MAX5943 Maxim Integrated Products, MAX5943 Datasheet - Page 13

• Manufacturer Part Number: MAX5943
• Description: FireWire Current Limiter and Low-Drop ORing Switch Controller
• Manufacturer: Maxim Integrated Products

Timing accuracy is guaranteed for R
4kΩ to 50kΩ. Large R
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
in R
power path. Use component values rated to handle
electrical stresses during the timeout period.
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).
GATE1 rises to 5.5V above SENSE when V
V
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
by the voltage drop, V
resistor. Q1 turns off rapidly when V
V
Set the MAX5943 ORing threshold (V
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
exceed V
device is not in a current fault shutdown.
Figure 4. Q1 Current Path During Inital Startup
OR
OR
V
IN
SENSE
, enhancing Q1. Q1’s low R
minus its hysteresis.
ON
UVLO
, Q1, Q2, and any other components in the
Power-Supply ORing (ONQ1 = Low)
IN
+
R
SENSE
and V
V
IS
______________________________________________________________________________________
GND
-
SENSE
FireWire Current Limiter and Low-Drop
TIM
ON_REF
IS
MAX5943
OR_ADJ
, across the current-sensing
values can be used, which
GATE1
, respectively, and the
DS(ON)
ONQ1
Q1
IS
OR
TIM
decreases below
GATE2
provides a very-
) by connecting
ranging from
Q2
IS
OUT
exceeds
TO
FireWire
PORT
ORing Switch Controller
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).
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
long as V
V
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).
Figure 5. MAX5943 ORing Controller Rerouting Load Current
ON_REF
V
V
INA
INB
FROM
BATTERY
SOURCE
FROM
POWER
SOURCE
I
I
INA
INB
= 0
threshold, and the MAX5943 is not in a cur-
ON
ON
IN
Reverse Current Override (ONQ1 = High)
is greater than V
IN
IN
R
R
SENSE
SENSE
GND
GND
SENSE GATE1
SENSE GATE1
MAX5943
MAX5943
OR_ADJ
OR_ADJ
ONQ1
ONQ1
UVLO
GATE2
GATE2
www.DataSheet4U.com
, V
ON
OUT
OUT
is above the
V
TO FireWire
PORT
INB
I
LOAD
> V
INA
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