MIC2589 Datasheet, PDF (12/17 Pages) Micrel Semiconductor – SINGLE-CHANNEL NEGATIVE HIGH VOLTAGE HOT SWAP POWER CONTROLLERS/SEQUENCERS

MIC2589/2595
Circuit Breaker Function
The MIC2589/89R and the MIC2595/95R employ an elec-
tronic circuit breaker that protects the external power MOSFET
and other system components against large-scale faults,
such as short circuits. The current-limit threshold is set via an
external resistor, R_SENSE, connected between the V_EE and
SENSE. For the MIC2589/89R and MIC2595/95R, a timer is
set via capacitor C_FILTER that determines the length of the
time delay (t_FLT) for which the device remains in current limit
before the circuit breaker is tripped. This programmable
delay prevents tripping of the circuit breaker because of high
inrush current charging bulk and distributed capacitive loads.
Whenever the voltage across R_SENSE exceeds 50mV, two
things happen:
1. A constant-current regulation loop is engaged which
is designed to hold the voltage across R_SENSE equal
to 50mV. This protects both the load and the
MIC2589/95 circuits from excessively high currents.
This current-regulation loop will engage in less than
1µs from the time at which the overvoltage condition
on R_SENSE occurs.
2. Capacitor C_FILTER is charged up to an internal
V_{CFILTER(TRIP)} threshold (= 1.25V) by an internal
95µA current source. If the voltage across C_FILTER
crosses this threshold, the circuit breaker trips and
the GATE pin is immediately pulled low by an internal
current pull-down. This operation turns off the
MOSFET quickly and disconnects the input from the
load. The value of C_FILTER should be selected to
allow the circuit’s minimum regulated value of I_OUT
to equal I_TRIP for somewhat longer than the time it
takes to charge the total load capacitance.
An initial value for C_FILTER is found by calculating the time it
will take for the MIC2589/95 to completely charge up the
output capacitive load. Assuming the load is enabled by the
PWRGD_X (or /PWRGD_X) signal(s) of the IC, the turn-on delay
time is derived from the following expression,
I = C × (dv/dt):
( ) _tTURN-ON
=
C_LOAD
× V_DD
I_LIMIT
–
V_EE
Using parametric values specific to the MIC2589/95, an
expression relating a design nominal value for C_FILTER to the
circuit’s turn-on delay time is:
( ) _{CFILTER(nom) =}
t_TURN-ON × I_CFILTER(typ)
V_CFILTER(typ)
=
t_TURN-ON × 95µA
1.25V
=
t _{TURN- ON}
× 76 × 10^–6
µF
sec
Substituting the variables above with the specification limits
of the MIC2589/95, an expression for the worst-case value for
C_FILTER is given by:
C_FILTER (max)
=
t _{TURN- ON}
×

135µA
1.17V

=
t _{TURN- ON}
×
_115.4
× 10^–6
µF
sec

Micrel
For example, in a system with a C_LOAD = 150µF, a maximum
(V_DD – V_EE) = 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
=
38.8mV
2A
= 19.4mΩ
(closest 1%
standard value is 19.6mΩ);
3. Using I_CHARGE = I_LIMIT = 2A, the application circuit turn-
on time is calculated:
t _{TURN- ON}
=
(150µF × 72V)
2A
=
5.4ms
(use
6
ms)
Allowing for capacitor tolerances and a nominal 6ms turn-on
time, an initial worst-case value for C_FILTER is:
C_{FILTER(WORST-CASE)} = 6ms x (115.4 × 10^–6µF/sec) = 692nF
The closest standard ±5% tolerance capacitor value is 698nF
and would be a good initial starting value for prototyping.
Whenever the hot swap controller is not in current limit, C_FILTER
is discharged to V_EE by an internal 4µA current source.
For the MIC2589R/95R devices, the circuit breaker automati-
cally resets after (20) t_FLT time constants (20 × t_FLT). If the fault
condition still exists, capacitor C_FILTER will again be charge up
to V_FILTER(TRIP) where the circuit breaker is tripped. Capacitor
C_FILTER will then be discharged by an internal 4µA current
source until the voltage across C_FILTER goes below
V_{FILTER(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 5% and the period of the auto-
restart cycle is given by:
t _AUTO-RESTART
=
20 ×
(C_FILTER ) × (1.25V
95µA
–
0.22V)
=
C_FILTER
×


216.8
ms 
µF 
The auto-restart period for the example above where the
worst-case C_FILTER was determined to be 698nF is:
t_AUTO-RESTART = 151ms
Current Sensing
As mentioned before, the MIC2589/89R and the MIC2595/95R
use an external, low-value resistor in series with the source
of the external MOSFET to measure the current flowing into
the load. The V_EE connection (Pin 7) to the IC is one input to
the device’s internal current sensing circuits and the SENSE
connection (Pin 9) is the other input.
The sense resistor is nominally valued at:
R_SENSE(nom)
=
V_TRIP(typ)
I_{HOT_SWAP}(nom)
where V_TRIP(typ) is the nominal circuit breaker threshold
voltage (= 50mV) and I_{HOT_SWAP}(nom) is the nominal hot
swap load current level to trip the internal circuit breaker in the
application.
M9999-031504
12
March 2004