Datasheet

MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
14 ______________________________________________________________________________________
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
long as V
IN
is greater than V
UVLO
, V
ON
is above the
V
ON_REF
threshold, and the MAX5943 is not in a cur-
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).
Undervoltage Lockout
The MAX5943 operates from 7.5V to 37V and has a
default 6.5V undervoltage lockout threshold. The external
MOSFETs remain off as long as V
IN
> 6.5V and V
ON
<
V
ON_REF
. The turn-on threshold is programmable by
connecting a resistor-divider at ON. When V
ON
> 1.24V
and V
IN
> 7.5V, GATE2 is enhanced to 5.5V, with respect
to OUT and the MAX5943 goes into normal operation.
Program the MAX5943’s turn-on voltage using the fol-
lowing formula:
where V
TURN_ON
is the desired turn-on voltage of the
MAX5943. Choose an R2 value in the 1kΩ range to
minimize error due to the ON bias current.
When operating the MAX5943, ON must remain above
its 1.24V threshold. If V
ON
falls below its 1.24V thresh-
old, the power-supply MOSFETs (Q1 and Q2) turn off
immediately (less than 1µs). This allows the power sup-
ply to disconnect in the event of a fault condition
detected by the system, such as shorting of the output
to a different power-supply voltage or a FireWire signal
pin. If possible transient conditions exist at the main
power-supply line (IN), place an RC filter at ON to
reject transient voltage dips and to prevent unneces-
sary power-supply interruptions.
The MAX5943 remains in low-current shutdown mode
when V
ON
is below the 0.4V threshold. Low-current
shutdown mode disables the MAX5943 resulting in less
than 10µA supply current. Shutdown places all logic
inputs in a high-impedance state allowing the inputs to
be connected to IN without drawing additional current
from the supply. An internal delay, t
SD
, allows Q1 and
Q2 to be turned off before the MAX5943 enters low-
current shutdown mode.
Fault Status Output (FAULT)
FAULT is a high-voltage open-drain output that pulls low
when a current-limit/circuit-breaker fault shutdown has
occurred. FAULT remains low until the next startup cycle.
R
V
R
TURN ON
1
124
12=−
⎛
⎝
⎜
⎞
⎠
⎟
×
_
.
SENSE GATE1
OR_ADJGNDLATCHILIM
GATE2
PORT1
OUTIN
MAX5943
TIM
ON
ONQ1
FAULT
SENSE GATE1
OR_ADJGNDLATCHILIM
GATE2
PORT2
OUTIN
MAX5943
TIM
ON
ONQ1
FAULT
Figure 7. MAX5943 Operating in Reverse-Current Mode
(Routing Power from Port 1 to Port 2)
MAX5943
R2
R1
V
IN
IN
GND
ON
Figure 8. Setting the MAX5943 Turn-On Voltage
MAX5943
R2
R1
V
IN
IN
LATCH
GND
ON
Figure 9. Resetting MAX5943 After a Latched Fault Condition