Micrel
MIC2586/MIC2586R
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 MIC2586/MIC2586R’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 MIC2586/MIC2586R’s GATE drive to
force a constant 12mV (typical) voltage drop across the
external sense resistor.
Circuit Breaker Operation
The MIC2586/MIC2586R 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 MIC2586/MIC2586R, 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 ovecurrent
response time is calculated using the following equation:
t
FLT
(ms)
=
C
FILTER
×
V
TIMERH
I
TIMERUP
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 MIC2586/MIC2586R to completely charge up
the output capacitive load. Assuming the load is enabled by
the PWRGDx (or /PWRGDx) signal(s) of the controller, 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 MIC2586/MIC2586R, an
expression relating a worse-case design value for C
TIMER
,
using the MIC2586/MIC2586R 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
=
(
1000
µ
F
×
72V
)
2A
Whenever the voltage across R
SENSE
exceeds the
MIC2586/MIC2586R’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 MIC2586/MIC2586R
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
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
October 2004
11
=
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 MIC2586 is not in current limit, C
TIMER
is
discharged to GND by an internal 3.5µA current sink
(I
TIMERDN
).
For the MIC2586R, 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 V
TIMERH
threshold, and if exceeded, trip the circuit breaker. Capacitor
C
TIMER
will then be discharged by I
TIMERDN
until the voltage
across C
TIMER
drops below the V
TIMERL
threshold, at which
time another start cycle is initiated. This will continue until
either of the following occurs: a) the fault condition is
removed, b) the input supply voltage power is
M9999-102204
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
