General Description
The MAX5955 and MAX5956 are +1V to +13.2V dual
hot-swap controllers with independent on/off control for
complete protection of dual-supply systems. They allow
the safe insertion and removal of circuit cards into live
backplanes. The MAX5955 and MAX5956 operate
down to 1V provided one of the inputs is above 2.7V.
The discharged filter capacitors of the circuit card pro-
vide low impedance to the live backplane. High inrush
currents from the backplane to the circuit card can burn
up connectors and components, or momentarily collapse
the backplane power supply leading to a system reset.
The MAX5955 and MAX5956 hot-swap controllers pre-
vent such problems by gradually ramping up the output
voltage and regulating the current to a preset limit when
the board is plugged in, allowing the system to stabilize
safely. After the startup cycle is completed, two on-chip
comparators provide VariableSpeed/BiLevel™ protection
against short-circuit and overcurrent faults, as well as
immunity against system noise and load transients. In the
event of a fault condition, the load is disconnected. The
MAX5955B and MAX5956B must be unlatched after a
fault, and the MAX5955A and MAX5956A automatically
restart after a fault.
The MAX5955 and MAX5956 integrate an on-board
charge pump to drive the gates of low-cost, external n-
channel MOSFETs. The devices offer integrated fea-
tures like startup current regulation and current glitch
protection to eliminate external timing resistors and
capacitors. These devices provide open-drain status
outputs, an adjustable startup timer, and adjustable
current limits. The MAX5955 provides output undervolt-
age/overvoltage protection for each channel, while the
MAX5956 provides undervoltage/overvoltage monitor-
ing for each channel.
The MAX5955 and MAX5956 are available in a space-
saving 16-pin QSOP package.
Applications
Features
oSafe Hot Swap for +1V to +13.2V Power Supplies
with VIN1 or VIN2 ≥ 2.7V
oIndependent On/Off Control for Each Channel
oInternal Charge Pumps Generate n-Channel
MOSFET Gate Drives
oInrush Current Regulated at Startup
oCircuit-Breaker Function
oAdjustable Circuit Breaker/Current-Limit
Threshold from 25mV to 100mV
oVariableSpeed/BiLevel Circuit Breaker Response
oAutoretry or Latched Fault Management
oStatus Outputs Indicate Fault/Safe Condition
oOutput Undervoltage and Overvoltage Monitoring
or Protection
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap
Controllers with Independent On/Off Control
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
PGOOD1 PGOOD2
ON2
IN2
SENSE2
GATE2
ON1
LIM2
MON2
TOP VIEW
MAX5955
MAX5956
QSOP
TIM
IN1
GND
SENSE1
GATE1
LIM1
MON1
Pin Configuration
19-3813; Rev 0; 9/05
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART TEMP RANGE PIN-PACKAGE
MAX5955AEEE -40°C to +85°C 16 QSOP
MAX5955AEEE+ -40°C to +85°C 16 QSOP
MAX5955AUEE 0°C to +85°C 16 QSOP
MAX5955AUEE+ 0°C to +85°C 16 QSOP
MAX5955BEEE -40°C to +85°C 16 QSOP
MAX5955BEEE+ -40°C to +85°C 16 QSOP
MAX5955BUEE 0°C to +85°C 16 QSOP
MAX5955BUEE+ 0°C to +85°C 16 QSOP
Variable Speed/BiLevel is a trademark of Maxim Integrated
Products, Inc.
+
Denotes lead-free package.
Ordering Information continued at end of data sheet.
Selector Guide and Typical Operating Circuit appear at end
of data sheet.
Base Station Line Cards
Network Switches,
Routers, Hubs
Solid-State Circuit
Breakers
RAID
Power-Supply Sequencing
Hot Plug-In Daughter
Cards
Portable Computer Device
Bays (Docking Stations)
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN_ = +1V to +13.2V provided at least one supply is higher than +2.7V, VON1 = VON2 = +2.7V, TA= TMIN to TMAX, unless otherwise
noted. Typical values are at VIN1 = +5V, VIN2 = +3.3V, and 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.
IN_ to GND...........................................................................+14V
GATE_ to GND ...........................................-0.3V to (VIN_ + 6.2V)
ON_, PGOOD_, TIM to GND.......................-0.3V to the higher of
(VIN1 + 0.3V) and (VIN2 + 0.3V)
SENSE_, MON_, LIM_ to GND ...................-0.3V to (VIN_ + 0.3V)
Current into Any Pin .........................................................±50mA
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW
Operating Temperature Range
MAX59_ _ _U_ _...................................................0°C to +85°C
MAX59_ _ _E_ _ ................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
POWER SUPPLIES
IN_ Input Voltage Range VIN Other VIN ≥ +2.7V 1.0 13.2 V
Supply Current IIN IIN1 + IIN2, VIN1 = +5V, VIN2 = +3.3V 1.2 2.3 mA
CURRENT CONTROL
TA = +25°C 22.5 25 27.5
LIM = GND TA = -40°C to +85°C 20.5 27.5
Slow-Comparator Threshold
(VIN_ - VSENSE_) (Note 2) VSC,TH
RLIM = 300kΩ80 100 130
mV
1mV overdrive 3 ms
Slow-Comparator Response Time
(Note 3) tSCD 10mV overdrive 110 µs
VSU,TH During startup 2 x VSC,TH
Fast-Comparator Threshold
(VIN_ - VSENSE_) VFC,TH VIN_ - VSENSE_; normal operation 4 x VSC,TH
mV
Fast-Comparator Response Time
(VIN_ - VSENSE_) tFCD 10mV overdrive, from overload condition 260 ns
SENSE Input Bias Current IB SENSE VSENSE_ = VIN_ 0.03 1 µA
MOSFET DRIVER
RTI M = 100kΩ 6 10.8 16
RTIM = 4kΩ (minimum value) 0.31 0.45 0.58
Startup Period (Note 4) tSTART
TIM floating 4 9 17
ms
C har g i ng , V
GATE
_ = + 5V , V
IN
_ = + 10V ( N ote 5) 65 100 130 µA
Average Gate Current IGATE Discharging, triggered by a fault or when
VON_ < 0.875V 3mA
VIN_ = 3V to 13.2V 4.8 5.4 6.0
Gate-Drive Voltage VDRIVE VGATE_ - VIN_,
IGATE_ < 1µA VIN_ = 2.7V to 3.0V 4.1 5.0 6.0 V
ON_ COMPARATOR
Low to high 0.85 0.875 0.90 V
ON_ Threshold VON_,TH Hysteresis 25 mV
ON_ Propagation Delay 10mV overdrive 50 µs
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN_ = +1V to +13.2V provided at least one supply is higher than +2.7V, VON1 = VON2 = +2.7V, TA= TMIN to TMAX, unless otherwise
noted. Typical values are at VIN1 = +5V, VIN2 = +3.3V, and TA= +25°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VON_ < 4.5V 0.03
VON_ > 4.5V 100ON_ Input Bias Current IBON VIN1 = VIN2 = +13.2V
VON_ = 4.5V 0.03 1
µA
ON_ Pulse-Width Low tUNLATCH To unlatch after a latched fault 100 µs
DIGITAL OUTPUT (PGOOD_)
Output Leakage Current VPGOOD_ = 13.2V 1 µA
Output Voltage Low VOL ISINK = 1mA 0.4 V
PGOOD_ Delay tPGDLY After tSTART, MON_ = VIN_ 0.75 ms
OUTPUT VOLTAGE MONITORS (MON1, MON2)
Overvoltage 657 687 707
MON_ Trip Threshold VMON Undervoltage 513 543 563 mV
MON_ Glitch Filter 20 µs
MON_ Input Bias Current VMON_ = 600mV 0.03 µA
UNDERVOLTAGE LOCKOUT (UVLO)
Startup is initiated when this threshold is
reached by VIN1 or VIN2, VON_ > 0.875V 2.10 2.4 2.67 V
UVLO Threshold VUVLO
Hysteresis 100 mV
UVLO Glitch Filter Reset Time VIN_ toggled below UVLO to unlatch after a
fault 100 µs
UVLO to Startup Delay tD
,
UVLO VIN_ step from 0 to 2.8V 20 37.5 66 ms
SHUTDOWN LATCH/RESTART
Autoretry Delay tRETRY Delay time to restart after fault shutdown 64 x tSTART ms
Note 1: All devices are 100% tested at TA= +25°C and TA= +85°C. Limits at TA= 0°C and -40°C are guaranteed by design.
Note 2: The MAX5955/MAX5956 slow-comparator threshold is adjustable. VSC,TH = RLIM 0.25µA + 25mV (see the
Typical
Operating Characteristics
).
Note 3: The current-limit slow-comparator response time is weighted against the amount of overcurrent; the higher the overcurrent
condition, the faster the response time (see the
Typical Operating Characteristics)
.
Note 4: The startup period (tSTART) is the time during which the slow comparator is ignored and the device acts as a current-limiter
by regulating the sense current with the fast comparator (see the
Startup Period
section).
Note 5: The current available at GATE is a function of VGATE (see the
Typical Operating Characteristics
).
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
4 _______________________________________________________________________________________
0
0.6
0.4
0.2
1.0
0.8
1.8
1.6
1.4
1.2
2.0
02468101214
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5955/56 toc01
VINX (V)
IIN (mA)
VINY = VON1 = VON2 = 2.7V
IINX + IINY
IINX
IINY
0
0.6
0.4
0.2
1.0
0.8
1.8
1.6
1.4
1.2
2.0
02468101214
TOTAL SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5955/56 toc02
VINX (V)
IIN (mA)
VINY = 5.0V
A) VON1 = VON2 = 3.3V
B) VON1 = VON2 = 1.5V
C) VON1 = VON2 = 0
A
C
B
0
0.6
0.4
0.2
0.8
1.0
1.2
1.4
1.6
1.8
2.0
-40 10-15 35 60 85
SUPPLY CURRENT
vs. TEMPERATURE
MAX5955/56 toc03
TEMPERATURE (°C)
IIN (mA)
VON1 = VIN1
VON2 = VIN2 IIN1 + IIN2
IIN2
IIN1
0
2
1
4
3
5
6
0682 4 10 12 14
GATE-DRIVE VOLTAGE
vs. INPUT VOLTAGE
MAX5955/56 toc04
VINX (V)
GATE-DRIVE VOLTAGE (V)
VINY = 2.7V
0
60
40
20
80
100
120
140
160
180
200
020
GATE CHARGE CURRENT
vs. GATE VOLTAGE
MAX5955/56 toc05
VGATEX (V)
GATE CHARGE CURRENT (µA)
51015
VINY = 2.7V
VINX = 13.2V
VINX = 5V
VINX = 1V
0
60
40
20
80
100
120
140
160
180
200
-40 10-15 35 60 85
GATE CHARGE CURRENT
vs. TEMPERATURE
MAX5955/56 toc06
TEMPERATURE (°C)
GATE CHARGE CURRENT (µA)
VINX = 13.2V
VINX = 5V
VINX = 1V
VINY = 2.7V
VGATEX = 0
0
2
1
4
3
5
6
01051520
GATE STRONG DISCHARGE CURRENT
vs. GATE VOLTAGE
MAX5955/56 toc07
VGATEX (V)
GATE DISCHARGE CURRENT (mA)
VINX = 13.2V
VINX = 5V
VINX = 1V
VON1 = VON2 = 0
VINY = 2.7V
VGATEX = VINX + 6.2V
0
2
1
4
3
5
6
-40 10-15 35 60 85
GATE STRONG DISCHARGE CURRENT
vs. TEMPERATURE
MAX5955/56 toc08
TEMPERATURE (°C)
GATE DISCHARGE CURRENT (mA)
VINX = 13.2V
VINX = 5V
VINX = 1V
VON1 = VON2 = 0
VINY = 2.7V
VGATEX = VINX + 6.2V
0.0001
0.001
0.1
0.01
1
10
0507525 100 125 150 175 200
TURN-OFF TIME vs. SENSE VOLTAGE
MAX5955/56 toc09
VIN - VSENSE (mV)
TURN-OFF TIME (ms)
SLOW-COMP. THRESHOLD
FAST-COMP. THRESHOLD
Typical Operating Characteristics
(Typical Operating Circuits, Q1 = Q2 = Fairchild FDB7090L, VIN1 = +5V, VIN2 = +3.3V, VON1 = VON2 = +2.7V, TA= +25°C, unless
otherwise noted. Channels 1 and 2 are identical in performance. Where characteristics are interchangeable, channels 1 and 2 are
referred to as X and Y.)
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
_______________________________________________________________________________________
5
0.1
1
10
20 30 3525 40 45 50 55 60 65 70 75 80
TURN-OFF TIME vs. SENSE VOLTAGE
(EXPANDED SCALE)
MAX5955/56 toc10
VIN - VSENSE (mV)
TURN-OFF TIME (ms)
SLOW-COMP. THRESHOLD
0
40
20
80
60
100
120
0 200100 300 400
SLOW-COMPARATOR THRESHOLD
vs. RLIM
MAX5955/56 toc11
RLIM (kΩ)
VSC,TH (mV)
0
20
10
40
30
50
60
STARTUP PERIOD vs. RTIM
MAX5955/56 toc12
RTIM (kΩ)
tSTART (ms)
0 200 300100 400 500 600
0
0
0
VPGOOD
5V/div
VSENSE - VIN
100mV/div
VGATE
5V/div
TURN-OFF TIME
SLOW-COMPARATOR FAULT
MAX5955/56 toc13
1ms/div
VIN = 5.0V
tSCD
26mV STEP
0
0
0VPGOOD
5V/div
VSENSE - VIN
100mV/div
VGATE
5V/div
TURN-OFF TIME
FAST-COMPARATOR FAULT
MAX5955/56 toc14
400ns/div
VIN = 5.0V
tFCD
125mV STEP
VON
2V/div
VPGOOD
2V/div
IOUT
5A/div
VOUT
5V/div
VGATE
5V/div
STARTUP WAVEFORMS
FAST TURN-ON
MAX5955/56 toc15
1ms/div
VIN = 5.0V, RSENSE = 10mΩ,
RTIM = 27kΩ, CBOARD = 1000µF
VON
2V/div
VPGOOD
2V/div
IOUT
5A/div
VOUT
5V/div
VGATE
5V/div
STARTUP
WAVEFORMS
SLOW TURN-ON
MAX5955/56 toc16
1ms/div
VIN = 5.0V, RSENSE = 10mΩ, RTIM = 47kΩ,
CBOARD = 1000µF, CGATE = 22nF
IOUT
5A/div
VOUT
5V/div
VGATE
5V/div
AUTORETRY DELAY
MAX5955/56 toc17
40ms/div
VIN = 5.0V, RSENSE = 10mΩ, RTIM = 47kΩ,
CBOARD = 1000µF, RBOARD = 1.4Ω
Typical Operating Characteristics (continued)
(Typical Operating Circuits, Q1 = Q2 = Fairchild FDB7090L, VIN1 = +5V, VIN2 = +3.3V, VON1 = VON2 = +2.7V, TA= +25°C, unless
otherwise noted. Channels 1 and 2 are identical in performance. Where characteristics are interchangeable, channels 1 and 2 are
referred to as X and Y.)
MAX5955/MAX5956
Detailed Description
The MAX5955 and MAX5956 are circuit breaker ICs for
hot-swap applications where a line card is inserted into
a live backplane. The MAX5955 and MAX5956 operate
down to 1V provided one of the inputs is above 2.7V.
Normally, when a line card is plugged into a live back-
plane, the card’s discharged filter capacitors provide
low impedance that can momentarily cause the main
power supply to collapse. The MAX5955 and MAX5956
reside either on the backplane or on the removable
card to provide inrush current limiting and short-circuit
protection. This is achieved by using external n-chan-
nel MOSFETs, external current-sense resistors, and two
on-chip comparators. The startup period and current-
limit threshold of the MAX5955/MAX5956 can be
adjusted with external resistors. Figure 1 shows the
MAX5955/MAX5956 functional diagram.
The MAX5955/MAX5956 pull both PGOODs low and
both external FETs off for an overcurrent condition. The
MAX5955 also pulls both PGOODs low and both external
FETs off (protection) for an undervoltage/overvoltage
fault, whereas, the MAX5956 ONLY pulls the corre-
sponding fault channel’s PGOOD_ low (monitoring).
When the overvoltage/undervoltage fault disappears on
the MAX5956, the corresponding PGOOD_ automatically
goes high impedance.
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
6 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1PGOOD1
Channel 1 Status Output (Open Drain, see the Absolute Maximum Ratings). PGOOD1 asserts high when hot
swap is successful and channel 1 is within regulation. PGOOD1 asserts low during startup, when ON1 is
low, when channel 1 is off, or when channel 1 has any fault condition.
2 TIM Startup Timer Setting. Connect a resistor from TIM to GND to set the startup period. Leave TIM unconnected
for the default startup period of 9ms.
3 IN1 Channel 1 Supply Input. Connect to a supply voltage of 1V to 13.2V.
4 SENSE1 Channel 1 Current-Sense Input. Connect RSENSE1 from IN1 to SENSE1. Connect to IN1 to disable circuit
breaker function of channel 1.
5 GATE1 Channel 1 Gate-Drive Output. Connect to the gate of an external n-channel MOSFET.
6 GND Ground
7 LIM1 Channel 1 Current-Limit Setting. Connect a resistor from LIM1 to GND to set the current trip level. Connect to
GND for the default 25mV threshold (see the Slow-Comparator Threshold, RLIM section).
8 MON1 Channel 1 Output-Voltage Monitor. Window comparator input. Connect through a resistive divider from
OUT1 to GND to set the channel 1 overvoltage and undervoltage threshold. Connect to IN1 to disable.
9 MON2 Channel 2 Output-Voltage Monitor. Window comparator input. Connect through a resistive divider from
OUT2 to GND to set the channel 2 overvoltage and undervoltage threshold. Connect to IN2 to disable.
10 LIM2 Channel 2 Current-Limit Setting. Connect a resistor from LIM2 to GND to set the current trip level. Connect to
GND for the default 25mV threshold (see the Slow-Comparator Threshold, RLIM section).
11 ON1 Channel 1 On/Off Control Input. Channel 1 is turned on when VON1 > 0.875V.
12 GATE2 Channel 2 Gate-Drive Output. Connect to the gate of an external n-channel MOSFET.
13 SENSE2 Channel 2 Current-Sense Input. Connect RSENSE2 from IN2 to SENSE2. Connect to IN2 to disable circuit-
breaker function of channel 2.
14 IN2 Channel 2 Supply Input. Connect to a supply voltage of 1V to 13.2V.
15 ON2 Channel 2 On/Off Control Input. Channel 2 is turned on when VON2 > 0.875V.
16 PGOOD2
Channel 2 Status Output (Open Drain, see the Absolute Maximum Ratings). PGOOD2 asserts high when hot
swap is successful and channel 2 is within regulation. PGOOD2 asserts low during startup, when VON2 is
low, when channel 2 is off, or when channel 2 has any fault condition.
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
_______________________________________________________________________________________ 7
UVLO
FAST COMP.
SLOW COMP.
FAST DISCHARGE
Q1
OUT1
2.4V 2.4V
BIAS AND
REFERENCES
TIMING
OSCILLATOR
STARTUP
OSCILLATOR
TO STARTUP
LOGIC BLOCKS
CHARGE
PUMP DEVICE CONTROL
LOGIC
VSC, TH VFS, TH
RLIM1
SENSE1
IN1
GATE1
RSENSE1
LIM1
RLIM2
N
3mA
687mV
543mV
MON1
ON1ON2 PGOOD2PGOOD1
UVLO
FAST COMP.
SLOW COMP.
FAST DISCHARGE
Q2
OUT2
CHARGE-PUMP
OSCILLATOR
TO STARTUP
LOGIC BLOCKS
CHARGE
PUMP
CURRENT CONTROL
AND
STARTUP LOGIC
CURRENT CONTROL
AND
STARTUP LOGIC
VSC, TH
VFS, TH
SENSE2
IN2
GATE2
RSENSE2
LIM2
100µA 3mA
687mV
543mV
MON2
MAX5955
MAX5956
0.875V
RTIM
TIM
Figure 1. Functional Diagram
MAX5955/MAX5956
Startup Period
RTIM sets the duration of the startup period from 0.45s
to 50ms (see the
Setting the Startup Period, RTIM
sec-
tion). The default startup period is fixed at 9ms when
TIM is floating. The startup period begins after the fol-
lowing three conditions are met:
1) VIN1 or VIN2 exceeds the UVLO threshold (2.4V) for
the UVLO to startup delay (37.5ms).
2) VON1 and VON2 exceed the ON threshold (0.875V).
3) The device is not latched or in its autoretry delay (see
the
Latched and Autoretry Overcurrent Fault
Management
section).
The MAX5955/MAX5956 limit the load current if an
overcurrent fault occurs during startup instead of com-
pletely turning off the external MOSFETs. The slow
comparator is disabled during the startup period and
the load current can be limited in two ways:
1) Slowly enhancing the MOSFETs by limiting the
MOSFET gate-charging current.
2) Limiting the voltage across the external current-
sense resistor.
During the startup period the gate-drive current is limit-
ed to 100µA and decreases with the increase of the
gate voltage (see the
Typical Operating Characteris-
tics
). This allows the controller to slowly enhance the
MOSFETs. If the fast comparator detects an overcur-
rent, the MAX5955/MAX5956 regulate the gate voltage
to ensure that the voltage across the sense resistor
does not exceed VSU,TH. This effectively regulates the
inrush current during startup. Figure 2 shows the start-
up waveforms. PGOOD_ goes high impedance 0.75ms
after the startup period if no fault condition is present.
VariableSpeed/BiLevel Fault Protection
VariableSpeed/BiLevel fault protection incorporates two
comparators with different thresholds and response
times to monitor the load current (Figure 3). During the
startup period, protection is provided by limiting the
load current. Protection is provided in normal operation
(after the startup period has expired) by discharging
both MOSFET gates with a strong 3mA pulldown cur-
rent in response to a fault condition. After a fault,
PGOOD_ is pulled low, the MAX5955B and MAX5956B
stay latched off and the MAX5955A and MAX5956A
automatically restart.
Slow-Comparator Startup Period
The slow comparator is disabled during the startup
period while the external MOSFETs are turning on.
Disabling the slow comparator allows the device to
ignore the higher-than-normal inrush current charging
the board capacitors when a card is first plugged into a
live backplane.
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
8 _______________________________________________________________________________________
tON
4.3V TO 5.8V
VGATE
VGATE
PGOOD
ON
VTH
VOUT
VOUT
ILOAD
tSTART + tPGDLY
CBOARD = LARGE
CBOARD = 0
VSU,TH
RSENSE
Figure 2. Startup Waveform
SENSE VOLTAGE (VIN - VSENSE)
TURN-OFF TIME
VSC,TH VFC,TH
(4 x VSC,TH)
3ms
110µs
260ns
SLOW
COMPARATOR
FAST
COMPARATOR
Figure 3. VariableSpeed/BiLevel Response
Slow-Comparator Normal Operation
After the startup period is complete, the slow compara-
tor is enabled and the device enters normal operation.
The comparator threshold voltage (VSC,TH) is adjustable
from 25mV to 100mV. The slow-comparator response
time decreases to a minimum of 100µs with a large
overdrive voltage. Response time is 3ms for a 1mV over-
drive. The variable speed response time allows the
MAX5955/MAX5956 to ignore low-amplitude momentary
glitches, thus increasing system noise immunity. After
an extended overcurrent condition, a fault is generated,
both PGOODs are pulled low and the MOSFET gates
are discharged with a strong 3mA pulldown current.
Fast-Comparator Startup Period
During the startup period, the fast comparator regulates
the gate voltage to ensure that the voltage across the
sense resistor does not exceed the startup fast-com-
parator threshold voltage (VSU,TH), VSU,TH is scaled to
two times the slow-comparator threshold (VSC,TH).
Fast-Comparator Normal Operation
In normal operation, if the load current reaches the fast-
comparator threshold, a fault is generated, both
PGOODs are pulled low, and the MOSFET gates are dis-
charged with a strong 3mA pulldown current. This hap-
pens in the event of a serious current overload or a dead
short. The fast-comparator threshold voltage (VFC,TH) is
scaled to four times the slow-comparator threshold
(VSC,TH). This comparator has a fast response time of
260ns (Figure 3).
Undervoltage Lockout (UVLO)
The UVLO prevents the MAX5955/MAX5956 from turning
on the external MOSFETs until one input voltage exceeds
the UVLO threshold (2.4V) for tD,UVLO. The MAX5955/
MAX5956 use power from the higher input voltage rail for
the charge pumps. This allows for more efficient charge-
pump operation. The UVLO protects the external
MOSFETs from an insufficient gate-drive voltage. tD,UVLO
ensures that the board is fully inserted into the backplane
and that the input voltages are stable. Any input voltage
transient on both supplies below the UVLO threshold
reinitiates the tD,UVLO and the startup period.
Latched and Autoretry
Overcurrent Fault Management
The MAX5955B/MAX5956B latch the external MOSFETs
off when an overcurrent fault is detected. Toggling ON_
below 0.875V or one of the supply voltages
below/above the UVLO threshold for at least 100µs
clears the fault latch and reinitiates the startup period.
Similarly, the MAX5955A/MAX5956A turn the external
MOSFETs off when an overcurrent fault is detected,
then automatically restart after the autoretry delay that
is internally set to 64 times tSTART. During the autoretry
delay, toggling ON_ below 0.875V does not clear the
fault latch. The autoretry can be overridden, causing
the startup period to begin immediately by toggling one
of the supply voltages below/above the UVLO thresh-
old. When toggling a supply voltage to clear a fault,
remember that the supply voltage must go below and
then above the UVLO threshold for at least 100µs
regardless of the final value of the supply voltage.
Output Overvoltage/Undervoltage Fault
Management
The MAX5955/MAX5956 monitor the output voltages with
the MON1 and MON2 window comparator inputs. These
voltage monitors are enabled after the startup period.
Once enabled, the voltage monitor detects a fault if
VMON_ is less than 543mV or greater than 687mV.
When the MAX5955 protection device detects an output
overvoltage/undervoltage fault on either MON1 or
MON2, both external MOSFET gates are discharged at
3mA and both PGOODs pull low. For the MAX5955A,
the part continuously attempts to restart after each
autoretry period. The part successfully restarts after the
fault is removed and after waiting the autoretry period.
For the MAX5955B, the GATEs are latched off until the
output voltage fault is removed and the fault latch is
cleared by toggling ON_ or by cycling one of the supply
voltages above/below the UVLO threshold.
When the MAX5956 monitoring device detects an out-
put overvoltage/undervoltage fault on either MON1 or
MON2, neither external MOSFET gates are affected,
but the PGOOD_ of the channel experiencing the fault
pulls low. Thus the fault is reported on the channel with
the problem, but the MAX5956 does not allow an output
overvoltage/undervoltage fault to disrupt operation by
shutting down the channels. The MAX5956’s PGOOD_
output immediately goes high impedance after the out-
put overvoltage/undervoltage fault is removed.
The voltage monitors do not react to output glitches of
less than 20µs. A capacitor from MON_ to GND increas-
es the effective glitch filter time. The voltage monitoring
function of the MAX5955/MAX5956 can be disabled by
connecting VIN1 to MON1 and VIN2 to MON2.
Status Outputs (PGOOD_)
The status output is an open-drain output that pulls low
in response to one of the following conditions:
• Overcurrent fault
• Output undervoltage/overvoltage fault
PGOOD_ goes low when the corresponding channel is
forced off (ON_ < 0.875V) (Table 1).
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
_______________________________________________________________________________________ 9
MAX5955/MAX5956
Applications Information
Component Selection
n-Channel MOSFET
Select the external MOSFETs according to the applica-
tion’s current levels. Table 2 lists some recommended
components. The MOSFET’s on-resistance (RDS(ON))
should be chosen low enough to have a minimum volt-
age drop at full load to limit the MOSFET power dissi-
pation. High RDS(ON) causes output ripple if there is a
pulsating load. Determine the device power rating to
accommodate a short-circuit condition on the board at
startup and when the device is in automatic-retry mode
(see the
MOSFET Thermal Considerations
section).
Using the MAX5955B/MAX5956B in latched mode allows
the use of MOSFETs with lower power ratings. A MOSFET
typically withstands single-shot pulses with higher dissi-
pation than the specified package rating. Table 3 lists
some recommended manufacturers and components.
Sense Resistor
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV. Select a sense resistor that causes
a drop equal to the slow-comparator threshold voltage at
a current level above the maximum normal operating
current. Typically, set the overload current at 1.2 to 1.5
times the full load current. The fast-comparator threshold
is four times the slow-comparator threshold in normal
operating mode. Choose the sense-resistor power rating
to be greater than (IOVERLOAD)2x VSC,TH.
Slow-Comparator Threshold, R
LIM
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV, allowing designers to fine-tune
the current-limit threshold for use with standard-value
sense resistors. Low slow-comparator thresholds allow
for increased efficiency by reducing the power dissi-
pated by the sense resistor. Furthermore, the low 25mV
slow-comparator threshold is beneficial when operating
with supply rails down to 1V because it allows a small
percentage of the overall output voltage to be used for
current sensing. The VariableSpeed/BiLevel fault pro-
tection feature offers inherent system immunity against
load transients and noise. This allows the slow-com-
parator threshold to be set close to the maximum nor-
mal operating level without experiencing nuisance
faults. To adjust the slow-comparator threshold, calcu-
late RLIM as follows:
where VTH is the desired slow-comparator threshold
voltage.
RVmV
A
LIM TH
=−25
025.µ
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
10 ______________________________________________________________________________________
PART OVERCURRENT
FAULT (VOUT1)
OVERCURRENT
FAULT (VOUT2)
OVER/UNDER-
VOLTAGE FAULT
(VOUT1)
OVER/UNDER-
VOLTAGE FAULT
(VOUT2)
PGOOD1/
PGOOD2
GATE1/
GATE2
Yes X X X Low/Low Off/Off
X Yes X X Low/Low Off/Off
X X Yes X Low/Low Off/Off
MAX5955
UV/OV
Protection X X X Yes Low/Low Off/Off
Yes X X X Low/Low Off/Off
X Yes X X Low/Low Off/Off
X X Yes No Low/High On/On
MAX5956
UV/OV
Monitor X X No Yes High/Low On/On
Table 1. Status Output Truth Table
PART NUMBER MANUFACTURER DESCRIPTION
IRF7413 11mΩ, 8 SO, 30V
IRF7401 22mΩ, 8 SO, 20V
IRL3502S
International
Rectifier
6mΩ, D2PAK, 20V
MMSF3300 20mΩ, 8 SO, 30V
MMSF5N02H 30mΩ, 8 SO, 20V
MTB60N05H
Motorola
14mΩ, D2PAK, 50V
FDS6670A 10mΩ, 8 SO, 30V
NDS8426A 13.5mΩ, 8 SO, 20V
FDB8030L
Fairchild
4.5mΩ, D2PAK, 30V
Table 2. Recommended n-Channel
MOSFETs
Setting the Startup Period, R
TIM
The startup period (tSTART) is adjustable from 0.45ms to
50ms. The adjustable startup period feature allows sys-
tems to be customized for MOSFET gate capacitance
and board capacitance (CBOARD). The startup period is
adjusted with the resistance connected from TIM to GND
(RTIM). RTIM must be between 4kΩand 500kΩ. The
startup period has a default value of 9ms when TIM is left
floating. Calculate RTIM with the following equation:
where tSTART is the desired startup period.
Startup Sequence
There are two ways of completing the startup sequence.
Case A describes a startup sequence that slowly turns
on the MOSFETs by limiting the gate charge. Case B
uses the current-limiting feature and turns on the
MOSFETs as fast as possible while still preventing a high
inrush current. The output voltage ramp-up time (tON) is
determined by the longer of the two timings, case A and
case B. Set the startup timer tSTART to be longer than tON
to guarantee enough time for the output voltage to settle.
Case A: Slow Turn-On (Without Current Limit)
There are two ways to turn on the MOSFETs without
reaching the fast-comparator current limit:
• If the board capacitance (CBOARD) is small, the
inrush current is low.
• If the gate capacitance is high, the MOSFETs turn
on slowly.
In both cases, the turn-on time is determined only by the
charge required to enhance the MOSFET. The small
gate-charging current of 100µA effectively limits the out-
put voltage dV/dt. Connecting an external capacitor
between GATE and GND extends turn-on time. The time
required to charge/discharge a MOSFET is as follows:
where:
CGATE is the external gate to ground capacitance
(Figure 4).
∆VGATE is the change in gate voltage.
QGATE is the MOSFET total gate charge.
IGATE is the gate-charging/discharging current.
In this case, the inrush current depends on the MOSFET
gate-to-drain capacitance (Crss) plus any additional
capacitance from GATE to GND (CGATE), and on any
load current (ILOAD) present during the startup period.
Example: Charging and Discharging Times Using
the Fairchild FDB7030L MOSFET
If VIN1 = 5V then GATE1 charges up to 10.4V (VIN1 +
VDRIVE); therefore ∆VGATE = 10.4V. The manufacturer’s
data sheet specifies that the FDB7030L has approxi-
mately 60nC of gate charge and Crss = 600pF. The
MAX5955/MAX5956 have a 100µA gate-charging cur-
rent and a 3mA strong discharging current.
IC
CC II
INRUSH BOARD
rss GATE GATE LOAD
=+×+
tCVQ
I
GATE GATE GATE
GATE
=×∆ +
Rt
pF
TIM START
=×128 800
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
______________________________________________________________________________________ 11
COMPONENT MANUFACTURER PHONE WEBSITE
Dale-Vishay 402-564-3131 www.vishay.com
Sense Resistors IRC 704-264-8861 www.irctt.com
Fairchild 888-522-5372 www.fairchildsemi.com
International Rectifier 310-233-3331 www.irf.com
MOSFETs
Motorola 602-244-3576 www.mot-sps.com/ppd
Table 3. Component Manufacturers
GATE
SENSE
GND
ON_
*
* REQUIRED COMPONENTS. SEE THE ON_ COMPARATORS SECTION.
RSENSE VOUT
CGATE
CBOARD
VIN
IN_
RPULLUP
PGOOD_ MAX5955
MAX5956
0.1µF
Figure 4. Operating with an External Gate Capacitor
MAX5955/MAX5956
CBOARD = 6µF and the load does not draw any current
during the startup period. With no gate capacitor the
inrush current, charge, and discharge times are:
With a 22nF gate capacitor the inrush current, charge,
and discharge times are:
Case B: Fast Turn-On (with Current Limit)
In applications where the board capacitance (CBOARD)
is high, the inrush current causes a voltage drop across
RSENSE that exceeds the startup fast-comparator
threshold. The fast comparator regulates the voltage
across the sense resistor to VSU,TH. This effectively
regulates the inrush current during startup. In this case,
the current charging CBOARD can be considered con-
stant and the turn-on time is:
The maximum inrush current in this case is:
Figure 2 shows the waveforms and timing diagrams for a
startup transient with current regulation (see
Typical
Operating Characteristics
)
.
When operating under this
condition, an external gate capacitor is not required.
ON_ Comparators
The ON_ comparators control the on/off function of the
MAX5955/MAX5956. ON_ allows independent control
over channel 1 and channel 2. Drive ON1 and ON2
high (> 0.875V) to enable channel 1 and channel 2,
respectively. Pull ON_ low (< 0.875V) to disable the
respective channel. An RC time delay must be added
to the ON_ inputs with delay set to at least 20µs. This
allows the internal circuits to stabilize after application
of a steeply rising VIN_.
Using the MAX5955/MAX5956 on the
Backplane
Using the MAX5955/MAX5956 on the backplane allows
multiple cards with different input capacitance to be
inserted into the same slot even if the card does not
have on-board hot-swap protection. The startup period
can be triggered if IN_ is connected to ON_ through a
trace on the card (Figure 5).
Input Transients
The voltage at IN1 or IN2 must be above the UVLO dur-
ing inrush and fault conditions. When a short-circuit con-
dition occurs on the board, the fast comparator trips
causing the external MOSFET gates to be discharged at
3mA. The main system power supply must be able to
sustain a temporary fault current, without dropping below
the UVLO threshold of 2.4V, until the external MOSFET is
completely off. If the main system power supply collapses
below UVLO, the MAX5955/MAX5956 force the device to
restart once the supply has recovered. The MOSFET is
turned off in a very short time resulting in a high di/dt. The
backplane delivering the power to the external card must
have low inductance to minimize voltage transients
caused by this high di/dt.
MOSFET Thermal Considerations
During normal operation, the external MOSFETs dissi-
pate little power. The MOSFET RDS(ON) is low when the
MOSFET is fully enhanced. The power dissipated in
normal operation is PD= ILOAD2x RDS(ON). The most
power dissipation occurs during the turn-on and turn-
off transients when the MOSFETs are in their linear
regions. Take into consideration the worst-case sce-
nario of a continuous short-circuit fault, consider these
two cases:
1) The single turn-on with the device latched after a
fault (MAX5955B/MAX5956B)
2) The continuous automatic retry after a fault
(MAX5955A/MAX5956A)
MOSFET manufacturers typically include the package
thermal resistance from junction to ambient (RθJA) and
thermal resistance from junction to case (RθJC), which
determine the startup time and the retry duty cycle
(d = tSTART/tSTART + tRETRY). Calculate the required
transient thermal resistance with the following equation:
where ISTART = VSU,TH / RSENSE
ZTT
VI
JA MAX JMAX A
IN START
θ()
≤×
−
IV
R
INRUSH SU TH
SENSE
=,
tCVR
V
ON BOARD IN SENSE
SU TH
=××
,
IF
pF nF AmA
tnF V nC
Ams
tnF V nC
mA ms
INRUSH
CHARGE
DISCHARGE
=µ
+×µ+=
=×+
µ=
=×+=
6
600 22 100 0 26 5
22 10 4 60
100 289
22 10 4 60
30 096
.
..
..
IF
pF AA
tVnC
Ams
tVnC
mA ms
INRUSH
CHARGE
DISCHARGE
=µ
+×µ+=
=×+
µ=
=×+=
6
600 0 100 0 1
0104 60
100 06
0104 60
3002
..
..
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
12 ______________________________________________________________________________________
Layout Considerations
To take full advantage of the switch response time to an
output fault condition, it is important to keep all traces as
short as possible and to maximize the high-current trace
dimensions to reduce the effect of undesirable parasitic
resistance and inductance. Place the MAX5955/
MAX5956 close to the card’s connector, and a 0.01µF
capacitor to GND should be placed as close as possible
to each VIN pin. Use a ground plane to minimize imped-
ance and inductance. Minimize the current-sense resis-
tor trace length (< 10mm), and ensure accurate current
sensing with Kelvin connections (Figure 6).
When the output is short circuited, the voltage drop
across the external MOSFET becomes large. Hence,
the power dissipation across the switch increases, as
does the die temperature. An efficient way to achieve
good power dissipation on a surface-mount package is
to lay out two copper pads directly under the MOSFET
package on both sides of the board. Connect the two
pads to the ground plane through vias, and use
enlarged copper mounting pads on the top side of the
board (refer to the MAX5956 EV Kit data sheet).
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
______________________________________________________________________________________ 13
PART OUTPUT UNDERVOLTAGE/OVERVOLTAGE
PROTECTION/MONITOR FAULT MANAGEMENT
MAX5955AEEE Protection Autoretry
MAX5955AUEE Protection Autoretry
MAX5955BEEE Protection Latched
MAX5955BUEE Protection Latched
MAX5956AEEE Monitor Autoretry
MAX5956AUEE Monitor Autoretry
MAX5956BEEE Monitor Latched
MAX5956BUEE Monitor Latched
Selector Guide
ON_
*
**
IN_ GATE_
VIN
VOUT
SENSE_
MAX5955
MAX5956
CBOARD
BACKPLANE
REMOVABLE CARD
WITH NO HOT-INSERTION
PROTECTION
* REQUIRED COMPONENTS. SEE THE ON_ COMPARATORS SECTION.
0.1µF
Figure 5. Using the MAX5955/MAX5956 on a Backplane
SENSE RESISTOR
HIGH-CURRENT PATH
MAX5955
MAX5956
Figure 6. Kelvin Connection for the Current-Sense Resistors
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
14 ______________________________________________________________________________________
MAX5955
MAX5956
VIN1
ON1
ON2
PGOOD1
PGOOD2
VIN2
GND
ON1
*
*
ON2
PGOOD1
PGOOD2
GND
IN1
0.1µF
SENSE1 GATE1
IN2 SENSE2 GATE2 LIM2
*
LIM1
*
TIM
* *
*
*
*
MON1
MON2
VOUT1
VOUT2
CBOARD2
CBOARD1
Q1
Q2
*OPTIONAL
*
*
0.1µF
* REQUIRED COMPONENTS. SEE THE ON_ COMPARATORS SECTION.
Typical Operating Circuit
Chip Information
TRANSISTOR COUNT: 3542
PROCESS: BiCMOS
Ordering Information (continued)
PART TEMP RANGE PIN-PACKAGE
MAX5956AEEE -40°C to +85°C 16 QSOP
MAX5956AEEE+ -40°C to +85°C 16 QSOP
MAX5956AUEE 0°C to +85°C 16 QSOP
MAX5956AUEE+ 0°C to +85°C 16 QSOP
MAX5956BEEE -40°C to +85°C 16 QSOP
MAX5956BEEE+ -40°C to +85°C 16 QSOP
MAX5956BUEE 0°C to +85°C 16 QSOP
MAX5956BUEE+ 0°C to +85°C 16 QSOP
+
Denotes lead-free package.
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
15
© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
Quijano
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.)
