Rainbow Electronics MAX5943 User Manual
Page 14
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
14
______________________________________________________________________________________
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.
R
V
R
TURN ON
1
1 24
1
2
=
−
×
_
.
V
INA
I
INA
I
INB
V
INB
V
INB
> V
INA
V
OUT
Figure 6. MAX5943 Power Switch Over ORing Timing Diagram
SENSE GATE1
OR_ADJ
GND
LATCH
ILIM
GATE2
PORT1
OUT
IN
MAX5943
TIM
ON
ONQ1
FAULT
SENSE GATE1
OR_ADJ
GND
LATCH
ILIM
GATE2
PORT2
OUT
IN
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