MIC2588 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
Power-Good (PWRGD or /PWRGD) Output
For the MIC2588-1 and the MIC2594-1, the Power-Good
output signal (PWRGD) will be high impedance when V_DRAIN
drops below V_PGTH, and will pull down to V_DRAIN when
V_DRAIN is above V_PGTH. For the MIC2588-2 and the
MIC2594-2, /PWRGD will pull down to the potential of the
V_DRAIN pin when V_DRAIN drops below V_PGTH, and will be high
impedance when V_DRAIN is above V_PGTH. Hence, the -1 parts
have an active-high PWRGD signal and the -2 parts have an
active-low /PWRGD output. Either PWRGD or /PWRGD may
be used as an enable signal for one or more subsequent
DC/DC converter modules or for other system uses as
desired. When used as an enable signal, the time necessary
for the PWRGD (or /PWRGD) signal to pull-up (when in high
impedance state) will depend upon the load (RC) that is
present on this output.
Circuit Breaker Function
The MIC2588 and the MIC2594 employ an electronic circuit
breaker that protects the MOSFET and other system compo-
nents against faults such as short circuits. The current limit
threshold is set via an external resistor, R_SENSE, connected
between the V_EE and SENSE pins. An internal 400µs timer
limits the length of time (t_FLT) for which the circuit can draw
current in excess of its programmed threshold before the
circuit breaker is tripped. This short delay prevents nuisance
tripping of the circuit breaker due to system transients while
providing rapid protection against large-scale transient faults.
Whenever the voltage across R_SENSE exceeds 50mV, two
things happen:
1. A constant-current regulation loop is engaged de-
signed to hold the voltage across R_SENSE equal to
50mV. This protects both the load and the MIC2588
circuit from excessively high currents. This loop will
engage in less than 1µs from the time at which the
overvoltage condition on R_SENSE occurs.
2. The internal 400µs timer is started. If the 400µs
timeout period is exceeded, 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.
Current Sensing
As mentioned before, the MIC2588 and the MIC2594 employ
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 to the IC from the negative supply
is also one input to the part’s internal current sensing circuits
and the SENSE input is the other input.
Sense Resistor Selection
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.
Micrel
To accommodate worst-case tolerances in the sense resistor
(for a ±1% initial tolerance, allow ±3% tolerance for variations
over time and temperature) and circuit breaker threshold
voltages, a slightly more detailed calculation must be used to
determine the minimum and maximum hot swap load
currents.
As the MIC2588/94’s minimum current limit threshold voltage
is 40mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:
( ) ^{IHOT_SWAP(min) =}
40mV
1.03 × R_SENSE(nom)
= ^38.8mV
R_SENSE(nom)
Keep in mind that the minimum hot swap load current should
be greater than the application circuit’s upper steady-state
load current boundary. Once the lower value of R_SENSE has
been calculated, it is good practice to check the maximum hot
swap load current (I_{HOT_SWAP}(max)) which the circuit may let
pass in the case of tolerance build-up in the opposite direc-
tion. Here, the worst-case maximum is found using a
V_TRIP(max) of 60mV and a sense resistor, 3% low in value:
( ) ^{IHOT_SWAP(max) =}
60mV
0.97 × R_SENSE(nom)
= ^61.9mV
R_SENSE(nom)
In this case, the application circuit must be sturdy enough to
operate over a ~1.6-to-1 range in hot swap load currents. For
example, if an MIC2594 circuit must pass a minimum hot
swap load current of 4A without nuisance trips, R_SENSE
should be set to
38.8mV
4A
= 9.7mΩ _,
and
the
nearest
1%
standard value is 9.76mΩ. At the other tolerance extremes,
I_{HOT_SWAP}(max) for the circuit in question is then simply
I_{HOT_SWAP}(max)
=
61.9mV
9.76mΩ
=
6.3A
With a knowledge of the application circuit’s maximum hot
swap load current, the power dissipation rating of the sense
resistor can be determined using P = I² × R. Here, the I is
I_{HOT_SWAP}(max) = 6.3A and the R is R_SENSE(min) =
(0.97)(R_SENSE(nom)) = 9.47mΩ. Thus, the sense resistor’s
maximum power dissipation is:
P_MAX = (6.3A)² × (9.47mΩ) = 0.376W
A 0.5Ω sense resistor is a good choice in this application.
Undervoltage/Overvoltage Detection—MIC2588
The MIC2588 has “UV” and “OV” input pins. These pins can be
used to detect input supply rail undervoltage and overvoltage
conditions. Undervoltage lockout prevents energizing the load
until the supply input is stable and within tolerance. In a similar
fashion, overvoltage turn-off prevents damage to sensitive
circuit components should the input voltage exceed normal
operational limits. Each of these pins is internally connected to
an analog comparator with 20mV of hysteresis. When the UV
pin falls below its V_UVL threshold or the OV pin is above its V_OVH
threshold, the GATE pin is immediately pulled low. The GATE
pin will be held low until UV exceeds its V_UVH threshold or OV
drops below its V_OVL threshold. The UV and OV circuit’s
threshold trip points are programmed using the resistor divider
December 2003
11
M9999-122303