MIC2589 Micrel Semiconductor, MIC2589 Datasheet - Page 12

• Manufacturer Part Number: MIC2589
• Description: (MIC2589 / MIC2595) SINGLE-CHANNEL NEGATIVE HIGH VOLTAGE HOT SWAP POWER CONTROLLERS/SEQUENCERS
• Manufacturer: Micrel Semiconductor

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. For the MIC2589/89R and MIC2595/95R, a timer is
set via capacitor C
time delay (t
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:
An initial value for C
will take for the MIC2589/95 to completely charge up the
output capacitive load. Assuming the load is enabled by the
PWRGD
time is derived from the following expression,
I = C
Using parametric values specific to the MIC2589/95, an
expression relating a design nominal value for C
circuit’s turn-on delay time is:
Substituting the variables above with the specification limits
of the MIC2589/95, an expression for the worst-case value for
C
M9999-031504
C
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1. A constant-current regulation loop is engaged which
2. Capacitor C
C
t
is designed to hold the voltage across R
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
1 s from the time at which the overvoltage condition
on R
V
95 A current source. If the voltage across C
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
allow the circuit’s minimum regulated value of I
to equal I
takes to charge the total load capacitance.
TURN-ON
(dv/dt):
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CFILTER(TRIP)
(nom)
is given by:
X
(or /PWRGD
SENSE
FLT
(max) t
) for which the device remains in current limit
TRIP
t
t
TURN-ON
TURN-ON
occurs.
C
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SENSE
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LOAD
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for somewhat longer than the time it
1.25V
threshold (= 1.25V) by an internal
V
X
CFILTER
TURN-ON
t
) signal(s) of the IC, the turn-on delay
TURN-ON
, connected between the V
I
that determines the length of the
LIMIT
is charged up to an internal
is found by calculating the time it
95 A
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I
V
CFILTER
DD
(typ)
SENSE
– V
should be selected to
t
135 A
1.17V
EE
(typ)
TURN-ON
115.4 10
exceeds 50mV, two
SENSE
76 10
–6
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sec
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equal
F
OUT
EE
–6
to the
sec
and
F
12
For example, in a system with a C
(V
–48V buss of 1.65A, the nominal circuit design equations
steps are:
1. Choose I
2. Select an
3. Using I
Allowing for capacitor tolerances and a nominal 6ms turn-on
time, an initial worst-case value for C
C
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
is discharged to V
For the MIC2589R/95R devices, the circuit breaker automati-
cally resets after (20) t
condition still exists, capacitor C
to V
C
source until the voltage across C
V
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:
The auto-restart period for the example above where the
worst-case C
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
the device’s internal current sensing circuits and the SENSE
connection (Pin 9) is the other input.
The sense resistor is nominally valued at:
where V
voltage (= 50mV) and I
swap load current level to trip the internal circuit breaker in the
application.
FILTER(RETRY)
FILTER(WORST-CASE)
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DD
standard value is 19.6m );
on time is calculated:
FILTER(TRIP)
– V
t
R
t
t
AUTO-RESTART
TURN-ON
AUTO-RESTART
SENSE
EE
will then be discharged by an internal 4 A current
TRIP
CHARGE
) = 72V, and a maximum load current on a nominal
LIMIT
(typ) is the nominal circuit breaker threshold
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R
(nom)
, at which time another start cycle is initiated.
SENSE
EE
where the circuit breaker is tripped. Capacitor
= I
EE
connection (Pin 7) to the IC is one input to
= I
150 F 72V
HOT_SWAP
was determined to be 698nF is:
by an internal 4 A current source.
LIMIT
FLT
= 151ms
= 6ms x (115.4 10
I
HOT_SWAP
2A
20
C
HOT_SWAP
38.8mV
time constants (20 t
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V
= 2A, the application circuit turn-
2A
TRIP
C
(nom) = 2A (1.65A + 20%);
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(typ)
LOAD
(nom)
(nom) is the nominal hot
19.4m
5.4ms (use 6 ms)
216.8
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will again be charge up
= 150 F, a maximum
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95 A
1.25V – 0.22V
–6
ms
is:
F
(closest 1%
F/sec) = 692nF
FLT
goes below
March 2004
). If the fault
Micrel
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