MIC1344 datasheet(13/17 Pages) MICREL

13 / 17 page

Micrel, Inc.

MIC1344

April 24, 2015

Revision 1.0

The ASEL\ and BSEL\ status pins indicate which channel

is selected and connected to the output. If ASEL\ is active

(low), then INA is selected and if BSEL\ is active (low),

then INB is selected. If both ASEL\ and BSEL\ are

inactive (open-drain high), then neither of the two inputs

is selected. This can occur if both INA and INB input

voltages are below the undervoltage lock out (UVLO)

voltage, or both inputs have been manually turned-off

(ENA = 0, ENB = 1).

If high-value pull-up resistors are used (>100k

Ω), then

minimal current will be injected into the digital input pins,

allowing them to stay at a digital low for a lower input

voltage below the UVLO voltage.

Each digital status output pin is capable of sinking 10mA,

which is sufficient current to drive LEDs, optocouplers,

etc.

Passing

the

status

the

MIC1344

through

optocouplers allows communicating the power status

across the isolation barrier of power systems.

Under Voltage Lockout

Both INA and INB have separate UVLO detection circuits.

The UVLO status of INA is reflected on the STAOK status

pin. Likewise, the UVLO status of INB is reflected on the

STBOK status pin. This feature can be used to monitor

the incoming voltage of the unselected input channel. If

the selected IN pin is below the UVLO voltage, then the

channel will be turned off, and only reverse leakage

current can flow.

Standard Hysteresis Mode

In automatic mode, the voltage of the inactive input must

be raised above the voltage of the active input by

≈300mV, in order for the MIC1344 to switch to the

inactive input.

High Hysteresis Mode

If four transitions between input channels occur within

≈8ms (INA → INB → INA → INB) while the part is in

automatic mode, then the 300mV switchover hysteresis

will increas

e to ≈600mV. This is the high hysteresis

mode. In the high hysteresis mode the part can only

switch between channels once per millisecond. The

higher hysteresis and the forced delay helps prevent the

part from chattering between inputs.

In the high hysteresis mode, if there are four additional

transitions in the next

≈8ms, then the status pin for the

selected channel will go low (STAOK if VINA selected

and STBOK if VINB is selected).

The part will exit the high hysteresis mode if there are

fewer than four transitions in a subsequent

≈8ms period,

or if a manual mode is selected.

Hot Swappable Inputs

The power to INA and/or INB can be connected to input

power and turned on or off without consequence to the

other input power supply, as long as the minimum and

maximum input voltage does not exceed the ratings of

the MIC1344. If the MIC1344 is set to automatic mode

(ENA = 0, ENB = 0), and power is applied to one of the

channels, the MIC1344 will wait until the input voltage is

above the UVLO threshold, then it will wait the initial

power up delay period, giving time for the input voltage to

stabilize, and then connect the selected input to the OUT

pin. Refer to the “Current Limiting” sub-section for details

of what occurs if the INA or INB input channels current

limit.

Cascading More than Two Input Power Sources

On the MIC1344, both inputs can be turned off using the

ENA = 0, ENB = 1, no input selected mode. This mode is

used when more than two power inputs are used. If there

input

voltage

either

INA

INB,

the

corresponding digital output status pins will stay active.

This allows detection of the presence of an input voltage

through the corresponding STAOK or STBOK pins, even

if the channel is not selected.

Reverse Voltage and On-Channel Voltage Detection

If one of the channels of the MIC1344 is turned off,

current is blocked from flowing from the OUT pin to the

unselected IN pin. For example; If INA is at 5V, and INA

is selected, only a very small leakage current (<10µA

maximum) will flow between OUT, and the unselected

INB.

If a channel is turned on and the OUT voltage exceeds

the corresponding IN voltage, then reverse current may

flow to the input. If the voltage at the OUT pin exceeds

the voltage at the correspondingly selected IN pin, then

the channel will be turned off and the corresponding

STAOK or STBOK pin will go active (low). For example, if

INA is selected and OUT exceeds INA

by ≥ 100mV, then

Channel A will be turned off and STAOK will go active,

indicating that a fault has occurred. The same sequence

occurs if Channel B is selected and OUT exceeds INB by

≥100mV. Once the differential between OUT and the

selected IN drops to

≈50mV, the selected channel will

automatically turn back on.

Output Capacitor

The output (OUT) requires sufficient capacitance that

when the MIC1344 switches between inputs (INA or INB),

that the output voltage does not unacceptably droop (go

low). The cause of the output voltage drooping is that the

MIC1344 switches “break before make”, which will

momentarily disconnect both inputs from the output. The

switchover

delay

specified

the

Electrical

Characteristics. A minimum of a 10µF ceramic chip

capacitor is recommended on the output of the MIC1344.

MIC1344 Datasheet(PDF) 13 Page - Micrel Semiconductor

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