MIC2589/2595
Micrel
Circuit Breaker Function
For example, in a system with a C
= 150µF, a maximum
LOAD
(V –V )=72V, andamaximumloadcurrentonanominal
–48V buss of 1.65A, the nominal circuit design equations
steps are:
The MIC2589/89R and the MIC2595/95R employ an elec-
troniccircuitbreakerthatprotectstheexternalpowerMOSFET
and other system components against large-scale faults,
such as short circuits. The current-limit threshold is set via an
DD
EE
1. Choose I
= I
(nom) = 2A (1.65A + 20%);
LIMIT
HOT_SWAP
external resistor, R
, connected between the V and
SENSE
EE
38.8mV
2A
standard value is 19.6mΩ);
SENSE. For the MIC2589/89R and MIC2595/95R, a timer is
set via capacitor C that determines the length of the
R
=
= 19.4mΩ
(closest 1%
2. Select an
SENSE
FILTER
time delay (t ) for which the device remains in current limit
FLT
3. Using I = I = 2A, the application circuit turn-
before the circuit breaker is tripped. This programmable
delay prevents tripping of the circuit breaker because of high
inrush current charging bulk and distributed capacitive loads.
Whenever the voltage across R
things happen:
CHARGE
LIMIT
on time is calculated:
150µF × 72V
(
)
= 5.4ms (use 6 ms)
exceeds 50mV, two
tTURN-ON
=
SENSE
2A
Allowing for capacitor tolerances and a nominal 6ms turn-on
time, an initial worst-case value for C is:
1. A constant-current regulation loop is engaged which
isdesignedtoholdthevoltageacrossR equal
FILTER
–6
SENSE
to 50mV. This protects both the load and the
MIC2589/95circuitsfromexcessivelyhighcurrents.
This current-regulation loop will engage in less than
1µs from the time at which the overvoltage condition
C
= 6ms x (115.4 × 10 µF/sec) = 692nF
FILTER(WORST-CASE)
The closest standard ±5% tolerance capacitor value is 698nF
and would be a good initial starting value for prototyping.
Wheneverthehotswapcontrollerisnotincurrentlimit, C
FILTER
on R
occurs.
SENSE
is discharged to V by an internal 4µA current source.
EE
2. Capacitor C
is charged up to an internal
FILTER
For the MIC2589R/95R devices, the circuit breaker automati-
V
threshold (= 1.25V) by an internal
CFILTER(TRIP)
cally resets after (20) t
time constants (20 × t ). If the fault
FLT
FLT
95µA current source. If the voltage across C
FILTER
condition still exists, capacitor C
will again be charge up
FILTER
crosses this threshold, the circuit breaker trips and
theGATEpinisimmediatelypulledlowbyaninternal
current pull-down. This operation turns off the
MOSFETquicklyanddisconnectstheinputfromthe
to V
C
where the circuit breaker is tripped. Capacitor
will then be discharged by an internal 4µA current
FILTER(TRIP)
FILTER
source until the voltage across C
goes below
FILTER
V
, at which time another start cycle is initiated.
FILTER(RETRY)
load. The value of C
should be selected to
FILTER
This will continue until the fault condition is removed or input
power is removed/cycled. The duty cycle of the auto-restart
function is therefore fixed at 5% and the period of the auto-
restart cycle is given by:
allow the circuit’s minimum regulated value of I
OUT
to equal I
for somewhat longer than the time it
TRIP
takes to charge the total load capacitance.
An initial value for C is found by calculating the time it
FILTER
will take for the MIC2589/95 to completely charge up the
output capacitive load. Assuming the load is enabled by the
PWRGD (or/PWRGD )signal(s)oftheIC,theturn-ondelay
time is derived from the following expression,
I = C × (dv/dt):
C
× 1.25V – 0.22V
(
)
(
)
FILTER
t
= 20 ×
AUTO-RESTART
95µA
X
X
ms
= C
× 216.8
FILTER
µF
C
× V – V
EE
(
)
The auto-restart period for the example above where the
LOAD
DD
t
=
TURN-ON
worst-case C
t
was determined to be 698nF is:
= 151ms
I
FILTER
LIMIT
AUTO-RESTART
Using parametric values specific to the MIC2589/95, an
expression relating a design nominal value for C
Current Sensing
to the
FILTER
circuit’s turn-on delay time is:
Asmentionedbefore,theMIC2589/89RandtheMIC2595/95R
use an external, low-value resistor in series with the source
of the external MOSFET to measure the current flowing into
t
×I
(typ)
(
)
TURN-ON
CFILTER
C
(nom) =
FILTER
the load. The V connection (Pin 7) to the IC is one input to
V
(typ)
EE
CFILTER
the device’s internal current sensing circuits and the SENSE
connection (Pin 9) is the other input.
t
× 95µA
µF
TURN-ON
–6
=
= t
× 76 ×10
TURN-ON
1.25V
sec
The sense resistor is nominally valued at:
Substituting the variables above with the specification limits
oftheMIC2589/95,anexpressionfortheworst-casevaluefor
VTRIP(typ)
RSENSE(nom) =
C
is given by:
IHOT_SWAP(nom)
FILTER
135µA
where V
voltage (= 50mV) and I
(typ) is the nominal circuit breaker threshold
TRIP
C
(max) = t
×
TURN-ON
FILTER
(nom) is the nominal hot
1.17V
HOT_SWAP
swaploadcurrentleveltotriptheinternalcircuitbreakerinthe
application.
µF
–6
= t
× 115.4 ×10
TURN-ON
sec
M9999-031504
12
March 2004
MICREL [ MICREL SEMICONDUCTOR ]
