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LTC4218 Hot Swap Controller FEATURES

DESCRIPTION
The LTC(R)4218 is a Hot SwapTM controller that allows a board to be safely inserted and removed from a live backplane. An internal high side switch driver controls the gate of an external N-channel MOSFET for supply voltages from 2.9V to 26.5V. A dedicated 12V version (LTC4218-12) contains preset 12V specific thresholds, while the standard LTC4218 allows adjustable thresholds. The LTC4218 provides an accurate (5%) current limit with current foldback limiting. The current limit threshold can be adjusted dynamically using an external pin. Additional features include a current monitor output that amplifies the sense voltage for ground referenced current sensing. Overvoltage, undervoltage and powergood monitoring are also provided.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Wide Operating Voltage Range: 2.9V to 26.5V Adjustable, 5% Accurate (15mV) Current Limit Current Monitor Output Adjustable Current Limit Timer Before Fault Powergood and Fault Outputs Adjustable Inrush Current Control 2% Accurate Undervoltage and Overvoltage Protection Available in 16-Lead SSOP and 16-Pin 5mm x 3mm DFN Packages
APPLICATIONS

RAID Systems ATCA, AMC, TCA Systems Server I/O Cards Industrial
TYPICAL APPLICATION
12V, 6A Card Resident Application
2m 12V Si7108DN VOUT 12V 6A 330F VIN 10V/DIV IIN 1A/DIV VOUT 10V/DIV
Power-Up Waveform
+
10 SENSE- SENSE+ VDD UV AUTO RETRY LTC4218DHC-12 FLT TIMER 0.1F 0.1F INTVCC GND IMON 20k PG 10k 12V
GATE SOURCE
1k 0.01F
PG 10V/DIV
4218 TA01b
25ms/DIV ADC
4218 TA01a
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LTC4218 ABSOLUTE MAXIMUM RATINGS
(Notes 1, 2)
Supply Voltage (VDD) ................................. -0.3V to 35V Input Voltages FB, OV, UV ............................................. -0.3V to 12V TIMER................................................... -0.3V to 3.5V SENSE- ............................ VDD - 10V or -0.3V to VDD SENSE+ ............................ VDD - 10V or -0.3V to VDD SOURCE........................................ - 5V to VDD + 0.3V Output Voltages ISET, IMON ................................................. -0.3V to 3V PG, FLT .................................................. -0.3V to 35V
INTVCC .................................................. -0.3V to 3.5V GATE (Note 3) ........................................ -0.3V to 35V Operating Temperature Range LTC4218C ................................................ 0C to 70C LTC4218I .............................................-40C to 85C Storage Temperature Range DHC Package .....................................-65C to 125C GN Package .......................................-65C to 150C Lead Temperature (Soldering, 10 sec) GN Package Only .............................................. 300C
PIN CONFIGURATION
TOP VIEW NC VDD UV OV TIMER INTVCC GND SOURCE 1 2 3 4 5 6 7 8 17 16 SENSE+ 15 SENSE- 14 ISET 13 IMON 12 FB 11 FLT 10 PG 9 GATE NC VDD UV OV TIMER INTVCC GND SOURCE 1 2 3 4 5 6 7 8 TOP VIEW 16 SENSE+ 15 SENSE- 14 ISET 13 IMON 12 FB 11 FLT 10 PG 9 GATE
DHC PACKAGE 16-LEAD (5mm x 3mm) PLASTIC DFN TJMAX = 125C, JA = 43C/W EXPOSED PAD (PIN 17) IS SUBSTRATE GND
GN PACKAGE 16-LEAD PLASTIC SSOP TJMAX = 150C, JA = 135C/W
ORDER INFORMATION
LEAD FREE FINISH LTC4218CDHC-12#PBF LTC4218IDHC-12#PBF LTC4218CGN#PBF LTC4218IGN#PBF TAPE AND REEL LTC4218CDHC-12#TRPBF LTC4218IDHC-12#TRPBF LTC4218CGN#TRPBF LTC4218IGN#TRPBF PART MARKING* 421812 421812 4218 4218 PACKAGE DESCRIPTION 16-Lead (5mm x 3mm) Plastic DFN 16-Lead (5mm x 3mm) Plastic DFN 16-Lead Plastic SSOP 16-Lead Plastic SSOP TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. 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/
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LTC4218 ELECTRICAL CHARACTERISTICS
SYMBOL DC Characteristics VDD IDD VDD(UVL) VDD(UVTH) VDD(UVHYST) VDD(OVTH) VDD(OVHYST) VSOURCE(PGTH) VSNS(TH) Input Supply Range Input Supply Current Input Supply Undervoltage Lockout Input Supply Undervoltage Threshold Input Supply Undervoltage Hysteresis Input Supply Overvoltage Threshold Input Supply Overvoltage Hysteresis SOURCE Powergood Threshold Current Limit Sense Voltage Threshold (VSENSE+ - VSENSE-) SENSE- Pin Input Current SENSE+ Pin Input Current External N-Channel Gate Drive (VGATE - VSOURCE) Gate High Threshold (VGATE - VSOURCE) External N-Channel Gate Pull-Up Current External N-Channel Gate Fast Pull-Down Current External N-Channel Gate Pull-Down Current Gate Drive On, VGATE = VSOURCE = 12V Fast Turn Off, VGATE = 18V, VSOURCE =12V Gate Drive Off, VGATE = 18V, VSOURCE =12V VIN = 1.2V, LTC4218 Only LTC4218-12 Only VIN Rising FET On VDD Rising LTC4218-12 Only VDD Rising LTC4218-12 Only LTC4218-12 Only VDD Rising LTC4218-12 Only LTC4218-12 Only VSOURCE Rising LTC4218-12 Only VFB = 1.23V VFB = 0V VFB = 1.23V, RSET = 20k VSENSE- = 12V VSENSE+ = 12V VDD = 2.9V to 26.5V (Note 3) IGATE = 0, -1A

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VDD = 12V unless otherwise noted.
PARAMETER CONDITIONS MIN 2.9 1.6 2.65 9.6 520 14.7 183 10.2 127 14.25 2.8 6.7 2.73 9.88 640 15.05 244 10.5 170 15 3.75 7.5 4 5.5 5 3.5 -19 100 200 6.15 4.2 -24 170 250 TYP MAX 26.5 5 2.85 10.2 760 15.4 305 10.8 213 15.75 4.7 8.325 10 20 6.5 4.8 -29 220 340 UNITS V mA V V mV V mV V mV mV mV mV A A V V A mA A
VSOURCE(PGHYST) SOURCE Powergood Hysteresis
ISENSE-(IN) ISENSE+(IN) VGATE VGATE-HIGH(TH) IGATE(UP) IGATE(FST) IGATE(DN) Inputs I(IN) R(IN) V(TH) VOV(HYST) VUV(HYST) VUV(RTH) VFB(HYST) RISET ISOURCE
OV, UV, FB Pin Input Current OV, UV, FB Pin Input Resistance OV, UV, FB Pin Threshold Voltage OV Pin Hysteresis UV Pin Hysteresis UV Pin Reset Threshold Voltage FB Pin Power Good Hysteresis ISET Pin Output Resistor SOURCE Pin Input Current

0 13 1.21 10 50 0.55 10 19.5 50 1 18 1.235 20 80 0.62 20 20 70 2 0 0.4 0 1.2 0.1 -80 1.4 1.235 0.21 -100 2
1 23 1.26 30 110 0.7 30 20.5 90 4 1 0.8 10 1.28 0.3 -120 2.6
A k V mV mV V mV k A A A V A V V A A
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VUV Falling

VSOURCE = VGATE = 12V, LTC4218-12 Only VSOURCE = VGATE = 12V, LTC4218 Only VSOURCE = VGATE = 0V IOUT = 2mA VOUT = 30V VTIMER Rising VTIMER Falling VTIMER = 0V VTIMER = 1.2V

Outputs V(OL) I(OH) VTIMER(H) VTIMER(L) ITIMER(UP) ITIMER(DN) PG, FLT Pin Output Low Voltage PG, FLT Pin Input Leakage Current TIMER Pin High Threshold TIMER Pin Low Threshold TIMER Pin Pull Up Current TIMER Pin Pull-Down Current
3
LTC4218 ELECTRICAL CHARACTERISTICS
SYMBOL ITIMER(RATIO) IMON(FS) IMON(OFF) GIMON AC Characteristics tPHL(GATE) tPHL(SENSE) Input High (OV), Input Low (UV) to GATE Low Propagation Delay VSENSE+ - VSENSE- High to GATE Low Propagation Delay Turn-On Delay VGATE < 16.5V Falling VFB = 0, Step (VSENSE+ - VSENSE-) to 60mV, CGATE = 1.5nF, VGATE < 16.5V Falling Step VUV to 2V, VGATE > 13V

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VDD = 12V unless otherwise noted.
PARAMETER TIMER Pin Current Ratio ITIMER(DN)/ ITIMER(UP) IMON Fullscale Output Current IMON Pin Offset Current IMON Pin Gain VSENSE+ - VSENSE- = 15mV VSENSE+ - VSENSE- = 1mV VSENSE+ - VSENSE- = 15mV and 1mV CONDITIONS

MIN 1.6 94 6.47
TYP 2 100 0 6.67 3 0.2
MAX 2.7 106 6 6.87 5 1
UNITS % A A A/mV s s
tD(ON)
50
100
150
ms
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 pins are positive, all voltages are referenced to GND unless otherwise specified.
Note 3: An internal clamp limits the GATE pin to a maximum of 6.5V above the SOURCE pin. Driving either GATE or SOURCE pin to voltages beyond the clamp may damage the device.
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LTC4218 TYPICAL PERFORMANCE CHARACTERISTICS
IDD vs VDD
2.0 3.5 3.0 1.8 85C IDD (mA) 1.6 25C 1.4 -40C 1.2 INTVCC (V) 2.5 2.0 1.5 1.0 0.5 0 0 5 10 15 VDD (V) 20 25 30
4218 G01
TA = 25C, VDD = 12V unless otherwise noted. UV Low-High Threshold vs Temperature
1.234
INTVCC Load Regulation
VDD = 5V UV LOW-HIGH HRESHOLD (V) -14 1.232
VDD = 3.3V
1.230
1.228
1.0
0
-2
-4
-8 ILOAD (mA)
-6
-10
-12
1.226 -50
-25
0 25 50 TEMPERATURE (C)
75
100
4218 G03
4218 G02
UV Hysteresis vs Temperature
0.10 TIMER PULL-UP CURRENT (A) -110
Timer Pull-Up Current vs Temperature
CURRENT LIMIT PROPAGATION DELAY (s) 1000
Current Limit Delay
CGATE = 10nF
-105
100
UV HYSTERESIS (V)
0.08
-100
10
0.06
-95
1
0.04 -50
-25
50 0 25 TEMPERATURE (C)
75
100
4218 G04
-90 -50
0.1 0 15 30 45 60 CURRENT LIMIT SENSE VOLTAGE (VSENSE+ - VSENSE-) (mV) 75
-25
50 0 25 TEMPERATURE (C)
75
100
4218 G05
4218 G06
Current Limit Threshold Foldback
16 CURRENT LIMIT SENSE VOLTAGE (VSENSE+ - VSENSE-) (mV) CURRENT LIMIT SENSE VOLTAGE (VDD - VSENSE) (mV) 14 12 10 8 6 4 2 0 0 0.2 0.4 0.6 0.8 FB VOLTAGE (V) 1.0 1.2
4218 G07
Current Limit Adjustment
16 14 12 RISET (k) 1k 10k 100k RSET () 1M 10M
4218 G08
ISET Resistor vs Temperature
22
21
10 8 6 4 2 0
20
19
18 -50
-25
50 0 25 TEMPERATURE (C)
75
100
4218 G09
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LTC4218 TYPICAL PERFORMANCE CHARACTERISTICS
GATE Pull-Up Current vs Temperature
-26.0 GATE DRIVE (VGATE - VSOURCE) (V) 7 VDD = 12V 6 5 4 3 VDD = 3.3V 2 1 0 -25 0 25 50 TEMPERATURE (C) 75 100
4218 G10
TA = 25C, VDD = 12V unless otherwise noted.
Gate Pull-Up Current vs Gate Drive
6.2 GATE DRIVE (VGATE - VSOURCE) (V) -25 -30
4218 G11
Gate Drive vs VDD
-25.5 IGATE PULL-UP (A)
6.0
5.8
-25.0
5.6
-24.5
5.4
-24.0 -50
5.2 0 -5 -10 -15 -20 IGATE (A) 0 5 10 15 VDD (V) 20 25 30
4218 G12
Gate Drive vs Temperature
6.15 GATE DRIVE (VGATE - VSOURCE) (V) 14 12 6.14 PG, FLT VOUT LOW (V) 10 8 6 4
PG, FLT VOUT Low vs ILOAD
105
IMON vs Temperature and VDD
VDD = 3.3V, 12V, 24V VSENSE+ - VSENSE- = 15mV
PG
FLT
100
6.12
IMON (A) 0 2 4 6 8 ILOAD (mA) 10 12
4218 G14
6.13
95
90
6.11 2 6.10 -50 0 -25 0 25 50 TEMPERATURE (C) 75 100
4218 G13
85
80 -50
-25
0 25 50 TEMPERATURE (C)
75
100
4218 G15
IMON vs Sense
100 4
VIMON vs Sense
75 IMON (A) VIMON (V)
3
RIMON = 100k RIMON = 40k
50
2
RIMON = 20k
25
1 RIMON = 10k
0 0 5 10 SENSE VOLTAGE (mV) 15
4218 G16
0 0 5 10 SENSE VOLTAGE (mV) 15
4218 G17
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LTC4218 PIN FUNCTIONS
Exposed Pad: Exposed pad may be left open or connected to device ground. FB: Foldback and Power Good Comparator Input. Connect this pin to an external resistive divider from SOURCE for the LTC4218 (adjustable version). The LTC4218-12 version uses a fixed internal divider with optional external adjustment. Open the pin if the LTC4218-12 thresholds for 12V operation are desired. If the voltage falls below 0.6V, the output power is considered bad and the current limit is reduced. If the voltage falls below 1.21V the PG pin will pull low to indicate the power is bad. FLT: Overcurrent Fault Indicator. Open drain output pulls low when an overcurrent fault has occurred and the circuit breaker trips. For overcurrent auto-retry tie to UV pin (see Applications Information for details). GATE: Gate Drive for External N-Channel FET. An internal 24A current source charges the gate of the external N-channel MOSFET. A resistor and capacitor network from this pin to ground sets the turn-on rate. During an undervoltage or overvoltage generated turn-off a 250A pull-down current turns the MOSFET off. During a short circuit or undervoltage lockout, a 170mA pull-down current source between GATE and SOURCE is activated. GND: Device Ground. IMON: Current Monitor Output. The current sourced from this pin is defined as the current sense voltage (between the SENSE+ and SENSE- pins) multiplied by 6.67A/mV. Placing a 20k resistor from this pin to GND creates a 0V to 2V voltage swing when the current sense voltage ranges from 0mV to 15mV. INTVCC: Internal 3V Supply Decoupling Output. This pin must have a 0.1F or larger capacitor. ISET: Current Limit Adjustment Pin. For 15mV current limit threshold, open this pin. This pin is driven by a 20k resistor in series with a voltage source. The pin voltage is used to generate the current limit threshold. The internal 20k resistor and an external resistor between ISET and ground create an attenuator that lowers the current limit value. NC: No Connection OV: Overvoltage Comparator Input. Connect this pin to an external resistive divider from VDD for the LTC4218 (adjustable version). The LTC4218-12 version uses a fixed internal divider with optional external adjustment for 12V operation. Open the pin if the LTC4218-12 thresholds are desired. If the voltage at this pin rises above 1.235V, an overvoltage is detected and the switch turns off. Tie to GND if unused. PG: Power Good Indicator. Open drain output pulls low when the FB pin drops below 1.21V indicating the power is bad. SENSE-: Current Sense Minus Input. Connect this pin to the opposite of VDD current sense resistor side. The current limit circuit controls the GATE pin to limit the sense voltage between the SENSE+ and SENSE- pins to 15mV or less depending on the voltage at the FB pin. SENSE+: Current Sense Plus Input. Connect this pin to the VDD side of the current sense resistor. SOURCE: N-Channel MOSFET Source Connection. Connect this pin to the source of the external N-channel MOSFET switch. This pin provides a return for the gate pull-down circuit. In the LTC4218-12 version, the powergood comparator monitors an internal resistive divider between the SOURCE pin and GND. TIMER: Timer Input. Connect a capacitor between this pin and ground to set a 12ms/F duration for current limit before the switch is turned off. If the UV pin is toggled low while the MOSFET switch is off, the switch will turn on again following a cool down time of 518ms/F duration. UV: Undervoltage Comparator Input. Tie high if unused. Connect this pin to an external resistive divider from VDD for the LTC4218 (adjustable version). The LTC4218-12 version drives the UV pin with an internal resistive divider from VDD. Open the pin if the preset LTC4218-12 thresholds for 12V operation are desired. If the UV pin voltage falls below 1.15V, an undervoltage is detected and the switch turns off. Pulling this pin below 0.62V resets the overcurrent fault and allows the switch to turn back on (see Applications Information for details). If overcurrent auto-retry is desired then tie this pin to the FLT pin. VDD: Supply Voltage. This pin has an undervoltage lockout threshold of 2.73V.
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LTC4218 FUNCTIONAL DIAGRAM
SENSE+ SENSE- GATE SOURCE
VDD CLAMP CS CHARGE PUMP AND GATE DRIVER
IMON
- +- +
ISET 20k 0.6V REFERENCE X1
CM FOLDBACK 0.6V SOURCE 150k 1.235V *
FB
+
UV PG LOGIC
140k UV 20k
*
-
*
0.62V
+
RST
VDD 224k OV 20k * 1.235V *
- +
OV
0.2V
+
TM1 INTVCC 100A
-
- -
UVLO1
2A
+
TM2 VDD 3.1V GEN 1.235V
VDD
-
TIMER
UVLO2
GND * DFN ONLY
EXPOSED PAD*
8
-
2.73V
+
+
20k *
VDD
- +
1.235V PG
FLT
INTVCC
2.65V
4218 BD
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LTC4218 OPERATION
The Functional Diagram displays the main circuits of the device. The LTC4218 is designed to turn a board's supply voltage on and off in a controlled manner, allowing the board to be safely inserted and removed from a live backplane. During normal operation, the charge pump and gate driver turn on the external N-channel pass FET's gate to provide power to the load. The current sense (CS) amplifier monitors the load current using the voltage sensed across the current sense resistor. The CS amplifier limits the current in the load by reducing the GATE-to-SOURCE voltage in an active control loop. It is simple to adjust the current limit threshold using the current setting (ISET) pin. This allows a different threshold during other times such as startup. A short circuit on the output to ground causes significant power dissipation during active current limiting. To limit this power, the foldback amplifier reduces the current limit value from 15mV to 3.75mV (referred to the SENSE+ minus SENSE- voltage) in a linear manner as the FB pin drops below 0.6V (see Typical Performance Characteristics). If an overcurrent condition persists, the TIMER pin ramps up with a 100A current source until the pin voltage exceeds 1.2V (comparator TM2). This indicates to the logic that it is time to turn off the MOSFET to prevent overheating. At this point the TIMER pin ramps down using the 2A current source until the voltage drops below 0.2V (Comparator TM1) which tells the logic to start an internal 100ms timer. At this point, the pass transistor has cooled and it is safe to turn it on again. The fixed 12V version, LTC4218-12, uses two separate internal dividers from VDD to drive the UV and OV pins. This version also features a divider from the SOURCE pin to drive the FB pin. The LTC4218-12 is available in a DFN package while the LTC4218 (adjustable version) is in a SSOP package. The output voltage is monitored using the FB pin and the PG comparator to determine if the power is available for the load. The power good condition is signaled by the PG pin using an open-drain pull-down transistor. The Functional Diagram shows the monitoring blocks of the LTC4218. The comparators on the left side include the UV and OV comparators. These comparators are used to determine if the external conditions are valid prior to turning on the MOSFET. But first, the undervoltage lockout circuits (UVLO1 and UVLO2) must validate the input supply and internally generated 3.1V supply (INTVCC) and generate the power up initialization to the logic circuits. If the external conditions remain valid for 100ms the MOSFET is allowed to turn on. Other monitoring features include the IMON current monitor. The current monitor (CM) outputs a current proportional to the sense resistor current. This current can drive an external resistor or other circuits for monitoring purposes.
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LTC4218 APPLICATIONS INFORMATION
The typical LTC4218 application is in a high availability system that uses a positive voltage supply to distribute power to individual cards. The basic application circuit is shown in Figure 1. External component selection is discussed in detail in the following sections.
RS 2m 12V R1 10 SENSE- GATE SOURCE SENSE+ VDD R4 140k UV R2 224k R3 20k R5 20k FLT OV LTC4218GN PG ISET RSET 20k CT 0.1F TIMER C1 0.1F INTVCC IMON GND RMON 20k
4218 F01
The pass transistor is turned on by charging up the GATE with a 24A charge pump generated current source (Figure 2).
VDD + 6.15 SLOPE = 24A/CGATE GATE
Q1 Si7108DN
VDD VOUT 12V 3A
SOURCE
RGATE 1k CGATE 0.01F R6 150k 12V R8 10k R7 20k
+
CL 330F
t1
t2
4218 F02
Figure 2. Supply Turn-On
FB
The voltage at the GATE pin rises with a slope equal to 24A/CGATE and the supply inrush current is set at: IINRUSH =
ADC
CL CGATE
* 24A
Figure 1. 3A, 12V Card Resident Application
Turn-On Sequence The power supply on a board is controlled by placing an external N-channel pass transistor (Q1) in the power path. Note the sense resistor (RS) detects current and the capacitor (CGATE) controls gate slew rate. Resistor R1 prevents high frequency oscillations in Q1 and resistor RGATE isolates CGATE during fast turn-off. Several conditions must be present before the external pass transistor can be turned on. First, the supply VDD must exceed its undervoltage lockout level. Next, the internally generated supply INTVCC must cross its 2.65V undervoltage threshold. This generates a 25s poweron-reset pulse which clears the logic's fault register and initializes internal latches. After the power-on-reset pulse, the LTC4218 will go through the following sequence. First, the UV and OV pins must indicate that the input power is within the acceptable range. All of these conditions must be satisfied for a duration of 100ms to ensure that any contact bounce during the insertion has ended.
When the GATE voltage reaches the MOSFET threshold voltage, the switch begins to turn on and the SOURCE voltage follows the GATE voltage as it increases. Once SOURCE reaches VDD, the GATE will ramp up until clamped by the 6.15V zener between GATE and SOURCE. As the SOURCE pin voltage rises, so will the FB pin which is monitoring it. If the voltage across the current sense resistor (RS) gets too high, the inrush current will be limited by the internal current limiting circuitry. Once the FB pin crosses its 1.235V threshold and the GATE to SOURCE voltage exceeds 4.2V, the PG pin will cease to pull low and indicate that the power is good. Turn-Off Sequence The switch can be turned off by a variety of conditions. A normal turn-off is initiated by the UV pin going below its 1.235V threshold. Additionally, several fault conditions will turn off the switch. These include an input overvoltage (OV pin) and overcurrent circuit breaker (SENSE pin). Normally, the switch is turned off with a 250A current pulling down the GATE pin to ground. With the switch turned off, the SOURCE pin voltage drops which pulls the FB pin below
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LTC4218 APPLICATIONS INFORMATION
its threshold. The PG then pulls low to indicate output power is no longer good. If VDD drops below 2.65V for greater than 5s or INTVCC drops below 2.5V for greater than 1s, a fast shutdown of the switch is initiated. The GATE is pulled down with a 170mA current to the SOURCE pin. Overcurrent Fault The LTC4218 features an adjustable current limit with foldback that protects the MOSFET when excessive load current happens. To protect the switch during active current limit, the available current is reduced as a function of the output voltage sensed by the FB pin. A graph in the Typical Performance Characteristics shows the current limit versus FB voltage. An overcurrent fault occurs when the current limit circuitry has been engaged for longer than the time-out delay set by the TIMER. Current limiting begins when the current sense voltage between the SENSE+ and SENSE- pins reaches 3.75mV to 15mV (depending on the foldback). The GATE pin is then brought down with a 170mA GATE-to-SOURCE current. The voltage on the GATE is regulated in order to limit the current sense voltage to less than 15mV. At this point, a circuit breaker time delay starts by charging the external timing capacitor from the TIMER pin with a 100A pull-up current. If the TIMER pin reaches its 1.2V threshold, the external switch turns off (with a 250A current from GATE to ground). Next, the FLT pin is pulled low to indicate an overcurrent fault has turned off the MOSFET. For a given the circuit breaker time delay, the equation for setting the timing capacitor's value is as follows: CT = TCB * 0.083[F/ms] After the switch is turned off, the TIMER pin begins discharging the timing capacitor with a 2A pull-down current. When the TIMER pin reaches its 0.2V threshold, the switch is allowed to turn on again if the overcurrent fault has been cleared. Bringing the UV pin below 0.6V and then high will clear the fault. If the TIMER pin is tied to INTVCC, then the switch is allowed to turn on again (after an internal 100ms delay) if the overcurrent fault is cleared. Tying the FLT pin to the UV pin allows the part to self-clear the fault and turn the MOSFET on as soon as TIMER pin has ramped below 0.2V. In this auto retry mode, the LTC4218 repeatedly tries to turn on after an overcurrent at a period determined by the capacitor on the TIMER pin. The waveform in Figure 3 shows how the output latches off following a short circuit. The drop across the sense resistor is 3.75mV as the timer ramps up.
VOUT 10V/DIV
IOUT 2A/DIV
VGATE 10V/DIV TIMER 2V/DIV 1ms/DIV
4218 F03
Figure 3. Short-Circuit Waveform
Current Limit Adjustment The default value of the active current limiting signal threshold is 15mV. The current limit threshold can be adjusted lower by placing a resistor on the ISET pin. As shown in the Functional Diagram the voltage at the ISET pin (via the clamp circuit) sets the CS amplifier's built-in offset voltage. This offset voltage directly determines the active current limit value. With the ISET pin open, the voltage at the ISET pin is determined by the buffered reference voltage. This voltage is set to 0.618V which corresponds to a 15mV current limit threshold. An external resistor placed between the ISET pin and ground forms a resistive divider with the internal 20k sourcing resistor. The divider acts to lower the voltage at the ISET pin and therefore lower the current limit threshold. The overall current limit threshold precision is reduced to 11% when using a 20k resistor to half the threshold.
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11
LTC4218 APPLICATIONS INFORMATION
Using a switch (connected to ground) in series with the external resistor allows the active current limit to change only when the switch is closed. This feature can be used when the startup current exceeds the typical maximum load current. Monitor MOSFET Current The current in the MOSFET passes through the sense resistor. The voltage on the sense resistor is converted to a current that is sourced out of the IMON pin. The gain of the ISENSE amplifier is 100A from IMON for 15mV on the sense resistor. This output current can be converted to a voltage using an external resistor to drive a comparator or ADC. The voltage compliance for the IMON pin is from 0V to INTVCC - 0.7V. A microcontroller with a built-in comparator can build a simple integrating single-slope ADC by resetting a capacitor that is charged with this current. When the capacitor voltage trips the comparator and the capacitor is reset, a timer is started. The time between resets will indicate the MOSFET current. Monitor OV and UV Faults Protecting the load from an overvoltage condition is the main function of the OV pin. In the LTC4218-12 an internal resistive divider (driving the OV pin) connects to a comparator to turn off the MOSFET when the VDD voltage exceeds 15.05V. If the VDD pin subsequently falls back below 14.8V, the switch will be allowed to turn on immediately. In the LTC4218, the OV pin threshold is 1.23V when rising and 1.21V when falling out of overvoltage. The UV pin functions as an undervoltage protection pin or as an "on" pin. In the LTC4218-12 the MOSFET turns off when VDD falls below 9.23V. If the VDD pin subsequently rises above 9.88V for 100ms, the switch will be allowed to turn on again. The LTC4218 UV turn on/off threshold is 1.23V (rising) and 1.15V (falling). In the case of an undervoltage or overvoltage, the MOSFET turns off and there is indication on the PG status pin. When the overvoltage is removed, the MOSFET's gate ramps up immediately. Powergood Indication In addition to setting the foldback current limit threshold, the FB pin is used to determine a powergood condition. The LTC4218-12 uses an internal resistive divider on the SOURCE pin to drive the FB pin. The PG comparator indicates logic high when SOURCE pin rises above 10.5V. If the SOURCE pin subsequently falls below 10.3V, the comparator toggles low. On the LTC4218 the PG comparator drives high when the FB pin rises above 1.23V and low when falls below 1.21V. Once the PG comparator is high, the GATE pin voltage is monitored with respect to the SOURCE pin. Once the GATE minus SOURCE voltage exceeds 4.2V, the PG pin goes high. This indicates to the system that it is safe to load the Output while the MOSFET is completely turned "on". The PG pin goes low when the GATE is commanded off (using the UV, OV or SENSE+/SENSE- pins) or when the PG comparator drives low. 12V Fixed Version In the LTC4218-12, the UV, OV and FB pins are driven by internal dividers which may need to be filtered to prevent false faults. By placing a bypass capacitor on these pins the faults are delayed by the RC time constant. Use the RIN value from the electrical table for this calculation. In cases where the fixed thresholds need a slight adjustment, placing a resistor from the UV or OV pins to VDD or GND will adjust the threshold up or down. Likewise, placing a resistor between FB pin to OUT or GND adjusts the threshold. Again, use the RIN value from the electrical table for this calculation. An example in Figure 4 raises the UV turn-on voltage from 9.88V to 10.5V. Increasing the UV level requires adding a resistor between UV and ground. The resistor, (RSHUNT1), can be calculated using electrical table parameters as follows: RSHUNT1 =
( VNEW - VOLD )
R(IN) * VOLD
=
18k * 9.88 = 287k (10.5 - 9.88)
4218fb
12
LTC4218 APPLICATIONS INFORMATION
LTC4218-12 VDD 12V OV RSHUNT2 R1 10 UV RSHUNT1
4218 F04
RS 2m
Q1 Si7108DN
+
CL 330F
VOUT 12V 6A
SENSE- SENSE+ VDD UV
GATE SOURCE 12V
RGATE 1k CGATE 0.01F
LTC4218DHC-12 FLT PG
R2 10k
Figure 4. Adjusting LTC4218-12 Thresholds
In this same figure the OV threshold is lowered from 15.05V to 13.5V. Decreasing the OV threshold requires adding a resistor between VDD and OV. This resistor can be calculated as follows: RSHUNT2 = R(IN) * VOLD V( TH) V -V NEW OV( TH) ( VOLD - VNEW )
CT 0.1F C1 0.1F
TIMER INTVCC GND IMON R3 20k
4218 F05
ADC
Figure 5. 6A, 12V Card Resident Application
(
) =

Calculate the time it takes to charge up COUT: tCHARGUP = CL * VIN 330F * 12V = = 4ms 1A IINRUSH
18k * 15.05 (13.5 - 1.235) = 1.736M 1.235 (15.05 - 13.5) Use the equation for RSHUNT1 for increasing the OV and FB thresholds. Likewise, use the equation for RSHUNT2 for decreasing the UV and FB thresholds. Design Example Consider the following design example (Figure 5): VIN = 12V, IMAX = 7.5A. IINRUSH = 1A, CL = 330F, VUVON = 9.88V, VOVOFF = 15.05V, VPWRGD = 10.5V. A current limit fault triggers an automatic restart of the power up sequence. The selection of the sense resistor, (RS), is set by the overcurrent threshold of 15mV: RS = 15mV/IMAX = 15mV/7.5A = 0.002 The MOSFET should be sized to handle the power dissipation during the inrush charging of the output capacitor COUT. The method used to determine the power in Q1 is the principal: EC = Energy in CL = Energy in Q1 Thus: EC = 1/2 CV2 = 1/2 (330F)(12)2 = 0.048J
The inrush current is set to 1A using CGATE: CGATE = CL IGATE(UP) IINRUSH = 330F 24A 0.01F 1A
The average power dissipated in the MOSFET: PDISS = EC/tCHARGUP = 0.048J/4ms = 12W The SOA (safe operating area) curves of candidate MOSFETs must be evaluated to ensure that the heat capacity of the package can stand 12W for 4ms. The SOA curves of the Vishay Siliconix Si7108DN provide 1.5A at 10V (15W) for 100ms, satisfying the requirement. Next, the power dissipated in the MOSFET during overcurrent must be limited. The active current limit uses a timer to prevent excessive energy dissipation in the MOSFET. The worst-case power occurs when the voltage versus current profile of the foldback current limit is at the maximum. This occurs when the current is 6A and the voltage is one half of 12V or (6V). See the Current Limit Sense Voltage vs FB Voltage in the Typical Performance curves to view this profile. In order to survive 36W, the MOSFET SOA dictates a maximum time at this power level. The Si7108DN allows 60W for 10ms or less. Therefore, it is acceptable to set the current limit timeout using CT to be 1.2ms: CT = 1.2ms/12[ms/F] = 0.1F
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13
LTC4218 APPLICATIONS INFORMATION
After the 1.2ms timeout the FLT pin needs to pull down on the UV pin to restart the power-up sequence. Since the default values for overvoltage, undervoltage and powergood thresholds for the 12V fixed version match the requirements, no external components are required for the UV, OV and FB pins. The final schematic results in very few external components. Resistor R1 (10) prevents high frequency oscillations in Q1 while RGATE of 1k isolates CGATE during fast turn-off. The pull-up resistor, (R2), connects to the PG pin while the 20k (R3) converts the IMON current to a voltage at a ratio: VIMON = 6.67A 2mV 0.267 V * * 20k = mV A A In Hot Swap applications where load currents can be 6A, narrow PCB tracks exhibit more resistances than wider tracks and operate at elevated temperatures. The minimum trace width for 1oz copper foil is 0.02" per amp to make sure the trace stays at a reasonable temperature. Using 0.03" per amp or wider is recommended. Note that 1oz copper exhibits a sheet resistance of about 0.5m/square. Small resistances add up quickly in high current applications It is also important to put C1, the bypass capacitor for the INTVCC pin, as close as possible between the INTVCC and GND. Place the 10 resistor as close as possible to Q1. This will limit the parasitic trace capacitance that leads to Q1 self-oscillation. Additional Applications The LTC4218 has a wide operating range from 2.9V to 26.5V. The UV, OV and PG thresholds are set with a few resistors. All other functions are independent of supply voltage. The last page includes a 24V application with a UV threshold of 19.8V, an OV threshold of 28.3V and a PG threshold of 20.75V. Figure 7 shows a 3.3V application with a UV threshold of 2.87V, an OV threshold of 3.77V and a PG threshold of 3.05V.
In addition, there is a 0.1F bypass (C1) on the INTVCC pin. 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 resistors and the power MOSFETs should include good thermal management techniques for optimal device power dissipation. A recommended PCB layout for the sense resistor and power MOSFET is illustrated in Figure 6.
RS Q1
RS 2m 3.3V
Q1 Si7104DN
R1 10
R5 14.7k RGATE 1k
+
VOUT 3.3V 6A CL 330F
R1
R2 17.4k
SENSE- GATE SOURCE SENSE+ VDD UV FLT OV R4 10k CT 0.1F C1 0.1F
4218 F06
CGATE 0.01F R6 10k
FB 3.3V LTC4218GN R7 10k PG
LTC4218
R3 3.16k
TIMER INTVCC GND IOUT RMON 20k
4218 F07
C
ADC
Figure 6. Recommended Layout
Figure 7. 3.3V, 6A Card Resident Application
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14
LTC4218 PACKAGE DESCRIPTION
DHC Package 16-Lead Plastic DFN (5mm x 3mm)
(Reference LTC DWG # 05-08-1706)
5.00 0.10 (2 SIDES) 0.65 0.05 3.50 0.05 3.00 0.10 (2 SIDES) 1.65 0.05 (2 SIDES) PACKAGE OUTLINE 1.65 0.10 (2 SIDES) PIN 1 NOTCH
(DHC16) DFN 1103
R = 0.115 TYP R = 0.20 TYP 9 16
0.40 0.10
2.20 0.05
PIN 1 TOP MARK (SEE NOTE 6) 8 0.200 REF 0.75 0.05 4.40 0.10 (2 SIDES) BOTTOM VIEW--EXPOSED PAD 1 0.25 0.05 0.50 BSC
0.25 0.05 0.50 BSC 4.40 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.00 - 0.05
NOTE: 1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WJED-1) IN JEDEC PACKAGE OUTLINE MO-229 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
GN Package 16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.045 .005
.189 - .196* (4.801 - 4.978) 16 15 14 13 12 11 10 9
.009 (0.229) REF
.254 MIN
.150 - .165
.229 - .244 (5.817 - 6.198)
.0165 .0015
.150 - .157** (3.810 - 3.988)
.0250 BSC
RECOMMENDED SOLDER PAD LAYOUT
1 .015 .004 x 45 (0.38 0.10) .007 - .0098 (0.178 - 0.249) .016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE .0532 - .0688 (1.35 - 1.75)
23
4
56
7
8
.004 - .0098 (0.102 - 0.249)
0 - 8 TYP
.008 - .012 (0.203 - 0.305) TYP
.0250 (0.635) BSC
GN16 (SSOP) 0204
4218fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC4218 TYPICAL APPLICATION
24V, 6A Card Resident Application
2m 24V * 10 158k Si7880ADP VOUT 24V 6A
+
330F 1k SENSE- SENSE+ GATE SOURCE FB 24V LTC4218GN 10k PG TIMER 0.1F INTVCC GND IMON 20k
4218 TA02
0.01F
215k
VDD UV FLT OV
10k
4.32k 10k
ADC
0.1F *DIODES INC., SMAJ24A
RELATED PARTS
PART NUMBER LTC1421 LTC1422 LTC1642A LTC1645 LTC1647-1/LTC1647-2/ LTC1647-3 LTC4210 LTC4211 LTC4212 LTC4214 LTC4215 LT4220 LTC4221 LTC4230 LTC4245 DESCRIPTION Dual Channel, Hot Swap Controller Single Channel, Hot Swap Controller Single Channel, Hot Swap Controller Dual Channel, Hot Swap Controller Dual Channel, Hot Swap Controllers Single Channel, Hot Swap Controller Single Channel, Hot Swap Controller Single Channel, Hot Swap Controller Negative Voltage, Hot Swap Controller Single Hot Swap Controller with ADC and I2C Interface Positive and Negative Voltage, Dual Channel, Hot Swap Controller Dual Hot Swap Controller/Sequencer Triple Channel, Hot Swap Controller COMMENTS Operates from 3V to 12V, Supports -12V, SSOP-24 Operates from 2.7V to 12V, SO-8 Operates from 3V to 16.5V, Overvoltage Protection up to 33V, SSOP-16 Operates from 3V to 12V, Power Sequencing, SO-8 or SO-14 Operates from 2.7V to 16.5V, SO-8 or SSOP-16 Operates from 2.7V to 16.5V, Active Current Limiting, SOT23-6 Operates from 2.5V to 16.5V, Multifunction Current Control, MSOP-8 or MSOP-10 Operates from 2.5V to 16.5V, Power-Up Timeout, MSOP-10 Operates from -6V to -16V, MSOP-10 Operates from 2.9V to 15V, Digitally Monitors Voltage and Current with 8-Bit ADC Operates from 2.7V to 16.5V, SSOP-16 Operates from 1V to 13.5V, Multifunction Current Control, SSOP-16 Operates from 1.7V to 16.5V, Multifunction Current Control, SSOP-20
Quad Hot Swap Controller with ADC and 3.3V, 5V and 12V for CompactPCI, or 3.3V, 3.3V Auxiliary and 12V for PCIExpress, Monitors Voltage and Current with 8-Bit ADC I2C Interface
4218fb
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1008 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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