MIC2580A Datasheet, PDF (18/21 Pages) Micrel Semiconductor – Hot-Swap PCI Power Controller

MIC2580A
Functional Description
Start-Up Sequence
The start-up sequence iniates after all four supplies are
connected to the inputs and then /ON is asserted by
transistioning from high to low. During the start-up sequence,
all four gates ramp up at a rate determined by C_SLEW. During
this time /PWRGD is deasserted until all four supplies are
within specified levels. When /PWRGD is asserted the power-
on-reset signal /POR timer begins. The time period is defined
by C_RST. Refer to Figure 1 for a timing diagram of the signals
during the start-up sequence.
During the start-up sequence, a current source charges
C_START at a constant rate until a threshold voltage of 2.4V is
reached. This period of time defines an interval during which
overcurrent events are ignored. This prevents high inrush
currents that normally occur when charging capacitance
erroneously asserting /FAULT. The magnitude of the start-
up time, t_START is defined by C_START.
The MIC2580A employs foldback current limiting that en-
sures the device starts up in current limit. This minimizes high
inrush currents due to ramping a voltage into capacitance
regardless of the size of the load capacitance.
Overcurrent Detection
The MIC2580A senses overcurrent via the use of external
sense resistors for the 5V and 3.3V supply rails. When the
sense voltage across these resistors is greater than or equal
to 50mV an overcurrent condition is detected. Therefore the
overcurrent set point is determined by I_LIMIT = 50mV/R_SENSE.
For the +12V and –12V supply rails overcurrent detection is
set internally at 1.3A and –0.5A respectively.
When an overcurrent condition is detected /FAULT is as-
serted only if the overcurrent condition lasts for a minimum
time period of 10µs. This delay prevents spurious noise from
the system erroneously tripping the circuit breaker and as-
serting /FAULT. Upon /FAULT being asserted an internal
latch is set that immediately turns off all four supplies to
prevent further damage to the system. Toggling /ON will reset
the latch and initiate another start-up sequence. Figures 2
and 3 depict the timing for two fault conditions.
Micrel, Inc.
Thermal Shutdown
The +12V and –12V internal MOSFET switches are protected
by current limit and overtemperature shutdown circuitry.
When the die temperature exceeds 160°C, /FAULT is as-
serted and all supplies are shut off. The power dissipated in
the MIC2580A is primarily due to the sum of current flowing
through the internal MOSFET switches and, to a lesser
extent, power dissipated due to the supply current. To com-
pute the total power dissipation of the MIC2580A the follow-
ing equation is used:
P_D(total) = P_{D(+12V switch)} + P_{D(–12V switch)} + P_D(supplies)
where:
P_D(switches) = R_DS(on) × I_OUT2
P_D(supplies) = V_SUPPLY × I_SUPPLY
To relate this to operating junction temperature:
T_J = P_D × θ_JA + T_A
where:
T_A = ambient temperature
θ_JA = package thermal resistance
Precharge Voltage
For CompactPCI applications, the MIC2580A-1.0BSM/BTS
integrates a 1V ±20% voltage source that satisfies
CompactPCI precharge requirements. The voltage source
can provide up to 10mA. For higher current, the MIC2580A-
1.6BSM/BTS integrates a 1.6V ±20% voltage source to bias
a NPN transistor.
+5V
MIC2580A-1.6
R
VPCHG
I > 10mA
DATA
BUS
Figure 5. Voltage Source
Turnoff
Deasserting /ON will turn off all four supplies. Alternatively
driving /FAULT low will turn off all supplies but will not latch
the supplies off. Releasing /FAULT will initiate a new start
sequence.
MIC2580A
18
February 2005