MIC2589_11 Datasheet, PDF (17/29 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controller/Sequencer

Micrel
current limit is defined as the overcurrent fault
timer, t_FLT, and is determined by the following
equation.
t_FLT
=
C_FILTER × V_{CFILTER(trip)}
I_{CFILTER(CHARGE)}
The value of C_FILTER should be selected to
allow the circuit’s minimum regulated value of
I_OUT to equal I_LIMIT for somewhat longer than
the time it takes to charge the total load
capacitance.
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).
An initial value for C_FILTER 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):
( ) _{t TURN−ON}
=
C_LOAD
× V_DD
I_LIMIT
−
V_EE
Using parametric values specific to the
MIC2589/MIC2595, an expression relating C_FILTER to
the circuit’s turn-on delay time is:
( ) _{CFILTER =}
t _TURN−ON × I_CFILTER
V_CFILTER
Substituting the variables above with the specification
limits of the MIC2589/MIC2595, an expression for the
worst-case value for C_FILTER is given by:
C_{FILTER(max )}
=
t _TURN−ON
× ⎜⎛ ^135µA ⎟⎞
⎝ ^1.17V ⎠
C_FILTER(max)
=
t _TURN−ON
× ⎜⎛115 × 10 ^−6
⎝
µF _⎟⎞
sec _⎠
For example, in a system with a C_LOAD = 1500µF, a
maximum (V_DD – V_EE) = 72V, and a maximum load
current on a nominal –48V buss of 2.5A, the nominal
circuit design equations steps are:
1. Choose I_LIMIT = I_{HOT_SWAP}(nom) = 3A (2.5A + 20%);
2.
Select an
R _SENSE
=
38.8mV
3A
= 12.9mΩ
(closest
1% standard value is 13.0mΩ);
3. Using I_CHARGE = I_LIMIT = 3A, the application circuit
turn-on time is calculated:
MIC2589/MIC2595
t _TURN−ON
=
(1500µF × 72V )
3A
=
36ms (use
40ms)
Allowing for capacitor tolerances and a nominal 40ms
turn-on time, an initial worst-case value for C_FILTER is:
C_{FILTER(WORST-CASE)} = 40ms×(115×10^–6µF/sec) = 4.6µF
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.
For the MIC2589R/MIC2595R devices, the circuit
breaker automatically resets after approximately 25
t_FLT time constants (23.75 × t_{FLT_AUTO}). If the fault
condition still exists, capacitor C_FILTER will again
charge up to V_{CFILTER(TRIP),} tripping the circuit breaker.
Capacitor C_FILTER will then be discharged by an
internal 4µA current source until the voltage across
C_FILTER goes below V_{CFILTER(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:
t_RETRY = t_FLT + t_{FLT_AUTO}
[ ( )] _tRETRY
= t_FLT
+
C_FILTER
×
V_{CFILTER(TRIP)} - V_{CFILTER(retry)}
I_{CFILTER(pull−down} )
The auto-restart period for the example above where
the worst-case C_FILTER was determined to be 4.7µF is:
t_AUTO-RESTART = 1.27s
No-Load Detection
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
V_{TRIP ), the output of the hot swap controller will}
R _SENSE
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, I_CNLD, will charge an external capacitor
C_NLD. An expression for the controller’s no-load time-
out delay is given by:
December 2005
17
M9999-120505
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