 range in hot swap load
currents. For example, if an MIC2587 circuit m ust pass a
minimum hot swap load current of 4A without nuisance
trips, RSENSE should be set to:
Ω=
=9.75m
4A
39mV
RSENSE(NOM)
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:
5.8A
9.76m
56.7mV
IAX)HOT_SWAP(M =
Ω
=
With a knowledge of the application circuit's maximum
hot swap load current, the power dissipation rating of the
sense res istor can be d eterm ined 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 li mit
would nominally fold back to 1.23A when the output is
shorted to ground.
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
19 Revision 2.0
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 MO SFET) flows directl y 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 w ay
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-pas s filter to a ttenuate higher f requenc y disturban ces
on the current sensing circuitry. However, individual
system analysis s hould be used to deter mine if filtering is
necessary and to select the appropriate cutoff frequency
for each specific application.
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
temper atur e f or a given copper plate ( e.g., 1o z. or 2o z. ) is
kept to a maximum of 10°C to 25°C. Also, these traces
should b e as sh or t as pos si ble in ord er to minim ize th e IR
drops between the input and the load. The feedback
network resistor values in Figure 8 are selected for a
+24V app licatio n. T he resistor s f or the f eedback (FB) and
ON pin networks should be placed close to the contro ller
and the as sociated traces should be as short as poss ible
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
connecti on t o th e p o wer an d gr ou nd pla nes wh en uti liz ing
multi-layer PCBs.
Figure 7. Current-Limit Sense Filter for Noisy Systems
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
20 Revision 2.0
Figure 8. Recommended PCB Layout for Sense Resistor,
Power MOSFET, Timer and Feedback Network
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
21 Revision 2.0
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
Vishay - Siliconix
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
Table 4. Resistor Vendors
Resistor Vendors Sense Resistors Contact Information
Vishay - Dale “WSL” and “WSR” Series www.vishay.com/docswsl_30100.pdf
Micrel, Inc.
MIC2587/MIC2587R
January 24, 2013
22 Revision 2.0
Package Information(1)
8-Pin SOIC (M)
Note:
1. Package i nformat i on is correct as of the publication date. For updates and most current inform ation, go t o www.micrel.com.
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