MIC2085 Datasheet, PDF (21/29 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
Auto-Retry Upon Overcurrent Faults
The MIC2085/86 can be configured for automatic restart
after a fault condition. Placing a diode between the ON
and/FAULT pins, as shown in Figure 9, will enable the
auto-restart capability of the controller. When an
application is configured for auto-retry, the overcurrent
timer should be set to minimize the duty cycle of the
overcurrent response to prevent thermal runaway of the
power MOSFET. See “MOSFET Transient Thermal
Issues” section for further detail. A limited duty cycle is
achieved when the overcurrent timer duration (t_OCSLOW) is
much less than the start-up delay timer duration (t_START)
and is calculated using the following equation:
Auto − Retry Duty Cycle = ^{t OCSLOW} × 100%
(10)
t _START
An InfiniBand™ Application Circuit
The circuit in Figure 10 depicts a single 50W
InfiniBand™ module using the MIC2085 controller. An
InfiniBand™ backplane distributes bulk power to multiple
plug-in modules that employ DC/DC converters for local
supply requirements. The circuit in Figure 10 distributes
12V from the backplane to the MIC2182 DC/DC
MIC2085/2086
converter that steps down +12V to+3.3V for local bias.
The pass transistor, Q1, isolates theMIC2182’s input
capacitance during module plug-in and allows the
backplane to accommodate additional plug-in modules
without affecting the other modules on the backplane.
The two control input signals are VBxEn_L (active LOW)
and a Local Power Enable (active HIGH). The MIC2085
in the circuit of Figure 10 performs a number of
functions. The gate output of Q1 is enabled by the two
bit input signal VBxEn_L, Local Power Enable = [0,1].
Also, the MIC2085 limits the drain current of Q1 to 7A,
monitors VB_In for an overvoltage condition greater than
16V, and enables the MIC2182 DC/DC converter
downstream to supply a local voltage rail. The
uncommitted comparator is used to monitor VB_In for an
undervoltage condition of less than 10V, indicated by a
logic LOW at the comparator output (COMPOUT).
COMPOUT may be used to control a downstream
device such as another DC/DC converter. Additionally,
the MIC2085 is configured for auto-retry upon an
overcurrent fault condition by placing a diode (D1)
between the /FAULT and ON pins of the controller.
May 2006
Figure 9. Auto-Retry Configuration
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M9999-050406
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