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