MIC2341R datasheet(21/32 Pages) MICREL

21 / 32 page

Micrel, Inc.

MIC2341/2341R

October 2007

M9999-102507-A

(408) 944-0800

Standby Mode

Standby mode is entered when one or more of the MAIN

supply inputs (12VIN and/or 3VIN) are below its

respective UVLO threshold or OFF. The MIC2341 also

supplies 3.3V auxiliary outputs (VAUX[A/B]), satisfying

PCI Express specifications. These outputs are fed via

the VSTBY[A/B] input pins and controlled by the

AUXEN[A/B] input pins. These outputs are independent

of the MAIN outputs (12VIN[A/B] and 3VIN[A/B]). Should

the MAIN supply inputs move below their respective

UVLO thresholds, VAUX[A/B] will still function as long as

VSTBY[A/B] is present. Prior to standby mode, ONA and

ONB inputs should be de-asserted or the MIC2341 will

assert the /FAULT_MAIN[A/B] and /INT output signals, if

an undervoltage condition on the MAIN supply inputs is

detected.

Circuit Breaker Function

The MIC2341 provides an electronic circuit breaker

function that protects against excessive loads, such as

short circuits, at each supply. When the current from one

or more of a slot’s MAIN outputs exceeds the current

limit threshold (ILIM = 50mV/RSENSE) for a duration greater

than tFLT, the circuit breaker is tripped and both MAIN

supplies (all outputs except VAUX[A/B]) are shut off.

Should the load current cause a MAIN output’s VSENSE to

exceed VTHFAST, the outputs are immediately shut off with

no delay. Undervoltage conditions on the MAIN supply

inputs also trip the circuit breaker, but only when the

MAIN outputs are enabled (to signal a supply input

brown-out condition).

The VAUX[A/B] outputs have a different circuit-breaker

function.

The

VAUX[A/B]

circuit

breakers

not

incorporate a fast-trip detector, instead they regulate the

output current into a fault to avoid exceeding their

operating current limit. The circuit breaker will trip due to

an overcurrent on VAUX[A/B] when the fault timer

expires. This use of the tFLT timer prevents the circuit

breaker from tripping prematurely due to brief current

transients.

Following a fault condition, the outputs can be turned on

again by toggling the ON[A/B] input high-low-high (if the

fault occurred on one of the MAIN outputs), or similarly

toggling the AUXEN[A/B] input (if the fault occurred on

the AUX outputs), or by cycling both ON[A/B] and

AUXEN[A/B] (if faults occurred on both the MAIN and

AUX outputs). When the circuit breaker trips, the

corresponding /FAULT_MAIN[A/B] or /FAULT_AUX[A/B]

will be asserted. At the same time, /INT will be asserted.

Note that /INT is only de-asserted by applying a high-to-

low transition on the corresponding slot’s ON[A/B] or

AUXEN[A/B] input.

The response time (tFLT) of the MIC2341’s primary

overcurrent detector is set by external capacitors at the

CFILTER[A/B] pins to GND. For Slot A, CFILTER[A] is

located at Pin 2; for Slot B, CFILTER[B] is located at Pin

35. For a given response time, the value for CFILTER[A/B] is

given by:

FILTER

FLT[A/B]

]

FILTER[A/B

(V)

(µA)

(ms)

(µF)

where tFLT[A/B] is the desired response time and quantities

IFILTER and VFILTER are specified in the MIC2341’s

“Electrical Characteristics” table.

Digital Filter and Auto-Retry Functions

New timers have been incorporated for additional

protection

for

overcurrent

situations.

many

applications,

external

power

MOSFETs

used

12VOUT[A/B] and at 3VOUT[A/B] have been damaged

during initial start-up and overcurrent conditions because

the response time of the primary OC detectors was set

too long (that is, an incorrect value for CFILTER[A/B]

was used). In these products, a digital filter delay

counter is introduced, and is internally set to a typical

delay time of 40 ms. As shown in the typical applications

circuit of the MIC2341, an external capacitor from

CFILTER[A/B] to ground is used to set the response

time of the primary overcurrent detectors to tFLT. At the

time a large inrush current causes the primary OC

detector

sense

overcurrent

condition,

CFILTER[A/B] charge-up sequence is initiated. At the

same time, the MIC2341’s digital filter delay counter is

also initiated and commences a count-up to 40 ms. The

MIC2341’s internal logic circuits have been designed to

trip the circuit breaker after tFLT or tDFLT, whichever delay

is smaller. If the overcurrent condition causes the

electronic circuit breaker to latch off and thereby

asserting a FAULT condition, the MIC2341 remains

latched-off awaiting system intervention (by toggling

ON[A/B] and/or AUXEN[A/B] and/or cycling the input

power supplies). Internal to the MIC2341R only, a

second, or auto-retry, delay counter is initiated and

commences a count-up to approximately 820 ms to

allow the external power MOSFET to cool before

attempting another power-up sequence.

Figure 6

illustrates the filter responses during an overcurrent fault.

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