FireWire Current Limiter and Low-Drop

ORing Switch Controller

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13

reduces timing accuracy. A maximum timeout period is

enforced to prevent arbitrarily long operation in current-

limit/circuit-breaker conditions.

Long durations in current-limit/circuit-breaker mode

may produce excessive heating and electrical stresses

power path. Use component values rated to handle

electrical stresses during the timeout period.

Power-Supply ORing (ONQ1 = Low)

GATE1 controls the MAX5943 ORing function. The

MAX5943 accomplishes a very-low-voltage-drop diode

ORing function using an n-channel power MOSFET, Q1.

Initially, GATE1 is off (ONQ1 = low) and the load current

conducts through the body diode of Q1 (Figure 4).

low-voltage drop across its source to drain, reducing volt-

age drop, power dissipation, and heat generation in the

power-supply path created by a traditional diode.

The MAX5943 continuously monitors the load current

OR_ADJ to GND for a 5mV threshold, or to IN for a

10mV threshold, or leave it open for a 7.5mV threshold.

Connect ONQ1 to GND for normal ORing function.

Drive ONQ1 high to force Q1 on, regardless of the load

current condition, as long as the voltages at IN and ON

device is not in a current fault shutdown.

When a higher voltage supply is connected at OUT

(Figure 5), the sense voltage drops below the program-

mable ORing threshold. The MAX5943 immediately

turns off Q1, blocking reverse current flow from OUT to

IN. The power is then routed from the higher supply

input to the load (Figure 6).

Reverse Current Override (ONQ1 = High)

Drive ONQ1 high to disable the power-supply ORing

function. Disabling the MAX5943 ORing function allows

reverse current flow through the power-supply system.

When ONQ1 is high, GATE1 remains fully enhanced as

rent-limit or circuit-breaker fault condition. The MAX5943

reverse-current flow feature permits a higher voltage

source connected to one port to route power to another

port, as long as the voltage at IN remains above the

minimum 7.5V operating range (see Figure 7).

SENSE

GATE1

OR_ADJ

ONQ1

GND

GATE2

OUT

Q1

Q2

IN

TO

FireWire

PORT

MAX5943

ON

+

-

Figure 4. Q1 Current Path During Inital Startup

SENSE GATE1

OR_ADJ

ONQ1

GND

GATE2

OUT

IN

FROM

BATTERY

SOURCE

TO FireWire

PORT

MAX5943

ON

SENSE GATE1

OR_ADJ

ONQ1

GND

GATE2

OUT

IN

FROM

POWER

SOURCE

MAX5943

ON

Figure 5. MAX5943 ORing Controller Rerouting Load Current