MIC2594 MICREL [Micrel Semiconductor], MIC2594 Datasheet - Page 11

• Manufacturer Part Number: MIC2594
• Description: Single-Channel, Negative High-Voltage Hot Swap Power Controllers
• Manufacturer: MICREL [Micrel Semiconductor]

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
drops below V
V
MIC2594-2, /PWRGD will pull down to the potential of the
V
impedance when V
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
between the V
limits the length of time (t
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
things happen:
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
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:
where V
voltage (= 50mV) and I
swap load current level to trip the internal circuit breaker in the
application.
December 2003
MIC2588/MIC2594
DRAIN
DRAIN
1. A constant-current regulation loop is engaged de-
2. The internal 400 s timer is started. If the 400 s
R
signed to hold the voltage across R
50mV. This protects both the load and the MIC2588
circuit from excessively high currents. This loop will
engage in less than 1 s from the time at which the
overvoltage condition on R
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.
pin when V
SENSE
is above V
TRIP
EE
(typ) is the nominal circuit breaker threshold
(nom)
connection to the IC from the negative supply
EE
PGTH
and SENSE pins. An internal 400 s timer
DRAIN
DRAIN
, and will pull down to V
PGTH
I
HOT_SWAP
HOT_SWAP
drops below V
is above V
FLT
V
. For the MIC2588-2 and the
TRIP
) for which the circuit can draw
(typ)
SENSE
SENSE
(nom)
(nom) is the nominal hot
PGTH
PGTH
occurs.
exceeds 50mV, two
. Hence, the -1 parts
SENSE
SENSE
, and will be high
DRAIN
, connected
equal to
DRAIN
when
11
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:
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
been calculated, it is good practice to check the maximum hot
swap load current (I
pass in the case of tolerance build-up in the opposite direc-
tion. Here, the worst-case maximum is found using a
V
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
should be set to
standard value is 9.76m . At the other tolerance extremes,
I
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 = I
I
(0.97)(R
maximum power dissipation is:
A 0.5 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
threshold, the GATE pin is immediately pulled low. The GATE
pin will be held low until UV exceeds its V
drops below its V
threshold trip points are programmed using the resistor divider
I
I
HOT_SWAP
HOT_SWAP
HOT_SWAP
HOT_SWAP
TRIP
(max) of 60mV and a sense resistor, 3% low in value:
P
I
HOT_SWAP
MAX
SENSE
(max) for the circuit in question is then simply
(min)
(max)
(max) = 6.3A and the R is R
= (6.3A)
(nom)) = 9.47m . Thus, the sense resistor’s
(max)
UVL
OVL
38.8mV
HOT_SWAP
1.03 R
2
0.97 R
threshold or the OV pin is above its V
4A
threshold. The UV and OV circuit’s
(9.47m ) = 0.376W
9.76m
61.9mV
40mV
60mV
SENSE
SENSE
(max)) which the circuit may let
9.7m
(nom)
(nom)
6.3A
, and the nearest 1%
2
UVH
R. Here, the I is
R
R
threshold or OV
SENSE
SENSE
SENSE
38.8mV
61.9mV
M9999-122303
SENSE
(nom)
(min) =
(nom)
SENSE
Micrel
OVH
has