General Description
The MAX5943 is a fully integrated power-management
IC for FireWire®applications. This device controls two
external power n-channel MOSFETs to regulate current
from the input power supply to the load and performs
low-voltage-drop, power-supply ORing. The MAX5943
operates over a 7.5V to 37V input range and provides
inrush and output current limit in addition to a diode
ORing function suitable for FireWire applications. The
MAX5943 FireWire/IEEE 1394™ protective circuit
(Figure 12) is UL®recognized.
The MAX5943 allows the safe insertion and removal of
FireWire peripherals into a live FireWire port by providing
inrush current control and an output current-limiting func-
tion. The ORing feature provides a very efficient way to
protect the FireWire port when connected to another
FireWire peripheral that sources a higher voltage. By turn-
ing off both external MOSFETS, the MAX5943 provides a
true bidirectional load disconnect. A low-current shut-
down mode disables the MAX5943, resulting in less than
10µA supply current. A pin-selectable input allows
latched or autoretry fault management after a fault.
The MAX5943A features a current-limit function that
actively limits the current drawn by the load with a pro-
grammable timeout. The MAX5943B–MAX5943E do not
actively limit the load current but provide a circuit-breaker
function. See the
Selector Guide
for the default and pro-
grammable circuit-breaker timeouts.
The MAX5943 operates over the -40°C to +85°C extended
temperature range and is available in a 16-pin QSOP
package.
Applications
FireWire Desktop/Notebook Port
FireWire Peripheral Device Current Limit
FireWire Hub
Hot Swap
Features
oProvides Safe Hot Swap for 7.5V to 37V Power
Supplies
oUL-Recognized FireWire/IEEE 1394 Protective
Circuit
oLow-Drop Power ORing with Ultra-Fast Turn-Off
oProgrammable Active Current Limit (MAX5943A)
oProgrammable Circuit Breaker
(MAX5943B–MAX5943E)
o±5% Current-Limit Sensing Accuracy
oProgrammable Current-Limit/Circuit-Breaker
Timeout
oFast Current-Limit Response to Output Short-
Circuit Conditions (MAX5943A)
oDrives External N-Channel Power MOSFETs
oOvercurrent Fault Status Output
oLatched or Autoretry Overcurrent Fault
Management
o10µA Shutdown Current
oON/OFF Control Allows Undervoltage Lockout
Programming
oFast Load Disconnect Control Through ON Input
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
________________________________________________________________
Maxim Integrated Products
1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
ON IN
SENSE
GATE1
N.C.
GATE2
OUT
N.C.
GND
TOP VIEW
MAX5943
QSOP
FAULT
TIM
OR_ADJ
ILIM
LATCH
N.C.
ONQ1
Pin Configuration
Ordering Information
19-3378; Rev 2; 10/07
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Typical Operating Circuit and Selector Guide appear at end
of data sheet.
FireWire is a registered trademark of Apple Computers, Inc.
IEEE 1394 is a trademark of the Institute of Electrical and
Electronics Engineers trademark.
UL is a registered trademark of Underwriters Laboratories, Inc.
PART TEMP RANGE PIN-PACKAGE
MAX5943AEEE -40°C to +85°C 16 QSOP
MAX5943BEEE -40°C to +85°C 16 QSOP
MAX5943CEEE -40°C to +85°C 16 QSOP
MAX5943DEEE -40°C to +85°C 16 QSOP
MAX5943EEEE -40°C to +85°C 16 QSOP
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN = 12V, TA= +25°C.)
(Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(Voltages referenced to GND, unless otherwise noted.)
IN ............................................................................-0.3V to +80V
OUT ........................................................................-0.3V to +80V
SENSE to IN..............................................................-1V to +0.3V
GATE1 to SENSE......................................................-0.3V to +8V
GATE2 to OUT (internally clamped).........................-0.3V to +8V
FAULT.....................................................................-0.3V to +85V
ON, TIM, ILIM, LATCH, OR_ADJ, ONQ1 ..........-0.3V to (IN + 0.3V)
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW
Operating Temperature Range ...........................-40°C to +85°C
Maximum Junction Temperature .....................................+150°C
Storage Temperature Range .............................-60°C to +150°C
ESD Rating (Human Body Model)......................................2000V
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
SUPPLIES
Supply Voltage Range VIN 7.5 37 V
Supply Current IIN VON = 3V, no load 1.3 2.3 mA
Shutdown Current ISD VON = 0.4V 5 10 µA
IN Default Undervoltage Lockout VUVLO VIN rising, ON = IN 6.2 6.5 6.8 V
IN Default Undervoltage Lockout
Hysteresis 0.35 V
ON Undervoltage Lockout
Threshold VON_REF VON rising 1.20 1.24 1.275 V
ON Undervoltage Lockout
Threshold Hysteresis 70 mV
ON Input Bias Current ION 1µA
GATE2 (current-limit/circuit-breaker control)
GATE2 Pullup Current IG2U (Note 2) 36 45 54 µA
7.5V ≤ VIN ≤ 8V 4.25 5.5 6.5
GATE2 High Voltage VG2 VGATE2 - VOUT 8V ≤ VIN ≤ 37V 4.5 5.5 6.5 V
ILIM = GND 47.5 50 52.5
ILIM = floating 37 39 41
Current-Limit/Circuit-Breaker
Threshold Voltage VTH VIN - VSENSE
ILIM = IN 55 58.5 61
mV
ILIM = GND 70 75 80
ILIM = floating 54 59 64
Fast Pulldown Current-Limit
Threshold Voltage (MAX5943A) VTHF VIN - VSENSE (Note 3)
ILIM = IN 83 88 93
mV
GATE2 Pulldown Current
(MAX5943A) IG2D VTH < (VIN - VSENSE) < VTHF,
t < tILIM 335 435 535 µA
GATE2 Fast Pulldown Current
(MAX5943A) IG2DF (VIN - VSENSE) > VTHF, VGATE2 = VOUT +
4.5V 80 125 200 mA
GATE2 Peak Pulldown Current
(MAX5943A) IG2DP (VIN - VSENSE) = 1V 1.0 A
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN = 12V, TA= +25°C.)
(Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
GATE2 Pulldown Current During
Any GATE2 Turn-Off Condition IG2OFF VGATE2 = VOUT + 4.5V (Note 4) 80 125 200 mA
GATE1 (ORing control)
OR_ADJ = GND 3 5 7
OR_ADJ = floating 5.5 7.5 9.5
ORing Switch Turn-On Current
Threshold Voltage VOR (VIN - VSENSE) rising
OR_ADJ = IN 8.5 10.5 12.8
mV
ORing Switch Turn-On Current
Threshold Voltage Hysteresis (VIN - VSENSE) falling 0.5 mV
GATE1 Pullup Current IG1U (Note 5) 36 45 54 µA
7.5V ≤ VIN ≤ 8V 4.25 5.5 6.5
GATE1 High Voltage VG1 VGATE1 - VSENSE 8V ≤ VIN ≤ 37V 4.5 5.5 6.5 V
GATE1 Turn-Off Switch On-
Resistance RG1 Measured between GATE1 and SENSE
when (VIN - VSENSE) < VOR 23Ω
LOGIC INPUTS
IIX Current into or out of input for tri-state logic
condition -3 +3
IIXL Current out of input for logic low 20
ILIM, OR_ADJ Input-Logic
Tristate Current Tolerance
(Note 6) IIH Current into input for logic high 20
µA
VIL VON > VON_REF 0.7 V
LATCH, ONQ1 Input-Logic Low
Voltage IIL LATCH, ONQ1 = GND, VON > VON_REF 1µA
LATCH, ONQ1 Input-Logic High
Voltage VIH VON > VON_REF 1.8 V
LATCH, ONQ1 Input Current IIH All voltages = VIN, VON > VON_REF 1µA
TIM, ILIM, LATCH, OR_ADJ,
OUT, ONQ1 Shutdown Mode
Input Current
IIH_SD All voltages = VIN, VON ≤ 0.4V 1 µA
FAULT Output Low Voltage VOL Low-impedance state, I FAULT = 5mA 0.12 0.25 V
FAULT Output High Leakage
Current IOH High-impedance state, V FAULT = 37V 1 µA
TIMING
TIM = IN 1.78 2.02 2.26 ms
RTIM = 4kΩ0.87 1.04 1.21 ms
RTIM = 50kΩ9.2 11.05 12.9 ms
Current-Limit Timeout
(MAX5943A) tILIM Current-limit mode
(Note 7)
TIM = floating 120 175 225 ms
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN = 12V, TA= +25°C.)
(Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TIM = IN or
TIM ≥ 3.4V 0.45 0.51 0.57 ms
RTIM = 4kΩ27 32.5 38 µs
RTIM = 50kΩ228 345 403 µs
Circuit-breaker mode,
MAX5943B
TIM = floating 3.75 5.5 7 ms
TIM = IN or
TIM ≥ 3.4V 0.89 1.00 1.13 ms
RTIM = 4kΩ54 65 76 µs
RTIM = 50kΩ575 690 806 µs
Circuit-breaker mode,
MAX5943C
TIM = floating 7.5 10.9 14 ms
TIM = IN 1.78 2.02 2.26 ms
RTIM = 4kΩ109 130 151 µs
RTIM = 50kΩ1.15 1.38 1.61 ms
Circuit-breaker mode,
MAX5943D
TIM = floating 15 21.9 28 ms
TIM = IN or
TIM ≥ 3.4V 3.56 4.05 4.52 ms
RTIM = 4kΩ217 260 303 µs
RTIM = 50kΩ2.3 2.76 3.23 ms
Circuit-Breaker Timeout
(MAX5943B–MAX5943E) (Note 7) tCB
Circuit-breaker mode,
MAX5943E
TIM = floating 30 44 56 ms
Automatic Restart Delay After
Current-Limit/Circuit-Breaker
Timeout
tOFF LATCH = GND (Note 8)
128 x
(tILIM
OR tCB
)
ms
GATE1 Turn-Off Time
(ORing Response Time) tOR From VOR trigger until (VGATE1 - VIN) < 1V,
CGATE1 = 10nF 100 ns
Turn-Off Response Time tON_OFF
Time from VON < (VON_OFF - Hysteresis)
until (VGATE2 - VOUT) < 1V, GATE2 = open
(Note 9)
0.34 0.6 µs
Minimum Delay from ON Low to
Low-Current Shutdown Mode tSD ON step from 1.3V to 0.4V 50 µs
Note 1: All min/max parameters are tested at +25°C and +85°C. Limits at -40°C are guaranteed by design.
Note 2: Measured at both VGATE2 = VOUT = 0V and VGATE2 = VOUT = VIN.
Note 3: The typical value of parameter VTHF is 1.5 x VTH.
Note 4: Current-Limit/Circuit-Breaker Timeout; IN or ON low.
Note 5: See the
Typical Operating Circuit
. Measured at VGATE1 = VSENSE.
Note 6: The tristate (“floating”) logic condition will be entered if the pin current is within these limits.
Note 7: RTIM is connected from TIM to GND. The maximum timeout period is enforced to prevent arbitrarily long operation in cur-
rent-limit mode. Therefore, the relationship between resistor value and timeout changes for large values of RTIM. See
Current-Limit Timeout vs. RTIM in the
Typical Operating Characteristics
for nominal values.
Note 8: Parameter tOFF is a direct multiple of tILIM (or tCB), so the limits for tOFF track the limits for tILIM (or tCB).
Note 9: If the ON voltage goes below VON_REF - Hysteresis, then GATE2 goes low immediately to disconnect power from the load
quickly (within tON_OFF).
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 5
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5943 toc01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
353020 251510
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
1.0
540
TA = +85°C
TA = +25°C
TA = -40°C
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5943 toc02
SUPPLY VOLTAGE (V)
SHUTDOWN SUPPLY CURRENT (μA)
353020 251510
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
0
540
CURRENT-LIMIT/CIRCUIT-BREAKER
THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5943 toc03
TEMPERATURE (°C)
THRESHOLD VOLTAGE (mV)
603510-15
46.5
47.0
47.5
48.0
48.5
49.0
49.5
50.0
50.5
51.0
51.5
52.0
46.0
-40 85
ILIM = GND
VIN = 32V
VIN = 12V
VIN = 7.5V
CURRENT-LIMIT/CIRCUIT-BREAKER
THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5943 toc04
TEMPERATURE (°C)
THRESHOLD VOLTAGE (mV)
603510-15
56.5
57.0
57.5
58.0
58.5
59.0
59.5
60.0
60.5
61.0
61.5
62.0
56.0
-40 85
ILIM = IN
VIN = 32V
VIN = 12V
VIN = 7.5V
CURRENT-LIMIT/CIRCUIT-BREAKER
THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5943 toc05
TEMPERATURE (°C)
THRESHOLD VOLTAGE (mV)
603510-15
36.5
37.0
37.5
38.0
38.5
39.0
39.5
40.0
40.5
41.0
41.5
42.0
36.0
-40 85
ILIM = FLOATING
TIMEOUT PERIOD vs. RTIM
MAX5943 toc06
RTIM (kΩ)
TIMEOUT PERIOD (μs)
10010
0.1
1
10
100
0.01
1 1000
MAX5943A
MAX5943E
ORing THRESHOLD VOLTAGE
vs. TEMPERATURE
MAX5943 toc07
TEMPERATURE (°C)
ORing THRESHOLD VOLTAGE (mV)
6035-15 10
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
4.0
-40 85
5.5
6.5
7.5
8.5
9.5
10.5
11.5
4.5
OR_ADJ = IN
OR_ADJ = OPEN
OR_ADJ = GND
GATE CURRENTS vs. TEMPERATURE
MAX5943 toc08
TEMPERATURE (°C)
GATE CURRENTS (μA)
603510-15-40 85
GATE2
44.5
45.0
45.5
46.0
46.5
47.0
44.0
GATE1
GATE1 DRIVE VOLTAGE
vs. TEMPERATURE
MAX5943 toc09
TEMPERATURE (°C)
GATE1 DRIVE VOLTAGE (V)
603510-15
5.233
5.236
5.239
5.242
5.245
5.248
5.251
5.254
5.257
5.260
5.230
-40 85
Typical Operating Characteristics
(VIN = +12V, TA= +25°C, unless otherwise noted.)
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VIN = +12V, TA= +25°C, unless otherwise noted.)
GATE2 DRIVE VOLTAGE
vs. TEMPERATURE
MAX5943 toc10
TEMPERATURE (°C)
GATE2 DRIVE VOLTAGE (V)
603510-15
5.203
5.206
5.209
5.212
5.215
5.218
5.221
5.224
5.227
5.230
5.200
-40 85
ON HIGH VOLTAGE vs. TEMPERATURE
MAX5943 toc11
TEMPERATURE (°C)
ON HIGH VOLTAGE (V)
603510-15
1.252
1.254
1.256
1.258
1.260
1.250
-40 85
ON LOW VOLTAGE vs. TEMPERATURE
MAX5943 toc12
TEMPERATURE (°C)
ON LOW VOLTAGE (V)
603510-15-40 85
1.182
1.184
1.186
1.188
1.190
1.192
1.194
1.196
1.198
1.200
1.180
FAULT LOW VOLTAGE
vs. TEMPERATURE
MAX5943 toc13
TEMPERATURE (°C)
FAULT LOW VOLTAGE (mV)
603510-15-40 85
20
40
60
80
100
120
140
160
180
200
0
IFAULT = 5mA
ORing THRESHOLD RESPONSE TIME
MAX5943 toc14
IIN
500mA/div
VOUT
5V/div
(VGATE - VSENSE)
5V/div
VIN
10V/div
100μs/div
OR_ADJ = IN
TURN-OFF RESPONSE TIME
MAX5943 toc15
IIN
1A/div
VOUT
10V/div
VGATE2
10V/div
VON
2V/div
200ns/div
MAX5943A SHORT-CIRCUIT EVENT
(VIN = 37V)
MAX5943 toc16
IIN
1A/div
VOUT
50V/div
VGATE2
50V/div
VIN
50V/div
400μs/div
TIM = IN
ILIM = GND
VFAULT
50V/div
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 7
MAX5943A SHORT-CIRCUIT EVENT
(VIN = 37V)
MAX5943 toc17
IIN
10A/div
VOUT
50V/div
VGATE2
50V/div
VIN
50V/div
100ns/div
ILIM = GND
VFAULT
50V/div
MAX5943A SHORT-CIRCUIT RESPONSE
(VIN = 8V)
MAX5943 toc18
IIN
1A/div
VOUT
10V/div
VGATE2
10V/div
VIN
10V/div
400μs/div
TIM = IN VFAULT
10V/div
MAX5943A SHORT-CIRCUIT RESPONSE
(VIN = 8V)
MAX5943 toc19
IIN
10A/div
VOUT
10V/div
VGATE2
10V/div
VIN
10V/div
100ns/div
VFAULT
10V/div
MAX5943E SHORT-CIRCUIT RESPONSE
(VIN = 24V)
MAX5943 toc20
IIN
166A/div
VOUT
50V/div
VGATE2
50V/div
VIN
50V/div
200μs/div
VFAULT
50V/div
MAX5943E SHORT-CIRCUIT RESPONSE
(VIN = 8V)
MAX5943 toc21
IIN
60A/div
VOUT
10V/div
VGATE2
10V/div
VIN
10V/div
100μs/div
VFAULT
10V/div
TIM = IN
ORing SWITCHOVER WAVEFORM
MAX5943 toc22
IIN1
500mA/div
VOUT
1V/div
200μs/div
IIN2
500mA/div
VIN1 = 12V VON2 RISING = 12V
VIN2 = 128V COUT = 0
Typical Operating Characteristics (continued)
(VIN = +12V, TA= +25°C, unless otherwise noted.)
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
8 _______________________________________________________________________________________
ORing SWITCHOVER WAVEFORM
(VIN2 RISING ABOVE VIN1)
MAX5943 toc23
IIN1
500mA/div
VOUT
1V/div
10μs/div
IIN2
500mA/div
VIN1 = 12V VON2 RISING = 12V
VIN2 = 13V COUT = 0
ORing SWITCHOVER WAVEFORM
(VIN2 FALLING BELOW VIN1)
MAX5943 toc24
IIN1
500mA/div
VOUT
1V/div
20μs/div
IIN2
500mA/div
VIN1 = 12V VON2 FALLING = 12V
VIN2 = 13V COUT = 0
MAX5943A HOT PLUG-IN LOAD CAPACITANCE
INTO ACTIVE PORT (NO FAULT)
MAX5943 toc25
IIN
2A/div
VFAULT
10V/div
400μs/div
VOUT
10V/div
CLOAD = 220μF
TIM = IN
VGATE2
10V/div
MAX5943A HOT PLUG-IN LOAD CAPACITANCE
INTO ACTIVE PORT (CURRENT-LIMIT TIMEOUT)
MAX5943 toc26
IIN
1A/div
VFAULT
10V/div
400μs/div
VOUT
10V/div
CLOAD = 470μF
TIM = IN
VGATE2
10V/div
Typical Operating Characteristics (continued)
(VIN = +12V, TA= +25°C, unless otherwise noted.)
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 9
Pin Description
PIN NAME FUNCTION
1ON
On/Off-Control Input. ON sets the undervoltage lockout threshold and resets the part after a fault latch.
Drive ON high to enable the device. Drive ON low to disable the device. Connect ON to IN for the default
undervoltage lockout threshold. Connect a resistor-divider from IN to ON and GND to program the desired
undervoltage lockout threshold.
2FAULT Current-Fault, Active-Low, Open-Drain Output. FAULT asserts and latches low after a current-limit/circuit-
breaker fault has exceeded the current-limit/circuit-breaker timeout period (see the
FAULT
section).
3 TIM Current-Limit/Circuit-Breaker Timeout Adjustment Input. Connect TIM to IN for the default timeout period or
connect a resistor from TIM to GND to program the current-limit/circuit-breaker timeout period.
4 ILIM
Current-Limit/Circuit-Breaker Threshold Input. Leave ILIM open for a 40mV circuit-breaker threshold,
connect to GND for a 50mV circuit-breaker threshold, or connect to IN for a 60mV circuit-breaker threshold
(see the Current Limiting section).
5 LATCH
Latch or Autoretry Fault Management Selection Input. Connect LATCH to IN to select latch-off mode
after a current-limit/circuit-breaker timeout. Connect LATCH to GND for autorestart mode after a current-
limit/circuit-breaker timeout.
6 OR_ADJ
ORing Switch Turn-On Current Threshold Adjustment Input. Select one of three ORing switch turn-on
threshold settings: connect OR_ADJ to GND, connect OR_ADJ to IN, or leave OR_ADJ floating (see the
Power-Supply ORing section).
7, 10, 13 N.C. No Connection. Not internally connected.
8 ONQ1
ORing Switch Manual Turn-On Input. Drive ONQ1 low to enable power-supply ORing. Drive ONQ1 high
to set GATE1 high to allow reverse current flow. ONQ1 is disabled when VIN is below the UVLO threshold,
VON is below the VON_REF threshold, and after a current/circuit-breaker fault.
9 GND Ground
11 OUT Output Voltage Sense. Connect to the output.
12 GATE2
Current-Limiter Switch Gate Drive Output. Connect GATE2 to the gate of the 2nd n-channel MOSFET.
GATE2 is a charge pump with a 45µA pullup current to 5.5V (typ) above OUT when active (see the Typical
Operating Circuit).
14 GATE1
ORing Switch Gate-Drive Output. Connect GATE1 to the gate of the 1st n-channel MOSFET. GATE1 is a
charge pump with a 45µA pullup current to 5.5V (typ) above SENSE when active (see the Typical
Operating Circuit).
15 SENSE Current-Sense Negative Input. Connect a current-sense resistor, RSENSE, from IN to SENSE (see the
Typical Operating Circuit).
16 IN Input Power and Current-Sense Positive Input. Connect IN to the positive terminal of the current-sense
resistor and to the power source (see the Typical Operating Circuit).
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 VON 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 (VIN) is above 7.5V and
VON is above the VON_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, VIS, 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 VIS is equal
to VTH. In normal operation, VIS drops below VTH 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
OUTGATE2
GATE2
DRIVE
ONQ1
OR_ADJ
ILIM
ON
RSENSE Q1 Q2
Figure 1. MAX5943 Functional Diagram
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 11
The MAX5943 current-limit/circuit-breaker timeout is
programmable by connecting an external resistor from
TIM to GND. The MAX5943A has a default 2ms current-
limit timeout. The MAX5943B, MAX5943C, MAX5943D,
and MAX5943E have 0.5ms, 1ms, 2ms, and 4ms
default circuit-breaker timeouts, respectively.
GATE1 controls the MAX5943 ORing function. Initially,
GATE1 is off (ONQ1 = low) and the load current con-
ducts through the body diode of Q1. GATE1 rises to
5.5V above SENSE when VIS exceeds VOR, thereby
enhancing Q1 and reducing the voltage drop, power
dissipation, and heat generation in the power-supply
path. When a voltage greater than VIN is connected at
OUT, the higher voltage source commences, providing
current to the load. The MAX5943 rapidly turns off
GATE1 upon VIS falling below VOR - hysteresis, thus
blocking the higher voltage from back-driving VIN.
GATE1 also turns off when the load current drops,
causing VIS to fall below the VOR hysteresis.
Current Limiting/Circuit Breaker
Current Limiting (MAX5943A Only)
The MAX5943A limits the load current by monitoring the
voltage across RSENSE at all times and by regulating the
current to the load as necessary to keep VIS to its pro-
grammable current-limit threshold voltage (VTH). Set the
maximum current limit (ILIMIT) by placing the appropriate
sense resistor between IN and SENSE, where ILIMIT =
VTH/RSENSE. When the load current is less than ILIMIT,
GATE2 rises to VG2 (5.5V) to fully enhance MOSFET Q2
(see the
Typical Operating Circuit)
.
When the load current attempts to draw more than
ILIMIT, the MAX5943A’s GATE2 pulldown current (IG2D)
regulates the current through Q2 causing OUT to act as
a constant-current source. The output current is limited
to the programmed ILIMIT. If the current-limit condition
persists after the current-limit timeout (tILIM) period has
expired, GATE2’s strong pulldown current (IG2 OFF)
quickly turns off GATE2 to disconnect the load from the
power supply. FAULT then asserts low (Figure 2a). If the
current-limit duration is less than tILIM, GATE2 pulls to
VG2 (Figure 2b). During an output short circuit or a
gross overload, the load current overshoots and causes
VIS to exceed the fast pulldown current-limit threshold
voltage (VTHF). The MAX5943A responds with much
stronger GATE2 pulldown currents (IG2DP and/or IG2DF)
to quickly bring the load current back down to the pro-
grammed current limit.
IOUT
VIN
VOUT
VGATE1
VGATE2
VFAULT
IOUT
VOUT
VGATE1
VGATE2
VFAULT
tILIM
< tILIM ILIMIT
ILIMIT
ILIMIT x RLOAD
ILIMIT x RLOAD
(a)
(b)
Figure 2. MAX5943A Current-Limit Timing Diagram
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
12 ______________________________________________________________________________________
MAX5943B–MAX5943E Circuit Breaker
The MAX5943B–MAX5943E provide a circuit-breaker
function that protects against excessive load currents
and short-circuit conditions. The load current is monitored
by sensing the voltage across an external sense resistor
(RSENSE) and disconnects the load from the power
source when the voltage across RSENSE,V
IS, exceeds
the programmed circuit-breaker thresholds (VTH) for
longer than the circuit-breaker timeout period, tCB.
Unlike the MAX5943A, when an output short circuit or
overcurrent occurs, the MAX5943B–MAX5943E keep
GATE2 fully enhanced. The short-circuit current is limit-
ed only by the input source impedance, RSENSE, Q1’s
RDS(ON), Q2’s RDS(ON), and any parasitic impedance in
the power path. If the excessive current condition per-
sists (as indicated by VIS > VTH) for longer than tCB,
GATE2 turns off with a strong pulldown (IG2 OFF) to
quickly disconnect the power source from the load and
FAULT asserts low (Figure 3).
Exercise caution when using the MAX5943B–MAX5943E.
Unlike the MAX5943A, the MAX5943B–MAX5943E do not
limit the load current during tCB. High current during tCB
may cause damage to external devices connected in the
power path.
Current-Limit/Circuit-Breaker Thresholds
The MAX5943 allows three selectable current-limit/cir-
cuit-breaker thresholds (VTH). Choose the voltage
threshold by connecting ILIM to the appropriate level;
see Table 1.
Current-Limit/Circuit-Breaker
Timeout Period (tILIM/tCB)
The MAX5943’s current-limit/circuit-breaker timeout
function disconnects power from the load if the current-
limit or circuit-breaker threshold voltage exceeds its
maximum limit for a programmable amount of time.
Connect TIM to IN for the MAX5943 default timeout
periods (see Table 2).
Adjust the fault timeout period (tILIM/ tCB) by connect-
ing a resistor (RTIM) from TIM to GND (see the
Typical
Operating Characteristics
). Calculate the fault-limit
timeout period using the following equation:
where RTIM is in kΩ, tILIM/tCB is in ms, and K = 1 for
MAX5943A, K = 32 for MAX5943B, K = 16 for
MAX5943C, K = 8 for MAX5943D, and K = 4 for the
MAX5943E.
tmstms Rk
K
ILIM CB TIM
() () . ()
==×025 Ω
IOUT
ILIMIT
tCB
VIN
VG2
VG2
VOUT
VGATE1
VGATE2
VFAULT
Figure 3. MAX5943B–MAX5943E Circuit-Breaker Timing Diagram
Table 1. Current-Limit/Circuit-Breaker
Trip Threshold Level
ILIM CONNECTION VTH (mV)
Open 40
GND 50
IN 60
PART
DEFAULT
TIMEOUT,
TIM = IN
(ms)
PROGRAMMABLE
TIMEOUT RANGE
(4kΩ < RTIM < 50kΩ)
MAXIMUM
TIMEOUT,
TIM =
FLOATING
(ms)
MAX5943A 2 1.04ms to 11.05ms 175
MAX5943B 0.5 32.5µs to 345µs 4.8
MAX5943C 1 65µs to 690µs 9.6
MAX5943D 2 130µs to 1.38ms 19.2
MAX5943E 4 260µs to 2.76ms 38.4
Table 2. MAX5943_ Timeout Periods
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 13
Timing accuracy is guaranteed for RTIM ranging from
4kΩto 50kΩ. Large RTIM values can be used, which
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 RSENSE, Q1, Q2, and any other components in the
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).
GATE1 rises to 5.5V above SENSE when VIS exceeds
VOR, enhancing Q1. Q1’s low RDS(ON) provides a very-
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, VIS, across the current-sensing
resistor. Q1 turns off rapidly when VIS decreases below
VOR minus its hysteresis.
Set the MAX5943 ORing threshold (VOR) by connecting
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 VUVLO and VON_REF, respectively, and the
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).
SENSE GATE1
OR_ADJ ONQ1GND
GATE2 OUT
Q1 Q2
IN
TO FireWire
PORT
MAX5943
ON
VIN RSENSE
+ VIS -
Figure 4. Q1 Current Path During Inital Startup
ILOAD
RSENSE
SENSE GATE1
OR_ADJ ONQ1GND
GATE2 OUTIN
VINA VINB > VINA
FROM
BATTERY
SOURCE
TO FireWire
PORT
IINA = 0
MAX5943
ON
RSENSE
SENSE GATE1
OR_ADJ ONQ1GND
GATE2 OUTIN
VINB FROM
POWER
SOURCE
IINB
MAX5943
ON
Figure 5. MAX5943 ORing Controller Rerouting Load Current
VINA
IINA
IINB
VINB
VINB > VINA
VOUT
Figure 6. MAX5943 Power Switch Over ORing Timing Diagram
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 VIN is greater than VUVLO, VON is above the
VON_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 VIN > 6.5V and VON <
VON_REF. The turn-on threshold is programmable by
connecting a resistor-divider at ON. When VON > 1.24V
and VIN > 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 VTURN_ON is the desired turn-on voltage of the
MAX5943. Choose an R2 value in the 1kΩrange to
minimize error due to the ON bias current.
When operating the MAX5943, ON must remain above
its 1.24V threshold. If VON 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 VON 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, tSD, 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.
RVR
TURN ON
1124 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
VIN
IN
GND
ON
Figure 8. Setting the MAX5943 Turn-On Voltage
MAX5943
R2
R1
VIN
IN
LATCH
GND
ON
Figure 9. Resetting MAX5943 After a Latched Fault Condition
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 15
Fault Management
The MAX5943 offers either latch or autoretry fault man-
agement configurable by the LATCH input. Connect
LATCH to IN for latch fault management or connect
LATCH to GND for autoretry fault management. In latch
fault management, FAULT latches low, GATE1 and
GATE2 latch off indefinitely. Cycle ON low and then
high to unlatch and restart the MAX5943. However, the
MAX5943 will not enter a startup cycle until tOFF has
expired. Figure 9 illustrates a way to reset the MAX5943
after a fault using a pushbutton switch.
In autoretry fault management, the MAX5943_ attempt
to restart after a tOFF of 128 x tILIM (or 128 x tCB) limit-
ing the duty cycle of the MOSFETs to 1/129 under con-
tinuous fault conditions. FAULT deasserts every time a
restart attempt is made.
Applications Information
Startup Consideration
MAX5943A
During startup, a large capacitor at OUT may result in a
charging current equivalent to the current limit. Choose a
current-limit timeout that will allow a successful startup.
The timeout can be approximated using the following
equation:
\where ILIMIT is the programmed current limit, COUT is
the capacitor at OUT, VIN is the supply voltage, and
ILOAD is the load current during startup. With IN = 12V,
COUT = 330µF, ILIMIT = 1.5A, and ILOAD = 0, the
MAX5943 commences by charging the output capacitor
with 1.5A for approximately 2.7ms. Therefore, the
MAX5943A current-limit timeout period (tILIM) should be
greater than 2.7ms for a successful startup. Otherwise,
the MAX5943A powers up in fault management mode
by exceeding the current-limit timeout period.
MAX5943B–MAX5943E
The MAX5943B–MAX5943E do not control the inrush
current during startup. Inrush current control can be
implemented by placing a resistor and capacitor at
GATE2 (Figure 10) to slowly ramp up the gate voltage,
thus limiting the inrush current. The inrush current can
be approximated using the following formula:
Where IG2U is GATE2’s 45µA sourcing current and
ILOAD is the load current at startup.
To prevent the MAX5943B–MAX5943E from starting up
in a fault condition set:
Optimizing for Short-Circuit Conditions
Choosing R
SENSE
Select a sense resistor that causes the circuit-breaker
voltage drop at a current-limit/circuit-breaker level
above the maximum normal operating current.
Typically, set the overload current at 1.2 to 1.5 times
the full load current.
Choose the sense-resistor power rating to accommodate
an overcurrent condition:
PRSENSE = I2LIMIT x RSENSE
where PRSENSE is the power dissipated across RSENSE
during a current-limit/circuit-breaker fault.
Under short-circuit conditions, it is imperative that the
appropriate sense resistor is utilized. Operating the
MAX5943B–MAX5943E at high input voltages can
cause very large currents during the circuit-breaker
timeout period. The peak current will be limited by the
saturation current of Q2 or the series resistance in the
power path (RTOTAL).
Using a 30mΩon-resistance MOSFET at GATE1 and
GATE2 and a 30mΩsense resistor results in a short-cir-
cuit current approximately equal to:
ISC = VIN/RTOTAL
where:
RTOTAL = RSENSE + 2 x (RON)
= 30mΩ+ 2 x (30mΩ) = 90mΩ
For example, an input voltage of 20V produces a current
at approximately 222A (or ISAT of Q2, whichever is less)
in the power path for the circuit-breaker timeout period.
Choose an RSENSE capable of handling the high power
dissipation during a short-circuit event.
IV
R
INRUSH TH
SENSE
<
IC
CII
INRUSH OUT
GATE G U LOAD
=×+
22
tCV
II
ILIM OUT IN
LIMIT LOAD
>×
−
Q1 Q2
CGATE
COUT
RSENSE
SENSE GATE1 GATE2
GND
OUT
LOAD
45μA
IN
+
-
VIN
ICILOAD
MAX5943B–MAX5943E
1kΩ
Figure 10. Controlling the MAX5943B–MAX5943E Inrush Current
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
16 ______________________________________________________________________________________
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (RDS(ON))
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High RDS(ON) also causes large output ripple if there is
a pulsating load. Determine the device power rating to
accommodate startup, a short-circuit condition, and
when the device is in autoretry mode.
During normal operation, the external MOSFETs dissipate
little power. The power dissipated in normal operation is:
P = ILOAD2x RDS(ON)
The most power dissipation occurs during a short-circuit
event, resulting in high power dissipated in Q2 (Figure
11) during the timeout period for the MAX5943A, where
the power dissipated across Q2 is:
PQ2 = (VIN - VIS – VQ1 ) x ILIMIT
For the MAX5943B–MAX5943E, a short-circuit event
results in high power dissipated in both Q1 and Q2 dur-
ing the timeout period (Figure 13) where the total power
dissipated in either MOSFET is:
P = ISC2x RDS(ON)
where:
ISC = VIN/REQ
and
REQ = RSENSE +RDS(ON1) + RDS(ON2)
VQ1 VQ2
VIS
SENSE GATE1 GATE2 OUTIN
+-
+- +-
VIN
MAX5943A
Figure 11. Power Dissipated Across MOSFETs During a Short-
Circuit Fault for MAX5943A
R2
0.020Ω
1%
0.25W
C3
1μF
C4
0.1μF
L1
FERRITE
250Ω
R3
1kΩ
R4
1kΩ
R5
100kΩ
R6
100kΩ
DMMT3906
Q1B
Q1A
GND
NC
OUT
GATE2
N.C.
GATE1
SENSE
IN
R1
200kΩ
C2
0.01μF
C1
1μF
J1
VIN
J2
GND
R7
100kΩ
TP1
VIN
TP2
GND
TP3
PORT 0
TP4
FAULT
R8
100kΩ
TP5
ON
TP6
VPULLUP
135
J3
N1
Si2318DS
N2A
Si7222DN N2B
246
PORT 0
ON
FAULT
TIM
ILIM
LATCH
OR_ADJ
N.C.
ONQ1
1
2
3
4
5
6
7
89
10
11
12
13
14
15
16
MAX5943AEEE
U1
Figure 12. UL-Recognized Single-Port FireWire Protective Circuit
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 17
The programmable timeout of the MAX5943 allows the
use of MOSFETs with low power ratings. A MOSFET
typically withstands single-shot pulses with higher dis-
sipation than the specified package rating.
A single-port FireWire protective circuit employing the
MAX5943A has been UL recognized as a result of
extensive safety testing by Underwriters Laboratories. It
is the circuits themselves, not any individual component
thereof, that are UL recognized. The circuit is further
described in Application Note 3984—
UL Recognized,
IEEE 1394 Single- and Dual-Port FireWire Protective
Circuits
available at www.maxim-ic.com/appnotes.cfm/
appnote_number/3984.
The circuit provides:
• 2.5A (adjustable) current limiting to the load at initial
connection or in overload fault condition
• 2ms (adjustable) current-limit timeout before load
disconnect for excessive load current
• Automatic-restart attempts after a fault shutdown
condition
• <0.8% current-limited duty cycle in continuous fault
condition
• 6.5V undervoltage shutdown protection
• Low-drop ORing in multiple power-source applica-
tions, and
• Fault reporting
RDS(ON)1 RDS(ON)1
RSENSE
SENSE GATE1 GATE2 OUT
ISC
IN
VIN
MAX5943B–MAX5943E
Figure 13. Power Dissipated Across MOSFETs During a Short-
Circuit Fault for MAX5943B–MAX5943E
Figure 14. Two-Port FireWire Power Provider Using MAX5943A and MAX5944
SENSE GATE1
OR_ADJONQ1GNDLATCH
GATE2 OUTIN
MAX5943A
TIM
ON
ILIM
FAULT
15mΩSI7958DP SI7958DP
SENSE GATE1
OR_ADJONQ1GNDLATCH
GATE2 OUTIN
MAX5943A
TIM
ON
ILIM
FAULT
15mΩSI7958DP SI7958DP
SENSEA GATE1A GATE2A OUTAINA
MAX5944
ONA
ONQ1
GND
ONB
FAULTA
FAULTB
30mΩSI7958DP SI7958DP
30mΩIRF7478 IRF7478
MAX5033
SGND
VIN
ON
FB
GND
VD
IN4148 IN4148
IN4148
BST
LX
PHY
POWER OUT
CABLE
ON
NOT
SLEEP
CURRENT-LIMIT
SENSE
PORT 1
SENSE
PORT 0
SENSE
CABLE
POWER
SOURCE
BATTERY-
POWER
SOURCE IN4148
CPA SENSE
PORT 0
PORT 1
SENSEB GATE1B GATE2B OUTBINB
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
18 ______________________________________________________________________________________
Chip Information
TRANSISTOR COUNT: 2324
PROCESS: BiCMOS
SENSE GATE1
OR_ADJ ONQ1GND
MAX5943 EQUIVALENT CIRCUIT
LATCHILIM
FAULT
ON
GATE2 OUT
Q1 Q2
LOAD DISCONNECT
SWITCH
VERY-LOW-DROP
ORing DIODE
CURRENT-LIMIT/CIRCUIT-BREAKER
AND INRUSH CONTROL
IN
TO
FireWire
PORT
FireWire
PORT
SUPPLY
MAX5943A
TIM
R3
+ VIS -
RSENSE
R1
R2
Typical Operating Circuit
TIMEOUT RANGE
MAX5943A
Current limit
1.04ms to 11.05ms
MAX5943B
Circuit breaker
MAX5943C
Circuit breaker
MAX5943D
Circuit breaker
130µs to 1.38ms
MAX5943E
Circuit breaker
260µs to 2.76ms
Selector Guide
30mΩSI7958DP SI7958DP
SENSE GATE1
OR_ADJ ONQ1GNDLATCHILIM
FAULT
ON
GATE2 OUT
Q1 Q2
IN
+
-
VIN = 12V
TO
FireWire
PORT
FireWire
PORT
SUPPLY
MAX5943A
TIM
4kΩ
10kΩ1kΩ
7.15kΩ
Typical Application Circuit
FireWire Power Management
The MAX5943 serves to regulate and protect FireWire
power over a system interface. The MAX5943 program-
mable features make it suitable for both power provider
and power receiver applications. Figure 14 shows a
high-end two-port FireWire power management system
using two MAX5943As and a dual-channel MAX5944
FireWire current-limiting IC.
Transient Protection
If IN or OUT experiences a fast voltage transient, the
drain-to-gate parasitic capacitance of Q1 and/or Q2
may be sufficient to briefly enhance one of the transis-
tors, allowing current glitch. If the circuit is subjected to
large transients, connect capacitors from the gate to
source across the appropriate MOSFET, preventing the
parasitic capacitance from turning on the device.
Caution: The additional gate-to-source capacitance
causes the turn-off time of the MOSFETs to increase
due to the additional charge of the capacitor. Use the
minimum capacitor value to prevent currents from flow-
ing in hot-plug situations.
MAX5943
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
QSOP.EPS
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 19
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
20
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 8/04 Initial release —
1 1/05 Corrected Table 2 and Figures 10, 11, and 12 12, 15, 16, 17
2 10/07
Added FireWire/IEEE and UL information, inserted new Figure 12, and updated
Package Outline 1, 16, 17, 18, 20