MIC2086 datasheet(21/29 Pages) MICREL

21 / 29 page

Micrel, Inc.

MIC2085/2086

May 2006

M9999-050406

(408) 955-1690

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 (tOCSLOW) is

much less than the start-up delay timer duration (tSTART)

and is calculated using the following equation:

100%

Cycle

Duty

Retry

Auto

START

OCSLOW ×

−

(10)

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

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.

Figure 9. Auto-Retry Configuration

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