MIC2587-2BM TR Micrel Inc, MIC2587-2BM TR Datasheet - Page 10

• Manufacturer Part Number: MIC2587-2BM TR
• Description: IC CTRLR HOT SWAP POS HV 8-SOIC
• Manufacturer: Micrel Inc
• Type: Hot-Swap Controllerr

Specifications of MIC2587-2BM TR

• Applications: General Purpose
• Internal Switch(s): No
• Voltage - Supply: 10 V ~ 80 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: MIC2587-2BMTR; MIC2587-2BMTR

the current-limit condition goes away before the TIMER pin
voltage rises above the V
state operation resumes.
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 MIC2587/MIC2587R’s FB pin
voltage also approaches 0V), the result is a proportionate
decrease in the maximum current allowed into the load.
This
characteristic is shown in Figure 1. Under excessive load
conditions (output and FB voltage equals 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 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
the
MIC2587/MIC2587R, a fault current timing circuit is set via
an external capacitor (C
the time delay (t
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 the following equation:
Whenever the voltage across R
MIC2587/MIC2587R’s nominal circuit breaker threshold
voltage of 47mV during steady-state operation, two things
occur:
Micrel
October 2004
1. A constant-current regulation loop will engage
2. Capacitor C
t
t
FLT
FLT
circuit’s
foldback
(ms)
within 1µs after an overcurrent condition is
detected by R
designed to hold the voltage across R
to 47mV. This feature protects both the load and
the MIC2587/MIC2587R circuits from excessively
high currents.
threshold (1.313V) by an internal 65µA current
source (I
such that the voltage across C
V
=
TIMERH
C
FILTER
=
20
I
TIMERUP
V
threshold, the circuit breaker trips and the
×
×
TIMERUP
CC
FLT
C
V
FILTER
current
TIMERH
TIMER
) for which the controller remains in
pin
). If the excessive current persists
(
TIMER
SENSE
µ
is then charged up to the V
F)
and
TIMERH
) that determines the length of
To prevent excessive power
, and the control loop is
limit
SENSE
SENSE
threshold, then steady-
subcircuit’s
, connected between
SENSE
TIMER
pin.
exceeds the
crosses the
SENSE
(4)
For
transfer
TIMERH
equal
the
10
An initial value for C
will take for the MIC2587/MIC2587R to completely charge
up the output capacitive load. Assuming the load is
enabled by the PWRGD (or /PWRGD) signal of the IC, the
turn-on delay time is derived from the following expression,
I = C × (dV/dt):
Using parametric values for the MIC2587/MIC2587R, an
expression relating a worse-case design value for C
using the MIC2587/MIC2587R specification limits, to the
circuit's turn-on delay time is:
For example, in a system with a C
maximum V
nominal +48V buss of 1.65A, the nominal circuit design
equations steps are:
Allowing for capacitor tolerances and a nominal 36ms turn-
on time, an initial worse-case value for C
The closest standard ±5% tolerance capacitor value is
3.3µF and would be a good initial starting value for
prototyping.
Whenever the MIC2587 is not in current limit, C
discharged to GND by an internal 3.5µA current sink
(I
For the MIC2587R, the circuit breaker automatically resets
after (20) t
exists, capacitor C
TIMERDN
1. Choose I
2. Select an R
3. Using I
C
C
C
C
t
t
TURN-ON
TURN-ON
TIMER(MAX)
TIMER(MAX)
TIMER
TIMER(MAX)
19.6mΩ);
turn-on time is calculated using Equation 5:
).
GATE pin is immediately pulled low by a 30mA
(minimum) internal current sink.
turns off the MOSFET quickly and disconnects the
input from the load. The value of C
selected to allow the circuit's minimum regulated
output current (I
longer than the time it takes to charge the total
load capacitance.
(MAX)
FLT_AUTO
=
=
CC
C
(
CHARGE
1000
=
=
=
=
= +72V, and a maximum load current on a
LOAD
LIMIT
t
0.036s
t
t
TURN
TURN
TURN
I
µ
time constants. If the fault condition still
LIMIT
2A
× V
SENSE
= I
TIMER
F × 72V
TIMER
-
-
-
V
ON
ON
ON
= I
CC(MAX)
TIMERH(MIN
HOT_SWAP(nom)
×
×
×
×
⎛
⎜
⎝
LIMIT
OUT
94
⎛
⎜
⎝
⎛
⎜
⎝
I
TIMERUP(MA
is found by calculating the time it
will begin to charge up to the
1.280V
94
(Closest 1% standard value is
120
)
×
) to equal I
= 36ms
×
10
= 2A, the application circuit
µ
10
)
A
6 -
6 -
⎞
⎟
⎠
sec
µ
X)
sec
µ
F
= 2A (1.65A + 20%);
F
⎞
⎟
⎠
MIC2587/MIC2587R
⎞
⎟
⎠
=
3.38
LOAD
LIMIT
TIMER
TIMER
µ
[phone]
This operation
M9999-102204
F
for somewhat
= 1000µF, a
is:
should be
(5)
(6)
TIMER
TIMER
is
,