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MIC2586-2BM

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
Micrel, Inc.
December 2012
13
M9999-122012
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 MIC2586/MIC2586R 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 MIC2586 and
MIC2586R 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 MIC2586/MIC2586R 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 filter capacitor is recommended to help reject
additional noise.
Start-Up Cycle
When the power supply voltage to the
MIC2586/MIC2586R is higher than the V
UVH
and the
V
ONH
threshold voltages, a start cycle is initiated. When
the controller is enabled, an internal 16µA current source
(I
GATEON
) is enabled and the GATE pin voltage rises from
0V with respect to ground at a rate equal to Equation 1:
GATE
GATEONGATE
C
I
dt
dV
=
Eq. 1
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 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.
C
GATE
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 MIC2586/MIC2586R'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 Equation 2:
GATE
GATEON
LOAD
INRUSH
C
I
CI ×=
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" subsection.
A capacitor connected to the controller’s TIMER pin sets
the value of overcurrent detector delay, t
FLT
, 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 C
TIMER
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 C
TIMER
is given in the "Circuit Breaker Operation"
subsection.
Overcurrent Protection
The MIC2586 and the MIC2586R 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 14) and the SENSE
connection (Pin 13) 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.
Micrel, Inc.
December 2012
14
M9999-122012
The nominal current limit is determined by Equation 3:
SENSE
TRIP(TYP)
LIMIT
R
V
I =
Eq. 3
where V
TRIP(TYP)
is the typical current limit threshold
specified in the datasheet and R
SENSE
is the value of the
selected sense resistor. As the MIC2586 and the
MIC2586R 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 V
TRIP
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 V
TIMERH
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 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
MIC2586/MIC2586R’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 MIC2586/MIC2586R’s GATE drive to
force a constant 12mV (typical) voltage drop across the
external sense resistor.
Circuit Breaker Operation
The MIC2586/MIC2586R employ an electronic circuit
breaker that protects the external N-channel 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, R
SENSE
,
connected between the circuit’s VCC pin and SENSE
pin. For the MIC2586/MIC2586R, a fault current timing
circuit is set via an external capacitor (C
TIMER
) that
determines the length of the time delay (t
FLT
) 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 Equation 4:
(μμFC20(ms)t
I
VC
(ms)t
FILTERFLT
TIMERUP
TIMERHFILTER
FLT
×=
×
=
Eq. 4
Whenever the voltage across R
SENSE
exceeds the
MIC2586/MIC2586R’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
R
SENSE
, and the control loop is designed to hold the
voltage across R
SENSE
equal to 47mV. This feature
protects both the load and the MIC2586/MIC2586R
circuits from excessively high currents.
2. Capacitor C
TIMER
is then charged up to the V
TIMERH
threshold (1.313V) by an internal 65µA current
source (I
TIMERUP
). If the excessive current persists
such that the voltage across C
TIMER
crosses the
V
TIMERH
threshold, the circuit breaker trips and the
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 C
TIMER
should be
selected to allow the circuit's minimum regulated
output current (I
OUT
) to equal I
LIMIT
for somewhat
longer than the time it takes to charge the total load
capacitance.
An initial value for C
TIMER
is found by calculating the time
it will take for the MIC2586/MIC2586R to completely
charge up the output capacitive load. Assuming the load
is enabled by the PWRGDx (or /PWRGDx) signal(s) of
the controller, the turn-on delay time is derived from the
following expression, I = C × (dV/dt):
LIMIT
CC(MAX)LOAD
ON-TURN
I
VC
t
×
=
Eq. 5
Micrel, Inc.
December 2012
15
M9999-122012
Using parametric values for the MIC2586/MIC2586R, an
expression relating a worse-case design value for
C
TIMER
, using the MIC2586/MIC2586R specification
limits, to the circuit's turn-on delay time is:
−
−
−
×
×
=
×=
×
=
sec
µF
10
94tC
1.280V
120µA
tC
V
It
C
6
ONTURNTIMER(MAX)
ON
TURN
TIMER(MAX)
)TIMERH(MIN
X)TIMERUP(MA
ON-TURN
TIMER(MAX)
Eq. 6
For example, in a system with a C
LOAD
= 1000µF, a
maximum V
CC
= +72V, and a maximum load current on a
nominal +48V buss of 1.65A, the nominal circuit design
equations steps are:
1. Choose I
LIMIT
= I
HOT_SWAP(NOM)
= 2A (1.65A + 20%)
2. Select an R
SENSE
(Closest 1% standard value is
19.6mΩ)
3. Using I
CHARGE
= I
LIMIT
= 2A, the application circuit
turn-on time is calculated using Equation 5:
( )
36ms
2A
72V1000μ0
t
ON-TURN
=
×
=
Allowing for capacitor tolerances and a nominal 36ms
turn-on time, an initial worse-case value for C
TIMER
is:
3.38µF
sec
μF
10940.036sC
6-
TIMER(MAX)
=××=
Eq. 7
The closest standard ±5% tolerance capacitor value is
3.3µF and would be a good initial starting value for
prototyping.
Whenever the MIC2586 is not in current limit, C
TIMER
is
discharged to GND by an internal 3.5µA current sink
(I
TIMERDN
).
For the MIC2586R, the circuit breaker automatically
resets after (20) t
FLT_AUTO
time constants. If the fault
condition still exists, capacitor C
TIMER
will begin to charge
up to the V
TIMERH
threshold, and if exceeded, trip the
circuit breaker.
Capacitor C
TIMER
will then be discharged by I
TIMERDN
until
the voltage across C
TIMER
drops below the V
TIMERL
threshold, at which time another start cycle is initiated.
This will continue until any of the following occurs:
a) The fault condition is removed.
b) The input supply voltage power is removed/cycled
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:
(
)
(
)
×
=
−
×
×=
×=
−
−
µF
ms
250C
t
I
V
VC
20t
t20t
TIMERRESTART
AUTO
TIMERUP
TIMERLTIMERHTIMER
RESTART
AUTO
FLT_AUTORTAUTO_RESTA
Eq. 8
The auto-restart period for the example above where the
worse-case C
TIMER
was calculated to be 3.3µF is:
825mst
RESTART-
AUTO
=
Input Undervoltage Lockout
The MIC2586/MIC2586R 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 V
UVH
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 V
UVL
threshold
voltage (7.5V typical).
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MIC2586-2BM

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IC CTRLR/SEQUENCE HOTSWAP 14SOIC
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