Rainbow Electronics MAX5943 - Manual (Page 10)

MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
10 ______________________________________________________________________________________
Detailed Description
The MAX5943 performs hot-swapping, power-supply
ORing, and current limiting for FireWire power-supply
applications. The MAX5943 incorporates two MOSFET
drivers (GATE1 and GATE2) to control two external
n-channel power MOSFETs to perform low-voltage-drop
power-supply ORing (GATE1) and hot swapping and
current limiting (GATE2) from the input power supply to
the load. A sense resistor provides accurate current-
limiting/circuit-breaker function. GATE1 and GATE2
provide true load disconnect to prevent current flowing
from either direction between IN and OUT.
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.
When the input supply voltage (V IN ) is above 7.5V and
V ON is above the V ON_REF threshold, the MAX5943
turns on, sourcing 45µA current from GATE2 (current
regulation) to enhance Q2 slowly. For the MAX5943A, if
the voltage across the current-sense resistor, V
IS , is
greater than the programmed current-limit threshold,
the MAX5943A regulates the GATE2 voltage to limit the
load current at the current-limit level so that V
IS is equal
to V TH . In normal operation, V IS drops below V TH and
GATE2 rises to approximately 5.5V above OUT. Since
the MAX5943B–MAX5943E do not regulate the load
current, a capacitor can be added from GATE2 to GND
to limit inrush current during startup (see the
Applications Information section).
SENSE
SENSE GATE1
6.5V
1.24V
GATE1 GATE2 OUT
MAX5943A–MAX5943E
POWER
SOURCE
(FAST CURRENT-
LIMIT
THRESHOLD)
CIRCUIT-BREAKER
(CURRENT-LIMIT)
THRESHOLD
OR
COMPARATOR
THRESHOLD
GND
LATCH
TIM
X128
FAULT
+
-
+
-
+
-
+
-
+
-
0.4V
SHUTDOWN
MODE
ON COMPARATOR
( ) MAX5943A ONLY.
UVLO COMPARATOR
+
-
FAULT
MANAGEMENT
LATCH OR
AUTORETRY
AUTORETRY
TIMEOUT
PERIOD
FAULT
TIMEOUT
GATE1
DRIVE
CP1 CP2
GATE
CHARGE
PUMPS
OUT GATE2
GATE2
DRIVE
ONQ1
OR_ADJ
ILIM
ON
R SENSE Q1 Q2
Figure 1. MAX5943 Functional Diagram