Micrel
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 MIC2587/MIC2587R’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 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
SENSE
, connected between
the circuit’s V
CC
pin and SENSE pin. For the
MIC2587/MIC2587R, 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 the following equation:
t
FLT
=
C
FILTER
×
V
TIMERH
I
TIMERUP
MIC2587/MIC2587R
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 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):
t
TURN-ON
=
C
LOAD
×
V
CC(MAX)
I
LIMIT
(5)
Using parametric values for the MIC2587/MIC2587R, an
expression relating a worse-case design value for C
TIMER
,
using the MIC2587/MIC2587R specification limits, to the
circuit's turn-on delay time is:
C
TIMER(MAX)
=
t
TURN-ON
×
I
TIMERUP(MAX)
V
TIMERH(MIN)
⎛
120
µ
A
⎞
C
TIMER(MAX)
=
t
TURN -ON
× ⎜
⎟
⎝
1.280V
⎠
µ
F
⎞
⎛
C
TIMER (MAX)
=
t
TURN -ON
× ⎜
94
×
10
- 6
⎟
sec
⎠
⎝
(6)
t
FLT
(ms)
=
20
×
C
FILTER
(
µ
F)
(4)
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:
t
TURN-ON
=
Whenever the voltage across R
SENSE
exceeds the
MIC2587/MIC2587R’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 MIC2587/MIC2587R 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
October 2004
10
(
1000
µ
F
×
72V
)
2A
=
36ms
Allowing for capacitor tolerances and a nominal 36ms turn-
on time, an initial worse-case value for C
TIMER
is:
µ
F
⎞
⎛
C
TIMER(MAX)
=
0.036s
× ⎜
94
×
10
- 6
⎟ =
3.38
µ
F
sec
⎠
⎝
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
TIMER
is
discharged to GND by an internal 3.5µA current sink
(I
TIMERDN
).
For the MIC2587R, 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
M9999-102204
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
