LTC4235
Dual 12V Ideal Diode-OR
and Single Hot Swap Controller
with Current Monitor
Description
Features
Ideal Diode-OR and Inrush Current Control for
Redundant Supplies
nn Low Loss Replacement for Power Schottky Diodes
nn Enables Safe Board Insertion into a Live Backplane
nn 9V to 14V Operation
nn Current Monitor Output
nn Controls N-Channel MOSFETs
nn Limits Peak Fault Current in ≤ 1µs
nn Adjustable Current Limit with Foldback
nn Adjustable Current Limit Fault Delay
nn 0.5µs Ideal Diode Turn-On and Turn-Off Time
nn Smooth Switchover without Oscillation
nn Fault and Power Good Outputs
nn LTC4235-1: Latch Off After Fault
nn LTC4235-2: Automatic Retry After Fault
nn 20-Pin 4mm x 5mm QFN Package
The LTC®4235 offers ideal diode-OR and Hot SwapTM
functions for two 12V power rails by controlling external
N-channel MOSFETs. MOSFETs acting as ideal diodes
replace two high power Schottky diodes and the associated heat sinks, saving power and board area. A Hot Swap
control MOSFET allows a board to be safely inserted and
removed from a live backplane by limiting inrush current.
The supply output is also protected against short-circuit
faults with a foldback current limit and circuit breaker.
nn
The LTC4235 regulates the forward voltage drop across
the MOSFETs to ensure smooth current transfer from one
supply to the other without oscillation. The ideal diodes
turn on quickly to reduce the load voltage droop during
supply switchover. If the input supply fails or is shorted,
a fast turn-off minimizes reverse-current transients.
A current sense amplifier translates the voltage across the
sense resistor to a ground referenced signal. The LTC4235
allows turn-on/off control, and reports fault and power
good status for the supply.
Applications
Redundant Power Supplies
High Availability Systems and Servers
nn Telecom and Network Infrastructure
nn
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Hot
Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of
their respective owners. Protected by U.S. Patents, including 7920013, 8022679.
nn
Typical Application
Ideal Diode-OR with Hot Swap Application
Smooth Supply Switchover
SiR158DP
12V
IN2
0.1µF
SiR158DP
12V
0.003Ω
0.1µF
13.7k
CPO1 IN1 DGATE1 CPO2
SiR158DP
+
0.1µF
IN2 DGATE2
REG
SENSE+
IN1
1V/DIV
CLOAD
IN2
1V/DIV
IIN1
2A/DIV
SENSE– HGATE OUT
ON
FAULT
2k
LTC4235
INTVCC
0.1µF
GND
D2OFF
IN1
IIN2
2A/DIV
PWRGD
IMON
EN
12V
7A
ADC
200ms/DIV
4235 TA01b
FTMR
0.1µF
4235 TA01a
4235f
For more information www.linear.com/LTC4235
1
LTC4235
Absolute Maximum Ratings (Notes 1, 2)
Supply Voltages
IN1, IN2................................................... –0.3V to 24V
INTVCC...................................................... –0.3V to 7V
REG............................SENSE+ – 5V to SENSE+ + 0.3V
Input Voltages
ON, D2OFF, EN ....................................... –0.3V to 24V
FTMR......................................–0.3V to INTVCC + 0.3V
SENSE+, SENSE–.................................... –0.3V to 24V
Output Voltages
IMON........................................................ –0.3V to 7V
FAULT, PWRGD....................................... –0.3V to 24V
CPO1, CPO2 (Note 3).............................. –0.3V to 35V
DGATE1, DGATE2 (Note 3)...................... –0.3V to 35V
HGATE (Note 4)...................................... –0.3V to 35V
OUT........................................................ –0.3V to 24V
Average Currents
FAULT, PWRGD.....................................................5mA
INTVCC................................................................10mA
Operating Ambient Temperature Range
LTC4235C................................................. 0°C to 70°C
LTC4235I..............................................–40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
Pin Configuration
OUT
HGATE
CPO1
DGATE1
TOP VIEW
20 19 18 17
SENSE– 1
16 PWRGD
SENSE+
15 FAULT
2
IN1 3
14 ON
21
INTVCC 4
13 D2OFF
11 IMON
9 10
EN
8
FTMR
7
CPO2
12 REG
IN2 6
DGATE2
GND 5
UFD PACKAGE
20-LEAD (4mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 43°C/W (NOTE 5)
EXPOSED PAD (PIN 21) PCB GND CONNECTION OPTIONAL
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC4235CUFD-1#PBF
LTC4235CUFD-1#TRPBF
42351
20-Lead (4mm x 5mm) Plastic QFN
0°C to 70°C
LTC4235CUFD-2#PBF
LTC4235CUFD-2#TRPBF
42352
20-Lead (4mm x 5mm) Plastic QFN
0°C to 70°C
LTC4235IUFD-1#PBF
LTC4235IUFD-1#TRPBF
42351
20-Lead (4mm x 5mm) Plastic QFN
–40°C to 85°C
LTC4235IUFD-2#PBF
LTC4235IUFD-2#TRPBF
42352
20-Lead (4mm x 5mm) Plastic QFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
4235f
For more information www.linear.com/LTC4235
LTC4235
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supplies
VIN
Input Supply Range
l
IIN
Input Supply Current
l
VINTVCC
Internal Regulator Voltage
VINTVCC(UVL)
Internal VCC Undervoltage Lockout
9
14
V
2.7
4
mA
I = 0, –500µA
l
4.5
5
5.5
V
INTVCC Rising
l
2.1
2.2
2.3
V
l
30
60
90
mV
l
2
15
28
mV
12
14
V
–90
–120
µA
∆VINTVCC(HYST) Internal VCC Undervoltage Lockout
Hysteresis
Ideal Diode Control
ΔVFWD(REG)
Forward Regulation Voltage
(VINn – VSENSE+)
ΔVDGATE
External N-Channel Gate Drive
(VDGATEn – VINn)
ΔVFWD = 0.15V; I = 0, –1µA
l
10
l
–50
ICPO(UP)
CPOn Pull-Up Current
CPO = IN = 12V
IDGATE(FPU)
DGATEn Fast Pull-Up Current
ΔVFWD = 0.2V, ΔVDGATE = 0V, CPO = 17V
IDGATE(FPD)
DGATEn Fast Pull-Down Current
ΔVFWD = –0.2V, ΔVDGATE = 5V
IDGATE2(DN)
DGATE2 Off Pull-Down Current
D2OFF = 2V, ΔVDGATE2 = 2.5V
l
100
200
µA
tON(DGATE)
DGATEn Turn-On Delay
ΔVFWD = 0.2V , CDGATE = 10nF
l
0.25
0.5
µs
tOFF(DGATE)
DGATEn Turn-Off Delay
ΔVFWD = –0.2V, CDGATE = 10nF
l
0.2
0.5
µs
tPLH(DGATE2)
D2OFF Low to DGATE2 High
l
50
100
µs
27.5
10.8
mV
mV
50
–1.5
A
1.5
A
Hot Swap Control
ΔVSENSE(TH)
Current Limit Sense Voltage Threshold
(VSENSE+ – VSENSE–)
OUT = 11V
OUT = 0V
l
l
22.5
5.8
25
8.3
VSENSE+(UVL)
SENSE+ Undervoltage Lockout
SENSE+ Rising
l
1.8
1.9
2
V
l
10
50
90
mV
∆VSENSE+(HYST) SENSE+ Undervoltage Lockout
Hysteresis
ISENSE+
SENSE+ Pin Current
SENSE+ = 12V
l
0.3
0.8
1.3
mA
ISENSE
–
SENSE– Pin Current
SENSE– = 12V
l
10
40
100
µA
ΔVHGATE
External N-Channel Gate Drive
(VHGATE – VOUT)
I = 0, –1µA
l
10
12
14
V
ΔVHGATE(H)
Gate High Threshold (VHGATE – VOUT)
l
3.6
4.2
4.8
V
IHGATE(UP)
External N-Channel Gate Pull-Up Current Gate Drive On, HGATE = 0V
l
–7
–10
–13
µA
IHGATE(DN)
External N-Channel Gate Pull-Down
Current
Gate Drive Off, OUT = 12V,
HGATE = OUT + 5V
l
1
2
4
mA
IHGATE(FPD)
External N-Channel Gate Fast Pull-Down
Current
Fast Turn-Off, OUT = 12V,
HGATE = OUT + 5V
l
100
200
350
mA
VOUT(PGTH)
OUT Power Good Threshold
OUT Rising
l
10.2
10.5
10.8
V
ΔVOUT(PGHYST)
OUT Power Good Hysteresis
l
110
tPHL(SENSE)
Sense Voltage (SENSE+ – SENSE–)
tOFF(HGATE)
High to HGATE Low
ON Low to HGATE Low
EN High to HGATE Low
SENSE+ Low to HGATE Low
tD(HGATE)
ON High, EN Low to HGATE Turn-On
Delay
tP(HGATE)
ON to HGATE Propagation Delay
170
240
mV
∆VSENSE = 200mV, CHGATE = 10nF
l
0.5
1
µs
SENSE+ UVLO
l
l
l
10
20
10
20
40
20
µs
µs
µs
100
150
ms
10
20
µs
l
ON = Step 0.8V to 2V
l
50
4235f
For more information www.linear.com/LTC4235
3
LTC4235
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VD2OFF(H,TH)
D2OFF Pin High Threshold
D2OFF Rising
l
1.21
1.235
1.26
V
VD2OFF(L,TH)
D2OFF Pin Low Threshold
D2OFF Falling
ΔVD2OFF(HYST)
D2OFF Pin Hysteresis
l
1.19
1.215
1.24
l
10
20
30
VON(TH)
ON Pin Threshold Voltage
ON Rising
l
1.21
1.235
1.26
VON(RESET)
ON Pin Fault Reset Threshold Voltage
ΔVON(HYST)
ON Pin Hysteresis
ON Falling
l
0.57
0.6
0.63
V
l
40
80
120
mV
IIN(LEAK)
Input Leakage Current (ON, D2OFF)
V = 5V
l
VEN(TH)
EN Pin Threshold Voltage
EN Rising
l
0
±1
µA
ΔVEN(HYST)
EN Pin Hysteresis
l
1.185
1.235
1.284
60
110
200
mV
IEN(UP)
EN Pull-Up Current
l
–7
VFTMR(H)
FTMR Pin High Threshold
l
1.198
–10
–13
µA
1.235
1.272
V
VFTMR(L)
FTMR Pin Low Threshold
l
0.15
0.2
0.25
V
Inputs
EN = 1V
V
mV
V
V
IFTMR(UP)
FTMR Pull-Up Current
FTMR = 1V, In Fault Mode
l
–80
–100
–120
µA
IFTMR(DN)
FTMR Pull-Down Current
FTMR = 2V, No Faults
l
1.3
2
2.7
µA
DRETRY
Auto-Retry Duty Cycle
l
0.07
0.15
0.23
%
tRST(ON)
ON Low to FAULT High
l
20
40
µs
100
2.5
170
4
µA
mA
0.15
0.4
0.4
1.2
V
V
Outputs
IOUT
OUT Pin Current
OUT = 11V, IN = 12V, ON = 2V
OUT = 13V, IN = 12V, ON = 2V
l
l
VOL
Output Low Voltage (FAULT, PWRGD)
I = 1mA
I = 3mA
l
l
VOH
Output High Voltage (FAULT, PWRGD)
I = –1µA
l
IOH
Input Leakage Current (FAULT, PWRGD)
V = 18V
l
IPU
Output Pull-Up Current (FAULT, PWRGD) V = 1.5V
l
ΔVREG
Floating Regulator Voltage
(VSENSE+ – VREG)
IREG = ±1µA
ΔVSENSE(FS)
Input Sense Voltage Full Scale
(VSENSE+ – VSENSE–)
VIMON(OS)
30
INTVCC
–1
INTVCC
– 0.5
V
0
±1
µA
–7
–10
–13
µA
l
3.6
4.1
4.6
V
SENSE+ = 12V
l
25
IMON Input Offset Voltage
ΔVSENSE = 0V
l
GIMON
IMON Voltage Gain
ΔVSENSE = 20mV and 5mV
l
99
VIMON(MAX)
IMON Maximum Output Voltage
ΔVSENSE = 70mV
l
3.5
VIMON(MIN)
IMON Minimum Output Voltage
ΔVSENSE = 200µV
l
RIMON(OUT)
IMON Output Resistance
ΔVSENSE = 200µV
l
Current Monitor
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into device pins are positive; all currents out of
the device pins are negative. All voltages are referenced to GND unless
otherwise specified.
4
15
mV
100
20
±150
µV
101
V/V
5.5
V
40
mV
27
kΩ
Note 3: An internal clamp limits the DGATE and CPO pins to a minimum of
10V above and a diode below IN. Driving these pins to voltages beyond the
clamp may damage the device.
Note 4: An internal clamp limits the HGATE pin to a minimum of 10V
above and a diode below OUT. Driving this pin to voltages beyond the
clamp may damage the device.
Note 5: Thermal resistance is specified when the exposed pad is soldered
to a 3" x 5", four layer, FR4 board.
4235f
For more information www.linear.com/LTC4235
LTC4235
Typical Performance Characteristics
SENSE+ Current vs Voltage
IN Supply Current vs Voltage
OUT Current vs Voltage
1.4
3.5
3
1.2
3
2.5
1
2
VOUT = 0V
ISENSE+ (mA)
VSENSE+ = VIN – 0.5V
VOUT = 12V
1.5
0.8
0.6
1
0.4
0.5
0.2
0
0
3
6
9
VIN (V)
12
15
0
18
14
0
3
6
9
12
15
4235 G01
VIN = 12V
12
9
VOUT (V)
15
18
4235 G03
CPO Voltage vs Current
VOUT = VIN
10
12
6
VCPO – VIN (V)
8
∆VHGATE (V)
∆VHGATE (V)
6
3
12
10
8
4
6
4
2
6
2
0
–2
–4
–6
IHGATE (µA)
–8
–10
4
–12
12
0
0
3
6
4235 G04
Diode Gate Voltage vs Current
9
VIN (V)
12
15
14
VSENSE+ = VIN – 0.15V
4
0.6
VSENSE+ = VIN – 0.15V
0.5
10
8
6
0
–20
–40
–60 –80 –100 –120 –140
IDGATE (µA)
4235 G07
4
–60 –80 –100 –120 –140
ICPO (µA)
FAULT, PWRGD
Output Low Voltage vs Current
2
0
–40
–20
4235 G06
OUTPUT LOW VOLTAGE (V)
∆VDGATE (V)
VIN = 12V
6
0
4235 G05
12
8
–2
18
Diode Gate Voltage vs IN Voltage
10
∆VDGATE (V)
0
4235 G02
Hot Swap Gate Voltage vs IN
Voltage
VOUT = VIN
8
–2
1
–0.5
18
10
0
1.5
0
14
12
2
0.5
VSENSE+ (V)
Hot Swap Gate Voltage vs Current
VIN = 12V, VSENSE+ = 11.5V
2.5
IOUT (mA)
3.5
IIN (mA)
TA = 25°C, VIN = 12V, unless otherwise noted.
0.4
0.3
0.2
0.1
0
3
6
9
VIN (V)
12
15
18
4235 G08
0
0
1
2
3
CURRENT (mA)
4
5
4235 G09
4235f
For more information www.linear.com/LTC4235
5
LTC4235
Typical Performance Characteristics
Current Limit Delay vs Sense
Voltage
Current Limit Threshold Foldback
100
15
10
5
10
1
0
2
4
6
8
10
0.1
12
4235 G10
25
20
15
10
0
0
–50
80
120
40
160
200
SENSE VOLTAGE (VSENSE+ – VSENSE –) (mV)
101
IMON VOLTAGE GAIN (V/V)
1
0
10
30
50
20
40
SENSE VOLTAGE (VSENSE+ – VSENSE–) (mV)
100
99.5
99
–50
100
120
100.5
2
75
IMON Propagation Delay vs
Sense Voltage
IMON PROPAGATION DELAY (µs)
5
3
0
50
25
TEMPERATURE (°C)
4235 G12
IMON Voltage Gain vs
Temperature
4
–25
4235 G11
IMON Voltage vs Sense Voltage
–25
25
0
50
TEMPERATURE (°C)
75
4235 G13
100
4235 G14
Ideal Diode Start-Up Waveform
on IN Power-Up
ON
5V/DIV
SENSE+
10V/DIV
HGATE
10V/DIV
OUT
10V/DIV
CPO
10V/DIV
100
80
60
40
20
0
0
1
3
5
2
4
SENSE VOLTAGE (VSENSE+ – VSENSE–) (mV)
4235 G15
HGATE Start-Up Waveform on ON
Toggling High
IN
10V/DIV
DGATE
PWRGD
10V/DIV
10ms/DIV
6
30
5
VOUT (V)
IMON VOLTAGE (V)
40
CHGATE = 10nF
INPUT OFFSET VOLTAGE (µV)
20
0
Current Sense Amplifier Input
Offset Voltage vs Temperature
35
25
CURRENT LIMIT DELAY (µs)
CURRENT LIMIT SENSE VOLTAGE
VSENSE+ – VSENSE– (mV)
30
0
TA = 25°C, VIN = 12V, unless otherwise noted.
4235 G16
20ms/DIV
4235 G17
4235f
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LTC4235
Pin Functions
CPO1, CPO2: Charge Pump Output. Connect a capacitor
from CPO1 or CPO2 to the corresponding IN1 or IN2 pin.
The value of this capacitor is approximately 10x the gate
capacitance (CISS) of the external MOSFET for ideal diode
control. The charge stored on this capacitor is used to pull
up the ideal diode MOSFET gate during a fast turn-on.
Leave this pin open if fast ideal diode turn-on is not needed.
DGATE1, DGATE2: Ideal Diode MOSFET Gate Drive Output. Connect this pin to the gate of an external N-channel
MOSFET for ideal diode control. An internal clamp limits
the gate voltage to 12V above and a diode voltage below
IN. During fast turn-on, a 1.5A pull-up charges DGATE from
CPO. During fast turn-off, a 1.5A pull-down discharges
DGATE to IN.
D2OFF: Control Input. A rising edge above 1.235V turns
off the external ideal diode MOSFET in the IN2 supply path
and a falling edge below 1.215V allows the MOSFET to be
turned on. Connect this pin to an external resistive divider
from IN1 to make IN1 the higher priority input supply when
IN1 and IN2 are equal.
EN: Enable Input. Ground this pin to enable Hot Swap
control. If this pin is pulled high, the Hot Swap MOSFET
is not allowed to turn on. A 10µA current source pulls
this pin up to a diode below INTVCC. Upon EN going low
when ON is high, there is a start-up delay of 100ms for
debounce, after which the fault is cleared.
FAULT: Overcurrent Fault Status Output. Output that pulls
low when the fault timer expires during an overcurrent
fault. Otherwise it is pulled high by a 10µA current source
to a diode below INTVCC. It may be pulled above INTVCC
using an external pull-up. Leave open if unused.
FTMR: Fault Timer Capacitor Terminal. Connect a capacitor
between this pin and ground to set a 12ms/µF duration
for current limit before the external Hot Swap MOSFET is
turned off. The duration of the off time is 8s/µF, resulting
in a 0.15% duty cycle.
GND: Device Ground.
HGATE: Hot Swap MOSFET Gate Drive Output. Connect
this pin to the gate of the external N-channel MOSFET for
Hot Swap control. An internal 10µA current source charges
the MOSFET gate. An internal clamp limits the gate voltage to 12V above and a diode voltage below OUT. During
an undervoltage generated turn-off, a 2mA pull-down
discharges HGATE to ground. During an output short or
INTVCC undervoltage lockout, a fast 200mA pull-down
discharges HGATE to OUT.
IN1, IN2: Positive Supply Input and Ideal Diode MOSFET
Gate Drive Return. Connect this pin to the power input
side of the external ideal diode MOSFET. The 5V INTVCC
supply is generated from IN1, IN2 and OUT via an internal
diode-OR. The voltage sensed at this pin is used to control
DGATE. The gate fast pull-down current returns through
this pin when DGATE is discharged.
INTVCC: Internal 5V Supply Decoupling Output. This pin
must have a 0.1µF or larger capacitor to GND. An external
load of less than 500µA can be connected at this pin. An
undervoltage lockout threshold of 2.2V will turn off both
MOSFETs.
IMON: Current Sense Monitoring Output. This pin voltage
is proportional to the sense voltage across the current
sense resistor with a voltage gain of 100. An internal 20k
resistor is connected from this pin to ground.
ON: ON Control Input. A rising edge above 1.235V turns
on the external Hot Swap MOSFET and a falling edge below
1.155V turns it off. Connect this pin to an external resistive
divider from SENSE+ to monitor the supply undervoltage
condition. Pulling the ON pin below 0.6V resets the fault
latch after an overcurrent fault. Tie to INTVCC if unused.
OUT: Hot Swap MOSFET Gate Drive Return. Connect this
pin to the output side of the external MOSFET. The gate fast
pull-down current returns through this pin when HGATE
is discharged. An internal resistive divider connected between this pin and GND is used for current limit foldback
and power good monitor for 12V operation. If the OUT
voltage falls below 10.33V, the PWRGD pin pulls high to
indicate the power is bad. If the voltage falls below 7.65V,
the output current limit is reduced.
4235f
For more information www.linear.com/LTC4235
7
LTC4235
Pin Functions
PWRGD: Power Status Output. Output that pulls low when
the OUT pin rises above 10.5V and the MOSFET gate drive
between HGATE and OUT exceeds 4.2V. Otherwise it is
pulled high by a 10µA current source to a diode below
INTVCC. It may be pulled above INTVCC using an external
pull-up. Leave open if unused.
REG: Internal Regulated Supply for Current Sense Amplifier. A 0.1µF or larger capacitor should be tied from REG to
SENSE+. This pin is not designed to drive external circuits.
8
SENSE+: Positive Current Sense Input. Connect this pin to
the diode-OR output of the external ideal diode MOSFETs
and input of the current sense resistor. The voltage sensed
at this pin is used for monitoring the current limit and
also to control DGATE for forward voltage regulation and
reverse turn-off. This pin has an undervoltage lockout
threshold of 1.9V that will turn off the Hot Swap MOSFET.
SENSE–: Negative Current Sense Input. Connect this pin
to the output of the current sense resistor. The current
limit circuit controls HGATE to limit the voltage between
SENSE+ and SENSE– to 25mV or less depending on the
voltage at the OUT pin.
4235f
For more information www.linear.com/LTC4235
LTC4235
Block Diagram
IN1 SENSE+
OUT
150k
20k
SENSE–
IN2
4.1V
FOLDBACK
0.9V
HGATE
200Ω
+
CL
–
GATE
DRIVER
12V
REG
+
CM
–
IMON
OUT
20k
10µA
CHARGE
PUMP 2
f = 2MHz
CHARGE
PUMP 1
f = 2MHz
CPO1
100µA
CPO2
100µA
DGATE1
+
–
+
GD2
–
GD1
12V
15mV
DGATE2
12V
15mV
INTVCC
5V LDO
INTVCC
10µA
0.6V
1.235V
RST
HGATE ON
PG1
FAULT RESET
+
–
EN
–
+
TM1
–
+
TM2
100µA
1.235V
FTMR
0.2V
GND
UVLO2
PG2
EN
INTVCC
–
+
ON
LOGIC
–
+
ON
–
+
UVLO1
–
+
1.235V
DOFF
DGATE2 OFF
–
+
1.235V
+
–
–
+
D2OFF
INTVCC
10µA
2.2V
SENSE+
1.9V
4.2V
HGATE
OUT
10.5V
INTVCC
10µA
PWRGD
FAULT
2µA
EXPOSED PAD
4235 BD
4235f
For more information www.linear.com/LTC4235
9
LTC4235
Operation
The LTC4235 functions as an input supply diode-OR with
inrush current limiting and overcurrent protection by
controlling the external N-channel MOSFETs (MD1, MD2
and MH) on a supply path. This allows boards to be safely
inserted and removed in systems with a backplane powered
by redundant supplies. The LTC4235 has a single Hot Swap
controller and two separate ideal diode controllers, each
providing independent control for the two input supplies.
When the LTC4235 is first powered up, the gates of the
external MOSFETs are held low, keeping them off. As the
DGATE2 pull-up can be disabled by the D2OFF pin, DGATE2
will pull high only when the D2OFF pin is pulled low. The
gate drive amplifier (GD1, GD2) monitors the voltage between the IN and SENSE+ pins and drives the respective
DGATE pin. The amplifier quickly pulls up the DGATE pin,
turning on the MOSFET for ideal diode control, when it
senses a large forward voltage drop. With the ideal diode
MOSFETs acting as input supply diode-OR, the SENSE+
pin voltage rises to the highest of the supplies at the IN1
and IN2 pins. An external capacitor connected at the CPO
pin provides the charge needed to quickly turn on the ideal
diode MOSFET. An internal charge pump charges up this
capacitor at device power-up. The DGATE pin sources
current from the CPO pin and sinks current into the IN
and GND pins.
Pulling the ON pin high and EN pin low initiates a 100ms
debounce timing cycle. After this timing cycle, a 10µA current source from the charge pump ramps up the HGATE
pin. When the Hot Swap MOSFET turns on, the inrush
current is limited at a level set by an external sense resistor
(RS) connected between the SENSE+ and SENSE– pins.
An active current limit amplifier (CL) servos the gate of
the MOSFET to 25mV or less across the current sense
resistor depending on the voltage at the OUT pin. Inrush
current can be further reduced, if desired, by adding a
capacitor from HGATE to GND. When OUT voltage rises
above 10.5V and the MOSFET’s gate drive (HGATE to OUT
voltage) exceeds 4.2V, the PWRGD pin pulls low.
10
The high side current sense amplifier (CM) provides accurate monitoring of current through the current sense
resistor. The sense voltage is amplified by 100 times and
level shifted from the positive rail to a ground-referred
output at the IMON pin. The output signal is analog and
may be used as is or measured with an ADC.
When the ideal diode MOSFET is turned on, the gate drive
amplifier controls DGATE to servo the forward voltage
drop (VIN – VSENSE+) across the MOSFET to 15mV. If the
load current causes more than 15mV of voltage drop,
the gate voltage rises to enhance the MOSFET. For large
output currents, the MOSFET’s gate is driven fully on and
the voltage drop is equal to ILOAD•RDS(ON) of the MOSFET.
In the case of an input supply short-circuit when the
MOSFETs are conducting, a large reverse current starts
flowing from the load towards the input. The gate drive
amplifier detects this failure condition and turns off the
ideal diode MOSFET by pulling down the DGATE pin.
In the case where an overcurrent fault occurs on the supply output, the current is limited with foldback. After a
delay set by 100µA charging the FTMR pin capacitor, the
fault timer expires and pulls the HGATE pin low, turning
off the Hot Swap MOSFET. The FAULT pin is also latched
low. At this point, the DGATE pin continues to pull high
and keeps the ideal diode MOSFET on.
Internal clamps limit both the DGATE to IN and CPO to IN
voltages to 12V. The same clamp also limits the DGATE
and CPO pins to a diode voltage below the IN pin. Another
internal clamp limits the HGATE to OUT voltage to 12V
and also clamps the HGATE pin to a diode voltage below
the OUT pin.
Power to the LTC4235 is supplied from either the IN or
OUT pins, through an internal diode-OR circuit to a low
dropout regulator (LDO). That LDO generates a 5V supply
at the INTVCC pin and powers the LTC4235’s internal low
voltage circuitry.
4235f
For more information www.linear.com/LTC4235
LTC4235
Applications Information
High availability systems often employ parallel-connected
power supplies or battery feeds to achieve redundancy and
enhance system reliability. Power ORing diodes are commonly used to connect these supplies at the point of load at
the expense of power loss due to significant diode forward
voltage drop. The LTC4235 minimizes this power loss by
using external N-channel MOSFETs as the pass elements,
allowing for a low voltage drop from the supply to the load
when the MOSFETs are turned on. When an input source
voltage drops below the output common supply voltage,
the appropriate MOSFET is turned off, thereby matching
the function and performance of an ideal diode. By adding
a current sense resistor and a Hot Swap MOSFET after
the parallel-connected ideal diode MOSFETs, the LTC4235
enhances the ideal diode performance with inrush current
limiting and overcurrent protection (see Figure 1). This
allows the board to be safely inserted and removed from
a live backplane without damaging the connector.
highest of the supplies at the IN and OUT pins to power the
device through the LDO. The diode-OR scheme permits the
device’s power to be kept alive by the OUT voltage when
the IN supplies have collapsed or shut off.
An undervoltage lockout circuit prevents all of the MOSFETs
from turning on until the INTVCC voltage exceeds 2.2V. A
0.1µF capacitor is recommended between the INTVCC and
GND pins, close to the device for bypassing. No external
supply should be connected at the INTVCC pin so as not
to affect the LDO’s operation. A small external load of less
than 500µA can be connected at the INTVCC pin.
Turn-On Sequence
The board power supply at the OUT pin is controlled
with external N-channel MOSFETs (MD1, MD2 and MH) in
Figure 1. The ideal diode MOSFETs connected in parallel
on the supply side function as a diode-OR, while MH on
the load side acts as a Hot Swap MOSFET controlling the
power supplied to the output load. The sense resistor RS
monitors the load current for overcurrent detection. The
HGATE capacitor CHG controls the gate slew rate to limit
the inrush current. Resistor RHG with CHG compensates
the current control loop, while RH prevents high frequency
oscillations in the Hot Swap MOSFET.
Internal VCC Supply
The LTC4235 operates with an input supply from 9V to
14V. The power supply to the device is internally regulated
at 5V by a low dropout regulator (LDO) with an output at
the INTVCC pin. An internal diode-OR circuit selects the
Figure 1. Card Resident Diode-OR with Hot Swap Application
MD1
SiR158DP
VIN1
12V
Z1
SMAJ15A
VIN2
12V
C2
0.1µF
RS
0.003Ω
MD2
SiR158DP
MH
SiR158DP
+
Z2
SMAJ15A
RH
10Ω
C3
0.1µF
RHG
1k
CHG
10nF
C4
0.1µF
R2
13.7k
R1
2k
C5
0.1µF
CPO1
ON
CARD
CONNECTOR
IN2 DGATE2
REG SENSE+
SENSE– HGATE
INTVCC
GND
D2OFF
C1
0.1µF
OUT
R3
100k
R4
100k
FAULT
PWRGD
LTC4235
EN
BACKPLANE
CONNECTOR
IN1 DGATE1 CPO2
CL
680µF
12V
7A
IMON
FTMR
CFT
0.1µF
ADC
4235 F01
4235f
For more information www.linear.com/LTC4235
11
LTC4235
Applications Information
During a normal power-up, the ideal diode MOSFETs turn
on first. As soon as the internally generated supply, INTVCC,
rises above its 2.2V undervoltage lockout threshold, the
internal charge pump is allowed to charge up the CPO
pins. Because the ideal diode MOSFETs are connected in
parallel as a diode-OR, the SENSE+ pin voltage approaches
the highest of the supplies at the IN1 and IN2 pins. The
MOSFET associated with the lower input supply voltage
will be turned off by the corresponding gate drive amplifier.
Before the Hot Swap MOSFET can be turned on, EN must
remain low and ON must remain high for a 100ms debounce
timing cycle to ensure that any contact bounces during the
insertion have ceased. At the end of the debounce cycle,
the internal fault latch is cleared. The Hot Swap MOSFET
is then allowed to turn on by charging up HGATE with a
10µA current source from the charge pump. The voltage
at the HGATE pin rises with a slope equal to 10µA/CHG and
the supply inrush current flowing into the load capacitor
CL is limited to:
I INRUSH =
CL
•10µA
CHG
The OUT voltage follows the HGATE voltage when the Hot
Swap MOSFET turns on. If the voltage across the current
sense resistor RS becomes too high based on the OUT pin
voltage, the inrush current will be limited by the internal
current limiting circuitry. Once the MOSFET gate overdrive
exceeds 4.2V and the OUT pin voltage is above 10.5V,
the PWRGD pin pulls low to indicate that the power is
good. Once OUT reaches the input supply voltage, HGATE
continues to ramp up. An internal 12V clamp limits the
HGATE voltage above OUT.
When the ideal diode MOSFET is turned on, the gate
drive amplifier controls the gate of the MOSFET to servo
the forward voltage drop across the MOSFET to 15mV.
If the load current causes more than 15mV of drop, the
MOSFET gate is driven fully on and the voltage drop is
equal to ILOAD•RDS(ON).
12
Turn-Off Sequence
The external MOSFETs can be turned off by a variety of
conditions. A normal turn-off for the Hot Swap MOSFET is
initiated by pulling the ON pin below its 1.155V threshold
(80mV ON pin hysteresis), or pulling the EN pin above its
1.235V threshold. Additionally, an overcurrent fault that
exceeds the fault timer period also turns off the Hot Swap
MOSFET. Normally, the LTC4235 turns off the MOSFET by
pulling the HGATE pin to ground with a 2mA current sink.
All of the MOSFETs turn off when INTVCC falls below its
undervoltage lockout threshold (2.2V). The DGATE pin is
pulled down with a 100µA current to one diode voltage
below the IN pin, while the HGATE pin is pulled down to
the OUT pin by a 200mA current. When D2OFF is pulled
high above 1.235V, the ideal diode MOSFET in the IN2
power path is turned off with DGATE2 pulled low by a
100µA current.
The gate drive amplifier controls the ideal diode MOSFET
to prevent reverse current when the input supply falls
below SENSE+. If the input supply collapses quickly, the
gate drive amplifier turns off the ideal diode MOSFET with
a fast pull-down circuit. If the input supply falls at a more
modest rate, the gate drive amplifier controls the MOSFET
to maintain SENSE+ at 15mV below IN.
Board Presence Detect with EN
If ON is high when the EN pin goes low, indicating a board
presence, the LTC4235 initiates a debounce timing cycle
for contact debounce. Upon board insertion, any bounces
on the EN pin restart the timing cycle. When the debounce
timing cycle is done, the internal fault latch is cleared. If
the EN pin remains low at the end of the timing cycle,
HGATE is charged up with a 10µA current source to turn
on the Hot Swap MOSFET.
If the EN pin goes high, indicating a board removal, the
HGATE pin is pulled low with a 2mA current sink after a
20µs delay, turning off the Hot Swap MOSFET without
clearing any latched fault.
4235f
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LTC4235
Applications Information
Overcurrent Fault
The LTC4235 features an adjustable current limit with
foldback that protects the external MOSFET against short
circuits or excessive load current. The voltage across the
external sense resistor RS is monitored by an active current limit amplifier. The amplifier controls the gate of the
Hot Swap MOSFET to reduce the load current as a function of the output voltage sensed by the OUT pin during
active current limit. A graph in the Typical Performance
Characteristics shows the current limit sense voltage
versus OUT voltage.
An overcurrent fault occurs when the output has been in
current limit for longer than the fault timer period configured
at the FTMR pin. Current limiting begins when the sense
voltage between the SENSE+ and SENSE – pins reaches
8.3mV to 25mV depending on the OUT pin voltage. The
gate of the Hot Swap MOSFET is brought under control by
the current limit amplifier and the output current is regulated to limit the sense voltage to less than 25mV. At this
point, the fault timer starts with a 100µA current charging
the FTMR pin capacitor. If the FTMR pin voltage exceeds
its 1.235V threshold, the external MOSFET turns off with
HGATE pulled to ground by 2mA and FAULT pulls low.
After the Hot Swap MOSFET turns off, the FTMR pin capacitor is discharged with a 2µA pull-down current until
its threshold reaches 0.2V. This is followed by a cool-off
period of 14 timing cycles as described in the FTMR Pin
Functions. Figure 2 shows an overcurrent fault on the
12V output.
OUT
10V/DIV
HGATE
10V/DIV
ILOAD
20A/DIV
OUT
10V/DIV
HGATE
10V/DIV
ILOAD
20A/DIV
5µs/DIV
4235 F03
Figure 3. Severe Short-Circuit on 12V Output
In the event of a severe short-circuit fault on the 12V output
as shown in Figure 3, the output current can surge to tens
of amperes. The LTC4235 responds within 1µs to bring the
current under control by pulling the HGATE to OUT voltage
down to zero volts. Almost immediately, the gate of the Hot
Swap MOSFET recovers rapidly due to the charge stored
in the RHG and CHG network and current is actively limited
until the fault timer expires. Due to parasitic supply lead
inductance, an input supply without any bypass capacitor may collapse during the high current surge and then
spike upwards when the current is interrupted. Figure 10
shows the input supply transient suppressors comprising
of Z1, RSNUB1, CSNUB1 and Z2, RSNUB2, CSNUB2 for the two
supplies if there is no input capacitance.
FTMR Pin Functions
An external capacitor CFT connected from the FTMR pin
to GND serves as fault timing when the supply output is
in active current limit. When the voltage across the sense
resistor exceeds the foldback current limit threshold (from
25mV to 8.3mV), FTMR pulls up with 100µA. Otherwise,
it pulls down with 2µA. The fault timer expires when the
1.235V FTMR threshold is exceeded, causing the FAULT
pin to pull low. For a given fault timer period, the equation
for setting the external capacitor CFT value is:
CFT = tFT • 0.083 [µF/ms]
200µs/DIV
4235 F02
Figure 2. Overcurrent Fault on 12V Output
After the fault timer expires, the FTMR pin capacitor pulls
down with 2µA from the 1.235V FTMR threshold until it
reaches 0.2V. Then, it completes 14 cooling cycles consisting of the FTMR pin capacitor charging to 1.235V with a
4235f
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13
LTC4235
Applications Information
100µA current and discharging to 0.2V with a 2µA current.
At that point, the HGATE pin voltage is allowed to start up
if the fault has been cleared as described in the Resetting
Fault section. When the latched fault is cleared during the
cool-off period, the FAULT pin pulls high. The total cool-off
time for the MOSFET after an overcurrent fault is:
tCOOL = CFT • 8 [s/µF]
After the cool-off period, the HGATE pin is only allowed
to pull up if the fault has been cleared for the latchoff
part. For the auto-retry part, the latched fault is cleared
automatically following the cool-off period and the HGATE
pin voltage is allowed to restart.
Resetting Fault (LTC4235-1)
For the latchoff part, an overcurrent fault is latched after
the fault timer expires and the FAULT pin is asserted low.
Only the Hot Swap MOSFET is turned off and the ideal
diode MOSFETs are not affected.
To reset a latched fault and restart the output, pull the
ON pin below 0.6V for more than 100µs and then high
above 1.235V. The fault latch resets and the FAULT pin
de-asserts on the falling edge of the ON pin. When ON
goes high again and the cool-off cycle has completed, a
debounce timing cycle is initiated before the HGATE pin
voltage restarts. Toggling the EN pin high and then low
again also resets a fault, but the FAULT pin pulls high at
the end of the debounce cycle before the HGATE pin voltage starts up. Bringing all the supplies below the INTVCC
undervoltage lockout threshold (2.2V) shuts off all the
MOSFETs and resets the fault latch. A debounce cycle is
initiated before a normal start-up when any of the supplies
is restored above the INTVCC UVLO threshold.
Auto-Retry after a Fault (LTC4235-2)
For the auto-retry part, the latched fault is reset automatically at the end of the cool-off period as described in the
FTMR Pin Functions section. At the end of the cool-off
period, the fault latch is cleared and FAULT pulls high.
The HGATE pin voltage is allowed to start up and turn on
the Hot Swap MOSFET. If the output short persists, the
supply powers up into a short with active current limiting
until the fault timer expires and FAULT again pulls low. A
14
new cool-off cycle begins with FTMR ramping down with
a 2µA current. The whole process repeats itself until the
output short is removed. Since tFT and tCOOL are a function
of FTMR capacitance CFT, the auto-retry cycle is equal to
0.15%, irrespective of CFT.
Figure 4 shows an auto-retry sequence after an over
current fault.
FTMR
2V/DIV
FAULT
10V/DIV
HGATE
20V/DIV
OUT
10V/DIV
100ms/DIV
4235 F04
Figure 4. Auto-Retry Sequence After a Fault
Monitor Undervoltage Fault
The ON pin functions as a turn-on control and an input
supply monitor. A resistive divider connected between
the supply diode-OR output (SENSE+) and GND at the
ON pin monitors the supply for undervoltage condition.
The undervoltage threshold is set by proper selection of
the resistors at the ON rising threshold voltage (1.235V).
For Figure 1, if R1 = 2k, R2 = 13.7k, the input supply
undervoltage threshold is set to 9.7V.
An undervoltage fault occurs if the diode-OR output supply falls below its undervoltage threshold. If the ON pin
voltage falls below 1.155V but remains above 0.6V, the
Hot Swap MOSFET is turned off by a 2mA pull-down from
HGATE to ground. The Hot Swap MOSFET turns back on
instantly without the debounce cycle when the diode-OR
output supply rises above its undervoltage threshold.
However, if the ON pin voltage drops below 0.6V, it turns
off the Hot Swap MOSFET and clears the fault latch. The
Hot Swap MOSFET turns back on only after a debounce
cycle when the diode-OR output supply is restored above
its undervoltage threshold.
4235f
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LTC4235
Applications Information
During the undervoltage fault condition, FAULT will not
be pulled low but PWRGD will be pulled high as HGATE
is pulled low. The ideal diode function controlled by the
ideal diode MOSFET is not affected by the undervoltage
(UV) fault condition.
Power Good Monitor
Internal circuitry monitors the MOSFET gate overdrive
between the HGATE and OUT pins. Also, an internal resistive divider that connects to OUT is used to determine a
power good condition. The power good comparator drives
high when the OUT pin rises above 10.5V, and drives low
when OUT falls below 10.33V. The power good status
for the input supply is reported via an open-drain output,
PWRGD. It is normally pulled high by an external pull-up
resistor or the internal 10µA pull-up.
The PWRGD pin pulls low when the OUT power good
comparator is high and the HGATE drive exceeds 4.2V. The
PWRGD pin goes high when the HGATE is turned off by the
ON or EN pins, or when the OUT power good comparator
drives low, or when INTVCC enters undervoltage lockout.
Current Sense Monitor
The current through the external sense resistor is monitored
by a LTC4235’s current sense amplifier at the SENSE+
and SENSE– pins (see Figure 5). The amplifier uses autozeroing circuitry to achieve an offset below 150µV over
temperature, sense voltage and input supply voltage. The
frequency of the auto-zero clock is 10kHz. An internal
resistor RIN is connected between the amplifier’s negative
input terminal and SENSE+ pin. The sense amplifier loop
forces the negative input terminal to have the same potential
as SENSE– and that develops a potential across RIN to be
the same as the sense voltage VSENSE. A corresponding
current, VSENSE/RIN, will flow through RIN. The high
impedance inputs of the sense amplifier will not conduct
this input current, allowing it to flow through an internal
MOSFET to a resistor ROUT connected between the IMON
and GND pins. The IMON output voltage is equal to (ROUT/
RIN) • VSENSE. The resistor ratio ROUT/RIN defines the voltage
gain of the sense amplifier and is set to 100 with RIN =
200Ω and ROUT = 20k. Full scale input sense voltage to
the sense amplifier is 25mV, corresponding to an output
of 2.5V. The output clamps at 3.5V if the allowable input
sense voltage range is exceeded.
IMON Output Filtering
A capacitor connected in parallel with ROUT will give a
low pass response. This will reduce unwanted noise at
the output, and may also be useful as a charge reservoir
to keep the output steady while driving a switching circuit
such as an ADC (see Figure 5). This output capacitor
COUT in parallel with ROUT will create a pole in the output
response at:
fC =
1
2 • π •ROUT •COUT
12V
0.1µF
LTC4235
REG
SENSE
VSENSE
+
SENSE–
RIN
200Ω
0.1µF
10µF
5V
HGATE
REF+
I LOAD
IMON VOUT
LOAD
IN
ROUT
20k
GND
0.1µF
ROUT
VOUT = ––––– • VSENSE = 100 • VSENSE
R IN
VCC
SCL
LTC2451
REF –
GND
2-WIRE I2C
INTERFACE
SDA
4235 F05
Figure 5. High Side Current Monitor with LTC2451 ADC
4235f
For more information www.linear.com/LTC4235
15
LTC4235
Applications Information
REG Pin Bypassing
MOSFET Selection
The LTC4235 has an internally regulated supply near
SENSE+ for internal bias of the current sense amplifier. It
is not intended for use as a supply or bias pin for external
circuitry. A 0.1µF capacitor should be connected between
the REG and SENSE+ pins. This capacitor should be located
very near to the device and close to the REG pin for the
best performance.
The LTC4235 drives N-channel MOSFETs to conduct the
load current. The important features of the MOSFETs are
on-resistance RDS(ON), the maximum drain-source voltage
BVDSS and the threshold voltage.
REG and IMON Start-Up
The start-up current of the current sense amplifier when
the LTC4235 is powered on consists of two parts: the
first is the current necessary to charge the REG bypass
capacitor, which is nominally 0.1µF. Since the REG voltage
charges to approximately 4.1V below the SENSE+ voltage,
this can require a significant amount of start-up current.
The second source is the output current that flows into
ROUT, which upon start-up may temporarily drive the
IMON output high for less than 2ms. This is a temporary
condition which will cease when the sense amplifier settles
into normal closed-loop operation.
CPO and DGATE Start-Up
The CPO and DGATE pin voltages are initially pulled up
to a diode below the IN pin when first powered up. CPO
starts ramping up 7µs after INTVCC clears its undervoltage lockout level. Another 40µs later, DGATE also starts
ramping up with CPO. The CPO ramp rate is determined
by the CPO pull-up current into the combined CPO and
DGATE pin capacitances. An internal clamp limits the CPO
pin voltage to 12V above the IN pin, while the final DGATE
pin voltage is determined by the gate drive amplifier. An
internal 12V clamp limits the DGATE pin voltage above IN.
CPO Capacitor Selection
The recommended value of the capacitor between the CPO
and IN pins is approximately 10× the input capacitance
CISS of the ideal diode MOSFET. A larger capacitor takes
a correspondingly longer time to charge up by the internal
charge pump. A smaller capacitor suffers more voltage
drop during a fast gate turn-on event as it shares charge
with the MOSFET gate capacitance.
16
The gate drive for the ideal diode and Hot Swap MOSFET
is guaranteed to be greater than 10V and is limited to 14V.
An external Zener diode can be used to clamp the potential
from the MOSFET’s gate to source if the rated breakdown
voltage is less than 14V.
The maximum allowable drain-source voltage BVDSS
must be higher than the supply voltage including supply
transients as the full supply voltage can appear across the
MOSFET. If an input or output is connected to ground, the
full supply voltage will appear across the MOSFET. The
RDS(ON) should be small enough to conduct the maximum
load current, and also stay within the MOSFET’s power
rating.
Supply Transient Protection
When the capacitances at the input and output are very
small, rapid changes in current during input or output
short-circuit events can cause transients that exceed the
24V absolute maximum ratings of the IN and OUT pins.
To minimize such spikes, use wider traces or heavier
trace plating to reduce the power trace inductance. Also,
bypass locally with a 10µF electrolytic and 0.1µF ceramic,
or alternatively clamp the input with a transient voltage
suppressor (Z1, Z2). A 100Ω, 0.1µF snubber damps the
response and eliminates ringing (See Figure 10).
Design Example
As a design example for selecting components, consider a
12V system with a 7A maximum load current for the two
supplies (see Figure 1).
First, select the appropriate value of the current sense
resistor RS for the 12V supply. Calculate the sense resistor
value based on the maximum load current ILOAD(MAX) and
4235f
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LTC4235
Applications Information
the lower limit for the current limit sense voltage threshold
ΔVSENSE(TH)(MIN).
ΔVSENSE(TH)(MIN)
=
ILOAD(MAX)
22.5mV
= 3.2mΩ
7A
Choose a 3mΩ sense resistor with a 1% tolerance.
Next, calculate the RDS(ON) of the ideal diode MOSFET to
achieve the desired forward drop at maximum load. Assuming a forward drop, ΔVFWD of 30mV across the MOSFET:
RDS(ON) ≤
ΔVFWD
ILOAD(MAX)
=
30mV
= 4.2mΩ
7A
The SiR158DP offers a good choice with a maximum
RDS(ON) of 1.8mΩ at VGS = 10V. The input capacitance
CISS of the SiR158DP is about 4980pF. Slightly exceeding
the 10× recommendation, a 0.1µF capacitor is selected
for C2 and C3 at the CPO pins.
Next, verify that the thermal ratings of the selected Hot
Swap MOSFET are not exceeded during power-up or an
overcurrent fault.
Assuming the MOSFET dissipates power due to inrush
current charging the load capacitor CL at power-up, the
energy dissipated in the MOSFET is the same as the energy
stored in the load capacitor, and is given by:
ECL =
1
• CL • VIN2
2
The inrush current is set to 1A by adding capacitance CHG
at the gate of the Hot Swap MOSFET.
CHG =
IINRUSH
2
tCHARGE
1 680µF • (12V )
= •
= 6W
2
8ms
The MOSFET selected must be able to tolerate 6W for 8ms
during power-up. The SOA curves of the SiR158DP provide
45W (1.5A at 30V) for 100ms. This is sufficient to satisfy
the requirement. The increase in junction temperature due
to the power dissipated in the MOSFET is ΔT = PAVG • ZthJC
where ZthJC is the junction-to-case thermal impedance.
Under this condition, the SiR158DP data sheet indicates
that the junction temperature will increase by 3°C using
ZthJC = 0.5°C/W (single pulse).
Next, the power dissipated in the MOSFET during an
overcurrent fault must be safely limited. The fault timer
capacitor (CFT) is used to prevent power dissipation in
the MOSFET from exceeding the SOA rating during active
current limit. A good way to determine a suitable value
for CFT is to superimpose the foldback current limit profile
shown in the Typical Performance Characteristics on the
MOSFET data sheet’s SOA curves.
For the SiR158DP MOSFET, this exercise yields the plot
in Figure 6.
100
IDM LIMITED
1ms
CL • VIN 680µF • 12V
=
= 8ms
IINRUSH
1A
CL • IHGATE(UP)
ECL
10
For CL = 680µF, the time it takes to charge up CL is calculated as:
tCHARGE =
PAVG =
680µF • 10µA
=
= 6.8nF
1A
Choose a practical value of 10nF for CHG.
ID – DRAIN CURRENT (A)
RS =
The average power dissipated in the MOSFET is calculated as:
10ms
ID LIMITED
100ms
LIMITED BY RDS(ON)*
1
1s
10s
0.1
MOSFET POWER
DISSIPATION CURVE
RESULTING FROM
FOLDBACK ACTIVE
CURRENT LIMIT
0.01
0.01
DC
BVDSS LIMITED
0.1
1
10
VDS – DRAIN-TO-SOURCE VOLTAGE (V)
100
4235 F06
* VGS > MINIMUM VGS AT WHICH RDS(ON) IS SPECIFIED
Figure 6. SiR158DP SOA with Design Example
MOSFET Power Dissipation Superimposed
4235f
For more information www.linear.com/LTC4235
17
LTC4235
Applications Information
As can be seen, the LTC4235’s foldback current limit profile
roughly coincides with the 100ms SOA contour. Since
this SOA plot is for an ambient temperature of 25°C only,
a maximum fault timer period of much less than 100ms
should be considered, such as 10ms or less. Selecting a
0.1µF ±10% value for CFT yields a maximum fault timer
period of 1.75ms which should be small enough to protect
the MOSFET during any overcurrent fault scenario.
⎛ VON(TH) ⎞
⎛ 1.235V ⎞
R1= ⎜
• 0.2% = 2.4k
• 0.2% = ⎜
⎟
⎝ 1µA ⎟⎠
⎝ I IN(LEAK) ⎠
Choose R1 to be 2k to achieve less than ±0.2% error and
calculating R2 yields:
⎛ VIN(UV)
⎞
R2 = ⎜
– 1⎟ • R1
⎝ VON(TH)
⎠
Next, select the values for the resistive divider at the ON
pin that defines the undervoltage threshold of 9.7V for the
12V supply at SENSE+. Since the leakage current for the
ON pin can be as high as ±1µA, the total resistance in the
divider should be low enough to minimize the resulting
offset error. Calculate the bottom resistor R1 based on
the following equation to obtain less than ±0.2% error
due to leakage current.
⎛ 9.7V
⎞
R2 = ⎜
– 1⎟ • 2k = 13.7k
⎝ 1.235V
⎠
The final components to consider are a 0.1µF bypass (C1)
at the INTVCC pin and a 0.1µF capacitor (C4) connected
between the REG and SENSE+ pins.
VIA TO GND PLANE
• ••
Z1
MD1
PowerPAK SO-8
IN1
W
•
CURRENT FLOW
TO LOAD
IN2
W
S
D
D
G
S
D
D
S
S
D
D
S
G
D
D
S
•
VIA TO IN1
VIA TO DGATE1
VIA TO C2 (CPO1)
MH
PowerPAK SO-8
RS
•
MD2
PowerPAK SO-8
•
S
D
20 19 18 17
S
D
S
D
G
D
•
•••
VIA TO DGATE2
TRACK WIDTH W:
0.03" PER AMPERE
ON 1oz Cu FOIL
C2
1
16
2
15
• C1
3
•
5
12
6
11
LTC4235UFD
4
OUT
14
13
C4
•
Z2
VIA TO C4 (REG)
W
RH
•
•
CURRENT FLOW
TO LOAD
VIA TO SENSE+
VIA TO GND PLANE
VIA TO GND PLANE
7
•
8
9
C3
10
4235 F08
Figure 7. Recommended PCB Layout for Power MOSFETs and Sense Resistor
18
4235f
For more information www.linear.com/LTC4235
LTC4235
Applications Information
PCB Layout Considerations
To achieve accurate current sensing, a Kelvin connection
for the sense resistor is recommended. The PCB layout
should be balanced and symmetrical to minimize wiring
errors. In addition, the PCB layout for the sense resistor
and the power MOSFET should include good thermal
management techniques for optimal device power dissipation. A recommended PCB layout is illustrated in Figure 7.
Connect the IN and OUT pin traces as close as possible to
the MOSFETs’ terminals. Keep the traces to the MOSFETs
wide and short to minimize resistive losses. The PCB traces
associated with the power path through the MOSFETs
should have low resistance. The suggested trace width for
1oz copper foil is 0.03" for each ampere of DC current to
keep PCB trace resistance, voltage drop and temperature
rise to a minimum. Note that the sheet resistance of 1oz
copper foil is approximately 0.5mΩ/square, and voltage
drops due to trace resistance add up quickly in high current applications.
It is also important to place the bypass capacitor C1 for
the INTVCC pin, as close as possible between INTVCC and
GND. Also place C2 near the CPO1 and IN1 pins, C3 near
the CPO2 and IN2 pins, and C4 near the REG and SENSE+
pins. The transient voltage suppressors Z1 and Z2, when
used, should be mounted close to the LTC4235 using
short lead lengths.
Prioritizing Supplies with D2OFF
Figure 8 shows an application where the IN1 supply
is passed to the output on the basis of priority, rather
than simply allowing the highest voltage to prevail. This
is achieved by connecting a resistive divider from IN1
at the D2OFF pin to suppress the turn-on of the ideal
diode MOSFET MD2 in the IN2 power path. When the IN1
supply voltage falls below 11.4V, it allows the ideal diode
MOSFET MD2, to turn on, causing the diode-OR output
to be switched from the main 12V supply at IN1 to the
auxiliary 12V supply at IN2. This configuration permits the
load to be supplied from a lower IN1 supply as compared
to IN2 until IN1 falls below the MD2 turn-on threshold. The
threshold value used should not allow the IN1 supply to
be operated at more than one diode voltage below IN2.
Otherwise, MD2 conducts through the MOSFET’s body
diode. The resistive divider connected from SENSE+ at
the ON pin provides the undervoltage threshold of 9.7V
for the diode-OR output supply.
MD1
SiR818DP
VMAIN
12V
Z1
SMAJ15A
VAUX
12V
C2
0.1µF
RS
0.004Ω
MD2
SiR818DP
MH
SiR818DP
+
Z2
SMAJ15A
RH
10Ω
C3
0.1µF
RHG
1k
CHG
10nF
C4
0.1µF
R2
13.7k
R1
2k
BACKPLANE
CONNECTOR
CARD
CONNECTOR
C5
0.1µF
IN1 DGATE1 CPO2
C6
0.1µF
IN2 DGATE2
REG SENSE+
SENSE– HGATE
12V
5A
VSENSE+
OUT
R6
100k
R5
100k
FAULT
PWRGD
LTC4235
EN
R4
21k
R3
2.49k
CPO1
ON
CL
470µF
IMON
D2OFF
GND
INTVCC
C1
0.1µF
Figure 8. Plug-In Card 12V Prioritized Power Supply at IN1
For more information www.linear.com/LTC4235
ADC
FTMR
CFT
0.1µF
4235 F08
4235f
19
LTC4235
Applications Information
MD1
SiR818DP
VMAIN
12V
Z1
SMAJ15A
VAUX
12V
R7 100Ω
C2
0.1µF
MD2
SiR818DP
RS
0.004Ω
MD3
SiR818DP
MH
SiR818DP
+
Z2
SMAJ15A
RH
10Ω
C3
0.1µF
R2
13.7k
C5
0.1µF
R1
2k
BACKPLANE
CONNECTOR
CARD
CONNECTOR
CPO1
ON
IN1 DGATE1 CPO2
C4
0.1µF
IN2 DGATE2
C6
0.1µF
R3
2.49k
REG SENSE+
SENSE– HGATE
VSENSE+
OUT
R6
100k
R5
100k
FAULT
PWRGD
LTC4235
EN
R4
20k
RHG
1k
CHG
10nF
CL
470µF
12V
5A
ADC
IMON
D2OFF
FTMR
GND
INTVCC
4235 F09
CFT
0.1µF
C1
0.1µF
Figure 9. 1V Supply Separation from IN2 for Prioritized Power Supply at IN1 Using Back-to-Back MOSFETs
MD1
SiR158DP
VIN1
12V
Z1
SMAJ15A
VIN2
12V
Z2
SMAJ15A
C2
0.1µF
RSNUB1
100Ω
CSNUB1
0.1µF
RS
0.002Ω
MD2
SiR158DP
MH
SiR158DP
+
RSNUB2
100Ω
CSNUB2
0.1µF
RH
10Ω
C3
0.1µF
PWREN
R1
10k
CPO1
ON
IN1 DGATE1 CPO2
CARD
CONNECTOR
REG SENSE
SENSE–
HGATE
OUT
R4
2.7k
D2
R3
2.7k
D1
FAULT
PWRGD
LTC4235
EN
BACKPLANE
CONNECTOR
+
IN2 DGATE2
VSENSE+
RHG
1k
CHG
10nF
C4
0.1µF
CL
220µF
12V
10A
IMON
GND
INTVCC
D2OFF
C1
0.1µF
ADC
4235 F09
FTMR
CFT
0.1µF
D1: GREEN LED LN1351C
D2: RED LED LN1261CAL
Figure 10. 12V, 10A Card Resident Application
20
4235f
For more information www.linear.com/LTC4235
LTC4235
Package Description
Please refer to http://www.linear.com/product/LTC4235#packaging for the most recent package drawings.
UFD Package
20-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1711 Rev B)
0.70 ±0.05
4.50 ±0.05
1.50 REF
3.10 ±0.05
2.65 ±0.05
3.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
2.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
(2 SIDES)
0.75 ±0.05
PIN 1 NOTCH
R = 0.20 OR
C = 0.35
1.50 REF
R = 0.05 TYP
19
20
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ±0.10
(2 SIDES)
2.50 REF
3.65 ±0.10
2.65 ±0.10
(UFD20) QFN 0506 REV B
0.200 REF
0.00 – 0.05
R = 0.115
TYP
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
4235f
For more information www.linear.com/LTC4235
21
LTC4235
Typical Application
12V, 5A Backplane Resident Ideal Diode-OR Application with Inrush Current Limiting
MD1
SiR158DP
VIN1
12V
BULK
SUPPLY
BYPASS
CAPACITOR
VIN2
12V
C2
0.1µF
RS
0.004Ω
MD2
SiR158DP
MH
SiR158DP
+
BULK
SUPPLY
BYPASS
CAPACITOR
RH
10Ω
C3
0.1µF
IN1 DGATE1 CPO2
REG SENSE+
IN2 DGATE2
SENSE– HGATE
R2
13.7k
R1
2k
C5
0.1µF
CL
1000µF
RHG
1k
CHG
10nF
C4
0.1µF
CPO1
12V
5A
OUT
FAULT
PWRGD
EN
ON
LTC4235
BACKPLANE
INTVCC
GND
D2OFF
IMON
FTMR
CFT
0.1µF
C1
0.1µF
PLUG-IN
CARD
ADC
4235 TA02
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PART NUMBER DESCRIPTION
COMMENTS
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Dual Ideal Diode and Hot Swap Controller
Operates from 2.9V to 18V, Controls Four N-Channels, SSOP-28 or QFN-28
LTC4229
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Operates from 2.9V to 18V, Controls Two N-Channels, SSOP-24 or QFN-24
LTC4352
Low Voltage Ideal Diode Controller
Operates from 0V to 18V, Controls N-Channel, MSOP-12 or DFN-12
LTC4353
Dual Low Voltage Ideal Diode Controller
Operates from 0V to 18V, Controls Two N-Channels, MSOP-16 or DFN-16
LTC4355
Positive High Voltage Ideal Diode-OR and Monitor Operates from 9V to 80V, Controls Two N-Channels, SO-16, DFN-14 or MSOP-16
LTC4357
Positive High Voltage Ideal Diode Controller
22 Linear Technology Corporation
Operates from 9V to 80V, Controls N-Channel, MSOP-8 or DFN-6
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC4235
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4235
4235f
LT 1115 • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2015
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