Micrel
MIC2589/MIC2595
current limit is defined as the overcurrent fault
timer, tFLT, and is determined by the following
equation.
1500µF × 72V
)
= 36ms (use 40ms)
tTURN−ON
=
3A
Allowing for capacitor tolerances and a nominal 40ms
turn-on time, an initial worst-case value for CFILTER is:
FILTER(WORST-CASE) = 40ms×(115×10–6µF/sec) = 4.6µF
CFILTER × VCFILTER(trip)
tFLT
=
ICFILTER(CHARGE)
C
The value of CFILTER should be selected to
allow the circuit’s minimum regulated value of
IOUT to equal ILIMIT for somewhat longer than
the time it takes to charge the total load
capacitance.
The closest standard ±10% tolerance capacitor value
is 4.7µF and would be a good initial starting value for
prototyping.
Whenever the hot swap controller is not in current
limit, CFILTER is discharged to VEE by an internal
4µA current source.
During startup, the CFILTER pin will begin to charge
once the GATE crosses 2.5V. In order to avoid false-
tripping of the circuit breaker by allowing the
overcurrent filter to time out, the overcurrent delay
must be set to exceed the time it takes to ramp the
GATE output above 5.5V (i.e., charge the output load
capacitance).
For the MIC2589R/MIC2595R devices, the circuit
breaker automatically resets after approximately 25
tFLT time constants (23.75 × tFLT_AUTO). If the fault
condition still exists, capacitor CFILTER will again
charge up to VCFILTER(TRIP), tripping the circuit breaker.
Capacitor CFILTER will then be discharged by an
internal 4µA current source until the voltage across
CFILTER goes below VCFILTER(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 4.25% and the period of
the auto-restart cycle is given by:
An initial value for CFILTER is found by calculating the
time it will take for the MIC2589/MIC2595 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 I = C × (dv/dt):
CLOAD
×
(
VDD − VEE
ILIMIT
values
)
tTURN−ON
=
tRETRY = tFLT + tFLT_AUTO
Using parametric
specific
to
the
[
CFILTER
×
(
VCFILTER(TRIP) - VCFILTER
)
]
(retry
)
MIC2589/MIC2595, an expression relating CFILTER to
the circuit’s turn-on delay time is:
tRETRY = tFLT +
ICFILTER(pull−down
)
(
tTURN−ON ×ICFILTER
)
CFILTER
=
The auto-restart period for the example above where
the worst-case CFILTER was determined to be 4.7µF is:
VCFILTER
Substituting the variables above with the specification
limits of the MIC2589/MIC2595, an expression for the
worst-case value for CFILTER is given by:
tAUTO-RESTART = 1.27s
No-Load Detection
135µA
1.17V
⎛
⎜
⎞
⎟
For applications in which a minimum load current will
always be present, the no-load detect capability of the
MIC2589 product family offers system designers the
ability to perform a shutdown operation on such fault
conditions, such as an unscheduled or unexpected
removal of PC boards from the system or on-board
fuse failure. As long as the minimum current drawn by
the load is at least 20% of the current limit (defined by
CFILTER(max) = tTURN−ON
×
⎝
⎛
⎠
µF
⎞
⎟
CFILTER(max) = tTURN−ON × 115 ×10−6
⎜
sec
⎝
⎠
For example, in a system with a CLOAD = 1500µF, a
maximum (VDD – VEE) = 72V, and a maximum load
current on a nominal –48V buss of 2.5A, the nominal
circuit design equations steps are:
VTRIP
), the output of the hot swap controller will
RSENSE
1. Choose ILIMIT = IHOT_SWAP(nom) = 3A (2.5A + 20%);
38.8mV
remain enabled. If the output current falls below 20%
of the actual current limit, the controller’s no-load
detection loop is enabled. In this loop, an internal
current source, ICNLD, will charge an external capacitor
2. Select an RSENSE
=
= 12.9mΩ (closest
3A
1% standard value is 13.0mΩ);
3. Using ICHARGE = ILIMIT = 3A, the application circuit
turn-on time is calculated:
CNLD. An expression for the controller’s no-load time-
out delay is given by:
17
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
