LTC4213
No RSENSE™
Electronic Circuit Breaker
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FEATURES
DESCRIPTIO
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The LTC®4213 is an Electronic Circuit Breaker. An overcurrent circuit breaker senses the voltage across the drain
and source terminals of an external N-channel MOSFET
with no need for a sense resistor. The advantages are a
lower cost and reduced voltage and power loss in the
switch path. An internal high-side driver controls the
external MOSFET gate.
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Fast 1µs Response Circuit Breaker
3 Selectable Circuit Breaker Thresholds
No Sense Resistor Required
Dual Level Overcurrent Fault Protection
Controls Load Voltages from 0V to 6V
High Side Drive for External N-Channel FET
Undervoltage Lockout
READY Pin Signals When Circuit Breaker Armed
Small Plastic (3mm x 2mm) DFN Package
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APPLICATIO S
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■
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Electronic Circuit Breaker
High-Side Switch
Hot Board Insertion
Two integrated comparators provide dual level overcurrent protection over the bias supply to ground common
mode range. The slow comparator has 16µs response
while the fast comparator trips in 1µs. The circuit breaker
has three selectable trip thresholds: 25mV, 50mV and
100mV. An ON pin controls the ON/OFF and resets circuit
breaker faults. READY signals the MOSFET is conducting
and the circuit breaker is armed. The LTC4213 operates
from VCC = 2.3V to 6V.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
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TYPICAL APPLICATIO
1.25V Electronic Circuit Breaker
SI4864DY
VIN
1.25V
VBIAS
2.3V TO 6V
VCC
VOUT
1.25V
3.5A
ON
IOUT
(50A/DIV)
SENSEP GATE SENSEN
VBIAS
LTC4213
OFF ON
Severe Overload Response
GND
ISEL
10k
VOUT
(1V/DIV)
READY
VGATE
(5V/DIV)
4213 TA01
VIN
(1V/DIV)
2µs/DIV
4213 TA01b
4213f
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LTC4213
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Bias Supply Voltage (VCC) ........................... –0.3V to 9V
Input Voltages
ON, SENSEP, SENSEN .............................– 0.3V to 9V
ISEL .......................................... – 0.3V to (VCC + 0.3V)
Output Voltages
GATE .....................................................– 0.3V to 15V
READY .....................................................– 0.3V to 9V
Operating Temperature Range
LTC4213C ............................................... 0°C to 70°C
LTC4213I ............................................. –40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10sec)................... 300°C
ORDER PART
NUMBER
TOP VIEW
8 VCC
READY 1
ON 2
ISEL 3
9
GND 4
LTC4213CDDB
LTC4213IDDB
7 SENSEP
6 SENSEN
5 GATE
DDB PART*
MARKING
DDB PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
TJMAX = 125°C, θJA = 250°C/W
EXPOSED PAD (PIN 9)
PCB CONNECTION OPTIONAL
LBHV
Consult LTC Marketing for parts specified with wider operating temperature ranges.
*The temperature grade is identified by a label on the shipping container.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, ISEL = 0 unless otherwise noted. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
VCC
Bias Supply Voltage
●
2.3
6
V
VSENSEP
SENSEP Voltage
●
0
6
V
ICC
VCC Supply Current
●
1.6
3
mA
VCC(UVLR)
VCC Undervoltage Lockout Release
∆VCC(UVHYST)
VCC Undervoltage Lockout Hysteresis
ISENSEP
SENSEP Input Current
ISENSEN
SENSEN Input Current
VCC Rising
MIN
TYP
MAX
UNITS
●
1.8
2.07
2.23
V
●
30
100
160
mV
15
40
80
µA
–1
±15
µA
40
80
µA
–1
±15
µA
VSENSEP = VSENSEN = 5V, Normal Mode
VSENSEP = VSENSEN = 0, Normal Mode
VSENSEP = VSENSEN = 5V, Normal Mode
15
VSENSEP = VSENSEN = 0, Normal Mode
VSENSEP = VSENSEN = 5V,
Reset Mode or Fault Mode
50
280
µA
VCB
Circuit Breaker Trip Voltage
VCB = VSENSEP – VSENSEN
ISEL = 0, VSENSEP = VCC
ISEL = Floated, VSENSEP = VCC
ISEL = VCC, VSENSEP = VCC
●
●
●
22.5
45
90
25
50
100
27.5
55
110
mV
mV
mV
VCB(FAST)
Fast Circuit Breaker Trip Voltage
VCB(FAST) = VSENSEP – VSENSEN
ISEL = 0, VSENSEP = VCC
ISEL = Floated, VSENSEP = VCC
ISEL = VCC, VSENSEP = VCC
●
●
●
63
126
252
100
175
325
115
200
371
mV
mV
mV
IGATE(UP)
GATE Pin Pull Up Current
VGATE = 0V
●
–50
–100
–150
µA
IGATE(DN)
GATE Pin Pull Down Current
∆VSENSEP – VSENSEN = 200mV, VGATE = 8V
●
10
40
∆VGSMAX
External N-Channel Gate Drive
VSENSEN = 0, VCC ≥ 2.97V, IGATE = –1µA
VSENSEN = 0, VCC = 2.3V, IGATE = –1µA
●
●
4.8
2.65
6.5
4.3
8
8
V
V
∆VGSARM
VGS Voltage to Arm Circuit Breaker
VSENSEN = 0, VCC ≥ 2.97V
VSENSEN = 0, VCC = 2.3V
●
●
4.4
2.5
5.4
3.5
7.6
7
V
V
mA
4213f
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LTC4213
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, ISEL = 0 unless otherwise noted. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
∆VGSMAX – ∆VGSARM
Difference Between ∆VGSMAX and
∆VGSARM
VSENSEN = 0, VCC ≥ 2.97V
VSENSEN = 0, VCC = 2.3V
●
●
VREADY(OL)
READY Pin Output Low Voltage
IREADY = 1.6mA, Pull Down Device On
●
0.2
0.4
V
IREADY(LEAK)
READY Pin Leakage Current
VREADY = 5V, Pull Down Device Off
●
0
±1
µA
VON(TH)
ON Pin High Threshold
ON Rising, GATE Pulls Up
●
0.76
0.8
0.84
V
∆VON(HYST)
ON Pin Hysteresis
ON Falling, GATE Pulls Down
10
40
90
VON(RST)
ON Pin Reset Threshold
ON Falling, Fault Reset, GATE Pull Down
●
0.36
0.4
0.44
V
ION(IN)
ON Pin Input Current
VON = 1.2V
●
0
±1
µA
∆VOV
Overvoltage Threshold
∆VOV = VSENSEP – VCC
0.41
0.7
1.1
V
tOV
Overvoltage Protection Trip Time
VSENSEP = VSENSEN = Step 5V to 6.2V
25
65
160
µs
tFAULT(SLOW)
VCB Trips to GATE Discharging
∆VSENSE Step 0mV to 50mV,
VSENSEN Falling, VCC = VSENSEP = 5V
●
7
16
27
µs
tFAULT(FAST)
VCB(FAST) Trips to GATE Discharging
∆VSENSE Step 0V to 0.3V, VSENSEN Falling,
VSENSEP = 5V
●
1
2.5
µs
tDEBOUNCE
Startup De-Bounce Time
VON = 0V to 2V Step to Gate Rising,
(Exiting Reset Mode)
27
60
130
µs
tREADY
READY Delay Time
VGATE = 0V to 8V Step to READY Rising,
VSENSEP = VSENSEN = 0
22
50
115
µs
tOFF
Turn-Off Time
VON = 2V to 0.6V Step to GATE Discharging
1.5
5
10
µs
tON
Turn-On Time
VON = 0.6V to 2V Step to GATE Rising,
(Normal Mode)
4
8
16
µs
tRESET
Reset Time
VON Step 2V to 0V
20
80
150
µs
●
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
MIN
TYP
0.3
0.15
1.1
0.8
MAX
UNITS
V
V
mV
Note 2: All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
4213f
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LTC4213
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TYPICAL PERFOR A CE CHARACTERISTICS
unless otherwise noted.
ICC vs Temperature
3.0
2.5
2.5
2.0
1.5
1.0
0.5
0
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
2.0
1.5
1.0
0.5
0
–50
6.0
–25
75
25
0
50
TEMPERATURE (°C)
100
2.2
2.1
VCC RISING
2.0
Normalized VCB vs VCC
1.8
1.7
–50
125
1.04
1.04
1.04
0.96
NORMALIZED VCB(FAST)
1.06
NORMALIZED VCB
1.06
0.98
1.02
1.00
0.98
0.96
0.94
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
6.0
75
0
50
25
TEMPERATURE (°C)
100
1.02
1.00
0.98
0.96
0.94
–50
–25
75
0
50
25
TEMPERATURE (°C)
4213 G04
100
125
0.94
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
4213 G05
Normalized VCB(FAST) vs
Temperature
5.5
6.0
4213 G06
IGATE(UP) vs VCC
1.06
125
Normalized VCB(FAST) vs VCC
1.06
1.00
–25
4213 G03
Normalized VCB vs Temperature
1.02
VCC FALLING
1.9
4213 G02
4213 G01
NORMALIZED VCB
VCC(UVLR) vs Temperature
2.3
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
3.0
BIAS SUPPLY CURRENT (mA)
BIAS SUPPLY CURRENT (mA)
ICC vs VCC
Specifications are at TA = 25°C. VCC = 5V
IGATE(UP) vs Temperature
104
104
102
102
1.00
0.98
IGATE(UP) (µA)
1.02
IGATE(UP) (µA)
NORMALIZED VCB(FAST)
1.04
100
98
100
98
0.96
0.94
–50
–25
75
0
50
25
TEMPERATURE (°C)
100
125
4213 G07
96
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
6.0
4213 G08
96
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
4213 G09
4213f
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LTC4213
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TYPICAL PERFOR A CE CHARACTERISTICS
unless otherwise noted.
∆VGSMAX and ∆VGSARM vs
Temperature
∆VGSMAX and ∆VGSARM vs VCC
8
∆VGSMAX (FOR 5VCC)
∆VGSARM
6
5
4
3
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
0.85
7
∆VGSARM (FOR 5VCC)
6
5
∆VGSMAX (FOR 2.5VCC)
∆VGSARM (FOR 2.5VCC)
–25
75
0
25
50
TEMPERATURE (°C)
100
4213 G10
LOW THRESHOLD
0.75
0.65
2.0
125
1.0
LOW THRESHOLD
0.75
0.70
–25
100
75
0
50
25
TEMPERATURE (°C)
125
0.72
0.70
0.68
0.66
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
100
80
80
tDEBOUNCE
tREADY
40
20
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
6.0
4213 G16
tDEBOUNCE AND tREADY (µs)
100
2.5
0.8
0.7
0.6
0.5
0.4
–50
6.0
–25
75
0
25
50
TEMPERATURE (°C)
100
125
4213 G15
tDEBOUNCE and tREADY vs
Temperature
tDEBOUNCE and tREADY vs VCC
0
2.0
0.9
4213 G14
4213 G13
60
5.5
tRESET vs VCC
120
100
60
tDEBOUNCE
40
tREADY
80
tRESET (µs)
0.65
–50
OVERVOLTAGE THRESHOLD (V)
OVERVOLTAGE THRESHOLD (V)
HIGH THRESHOLD
6.0
5.5
∆VOV vs Temperature
0.74
0.90
0.80
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
4213 G12
∆VOV vs VCC
0.85
2.5
4213 G11
VON(TH) vs Temperature
ON PIN THRESHOLD (V)
HIGH THRESHOLD
0.80
0.70
4
3
–50
6.0
ON PIN THRESHOLD (V)
7
∆VGSMAX AND ∆VGSARM (V)
∆VGSMAX AND ∆VGSARM (V)
VON(TH) vs VCC
0.90
8
∆VGSMAX
tDEBOUNCE AND tREADY (µs)
Specifications are at TA = 25°C. VCC = 5V
60
40
20
0
–50
20
–25
75
0
50
25
TEMPERATURE (°C)
100
125
4213 G17
0
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
6.0
4213 G18
4213f
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LTC4213
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TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25°C. VCC = 5V
unless otherwise noted.
tRESET vs Temperature
tFAULT(SLOW) vs VCC
100
tFAULT(SLOW) vs Temperature
22
22
20
20
80
tFAULT(SLOW) (µs)
tFAULT(SLOW) (µs)
tRESET (µs)
90
18
16
14
18
16
14
70
12
60
–50
–25
75
0
50
25
TEMPERATURE (°C)
100
125
12
10
2.0
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
4213 G19
75
0
50
25
TEMPERATURE (°C)
–25
100
125
4213 G21
tFAULT(FAST) vs Temperature
1.3
1.3
1.2
1.2
1.1
1.1
tFAULT(FAST) (µs)
tFAULT(FAST) (µs)
10
–50
4213 G20
tFAULT(FAST) vs VCC
1.0
0.9
0.8
0.7
2.0
6.0
1.0
0.9
0.8
2.5
3.0 3.5 4.0 4.5 5.0
BIAS SUPPLY VOLTAGE (V)
5.5
6.0
4213 G22
0.7
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
4213 G23
4213f
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LTC4213
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PI FU CTIO S
READY (Pin 1): READY Status Output. Open drain output
that goes high impedance when the external MOSFET is on
and the circuit breaker is armed. Otherwise this pin pulls
low.
ON (Pin 2): ON Control Input. The LTC4213 is in reset
mode when the ON pin is below 0.4V. When the ON pin
increases above 0.8V, the device starts up and the GATE
pulls up with a 100µA current source. When the ON pin
drops below 0.76V, the GATE pulls down. To reset a circuit
breaker fault, the ON pin must go below 0.4V.
ISEL (Pin 3): Threshold Select Input. With the ISEL pin
grounded, float or tied to VCC the VCB is set to 25mV, 50mV
or 100mV, respectively. The corresponding VCB(FAST)
values are 100mV, 175mV and 325mV.
GND (Pin 4): Device Ground.
GATE (Pin 5): GATE Drive Output. An internal charge
pump supplies 100µA pull-up current to the gate of the
external N-channel MOSFET. Internal circuitry limits the
voltage between the GATE and SENSEN pins to a safe gate
drive voltage of less than 8V. When the circuit breaker
trips, the GATE pin abruptly pulls to GND.
SENSEN (Pin 6): Circuit Breaker Negative Sense Input.
Connect this pin to the source of the external MOSFET.
During reset or fault mode, the SENSEN pin discharges the
output to ground with 280µA.
SENSEP (Pin 7): Circuit Breaker Positive Sense Input.
Connect this pin to the drain of external N-channel MOSFET.
The circuit breaker trips when the voltage across SENSEP
and SENSEN exceeds VCB. The input common mode range
of the circuit breaker is from ground to VCC + 0.2V when
VCC < 2.5V. For VCC ≥ 2.5V, the input common mode range
is from ground to VCC + 0.4V.
VCC (Pin 8): Bias Supply Voltage Input. Normal operation
is between 2.3V and 6V. An internal under-voltage lockout
circuit disables the device when VCC < 2.07V.
Exposed Pad (Pin 9): Exposed pad may be left open or
connected to device ground.
4213f
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LTC4213
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BLOCK DIAGRA
+
–
VCB(FAST)
VCB
+
–
SLOWCOMP
8
+
–
+
–
+
VCB
VCB(FAST)
100mV 325mV
50mV
175mV
25mV
100mV
SENSEN
–
3
6
+
ISEL
7
FASTCOMP
VCC
16µs
DELAY
1µs
DELAY
CB TRIPS
1
CB TRIPS
VCC
OVCOMP
CHARGE
PUMP
280µA
READY
VCC
0.7V
–
SENSEP
RESET OR
FAULT MODE
65µs
DELAY
BLANK
100µA
OV TRIPS
GATE ON
GATE
5
LOGIC
50µs
DELAY
6.5V
CLAMP
CIRCUIT
+
ARM
RESET
STARTUP
NORMAL MODE
GATE ON/OFF
ARM
COMP
–
+–
VGSARM
80µs
DELAY
–
COMP1
0.4V
+
60µs
DELAY
UV COMP
+
–
VCC
2.07V
2
ON
8µs 5µs
DELAY
+
COMP2
GATEOFF
SENSEN
–
0.8V
4
GND
4213 BD
4213f
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LTC4213
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TI I G DIAGRA
1
2
3
4
5
VON(TH)
6
VON(TH)
VON(TH) – VON(HYST)
VON
VGSMAX
VGSMAX – 0.3V
VGATE
0.3V
0.3V
tDEBOUNCE
1
tOFF
2
3
tON
4
5
1.2V
∆VSENSE
VGATE
VGSMAX
VGSMAX – 0.3V
0.3V
VON
VON(TH)
VON(RST)
4213 TD
tFAULT(FAST)
tRESET
4213f
9
LTC4213
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OPERATIO
Overview
The LTC4213 is an Electronic Circuit Breaker (ECB) that
senses load current with the the RDSON of the external
MOSFET instead of using an external sense resistor. This
no RSENSE method is less precise than RSENSE method due
to the variation of RDSON. However, the advantages are
less complex, lower cost and reduce voltage and power
loss in the switch path owing to the absence of a sense
resistor. Without the external sense resistor voltage drop,
the VOUT improvement can be quite significant especially
in the low voltage applications. The LTC4213 is designed
to operate over a bias supply range from 2.3V to 6V. When
bias supply voltage and the ON pin are sufficiently high, the
GATE pin starts charging after an internal debounce delay
of 60µs. During the GATE ramp-up, the circuit breaker is
not armed until the external MOSFET is fully turned on.
Once the circuit breaker is armed, the LTC4213 monitors
the load current through the RDSON of the external MOSFET.
Circuit Breaker Function
The LTC4213 provides dual level and dual response time
circuit breaker functions for overcurrent protection.
The LTC4213 circuit breaker function block consists of
two comparators, SLOWCOMP and FASTCOMP. The
thresholds of SLOWCOMP and FASTCOMP are VCB and
VCB(FAST). The ISEL pin selects one of the three settings:
1. VCB = 25mV and VCB(FAST) = 100mV with ISEL at GND
2. VCB = 50mV and VCB(FAST) = 175mV with ISEL floating
3. VCB = 100mV and VCB(FAST) = 325mV with ISEL at VCC
ISEL can be stepped dynamically, such as to allow a higher
circuit breaker threshold at startup and a lower threshold
after supply current has settled. The inputs of the comparators are SENSEP and SENSEN pins. The voltage
across the drain and source of the external MOSFET is
sensed at SENSEP and SENSEN.
∆VSENSE = VSENSEP − VSENSEN
(1)
When ∆VSENSE exceeds the VCB threshold but is less than
VCB(FAST), the comparator SLOWCOMP trips the circuit
breaker after a 16µs delay. If ∆VSENSE is greater than
VCB(FAST), the comparator FASTCOMP trips the circuit
breaker in 1µs.
A severe short circuit condition can cause the load supply
to dip substantially. This does not pose a problem for the
LTC4213 as the input stages of the current limit comparators are common mode to ground.
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APPLICATIO S I FOR ATIO
Figure 1 shows an electronic circuit breaker (ECB) application. An external auxiliary supply biases the VCC pin and
the internal circuitry. A VIN load supply powers the load via
an external MOSFET. The SENSEP and SENSEN pins
Q1
SI4864DY
VIN
1.25V
CIN
100µF
VBIAS
2.5V
OFF ON
+
CLOAD
100µF
VCC SENSEP GATE SENSEN
C1
0.1µF
LTC4213
ON
GND
ISEL
VOUT
1.25V
3.5A
+
VCC
R4
10k
READY
4213 F01
sense the load current at the drain and source of the
external MOSFET. In ECB applications, large input bypass
capacitors are usually recommended for good transient
performance.
Undervoltage Lockout
An internal undervoltage lockout (UVLO) circuit resets the
LTC4213 if the VCC supply is too low for normal operation.
The UVLO comparator (UVCOMP) has a low-to-high threshold of 2.07V and 100mV of hysteresis. UVLO shares the
glitch filters for both low-to-high transition (startup) and
high-to-low transition (reset) with the ON pin comparators. Above 2.07V bias supply voltage, the LTC4213 starts
if the ON pin conditions are met. Short, shallow bus bias
Figure 1. LTC4213 Electronic Circuit Breaker Application
4213f
10
LTC4213
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APPLICATIO S I FOR ATIO
supply transient dips below 1.97V of less than 80µs are
ignored.
ON Function
When VON is below comparator COMP1’s threshold of
0.4V for 80µs, the device resets. The system leaves reset
mode if the ON pin rises above comparator COMP2’s
threshold of 0.8V and the UVLO condition is met. Leaving
reset mode, the GATE pin starts up after a tDEBOUNCE delay
of 60µs. When ON goes below 0.76V, the GATE shuts off
after a 5µs glitch filter delay. The output is discharged by
the external load when VON is in between 0.4V to 0.8V. At
this state, the ON pin can re-enable the GATE if VON
exceeds 0.8V for more than 8µs. Alternatively, the device
resets if the ON pin is brought below 0.4V for 80µs. Once
reset, the GATE pin restarts only after the tDEBOUNCE 60µs
delay at VON rising above 0.8V. To protect the ON pin from
overvoltage stress due to supply transients, a series
resistor of greater than 10k is recommended when the ON
pin is connected directly to the supply. An external resistive divider at the ON pin can be used with COMP2 to set
a supply undervoltage lockout value higher than the internal UVLO circuit. An RC filter can be implemented at the
ON pin to increase the powerup delay time beyond the
internal 60µs delay.
Gate Function
The GATE pin is held low in reset mode. 60µs after leaving
reset mode, the GATE pin is charged up by an internal
100µA current source. The circuit breaker arms when
VGATE > VSENSEN + ∆VGSARM. In normal mode operation,
the GATE peak voltage is internally clamped to ∆VGSMAX
above the SENSEN pin. When the circuit breaker trips, an
internal MOSFET shorts the GATE pin to GND, turning off
the external MOSFET.
READY Status
The READY pin is held low during reset and at startup. It
is pulled high by an external pullup resistor 50µs after the
circuit breaker arms. The READY pin pulls low if the circuit
breaker trips or the ON pin is pulled below 0.76V, or VCC
drops below undervoltage lockout.
∆VGSARM and VGSMAX
Each MOSFET has a recommended VGS drive voltage
where the channel is deemed fully enhanced and RDSON is
minimized. Driving beyond this recommended VGS voltage yields a marginal decrease in RDSON. At startup, the
gate voltage starts at ground potential. The GATE ramps
past the MOSFET threshold and the load current begins to
flow. When VGS exceeds ∆VGSARM, the circuit breaker is
armed and enabled. The chosen MOSFET should have a
recommended minimum VGS drive level that is lower than
∆VGSARM. Finally, VGS reaches a maximum at ∆VGSMAX.
Trip and Reset Circuit Breaker
Figure 2 shows the timing diagram of VGATE and VREADY
after a fault condition. A tripped circuit breaker can be reset
either by cycling the VCC bias supply below UVLO threshold or pulling ON below 0.4V for >tRESET. Figure 3 shows
the timing diagram for a tripped circuit breaker being reset
by the ON pin.
Calculating Current Limit
The fault current limit is determined by the RDSON of the
MOSFET and the circuit breaker voltage VCB.
ILIMIT =
VCB
RDSON
(2)
The RDSON value depends on the manufacturer’s distribution, VGS and junction temperature. Short Kelvin-sense
connections between the MOSFET drain and source to
the LTC4213 SENSEP and SENSEN pins are strongly
recommended.
For a selected MOSFET, the nominal load limit current is
given by:
ILIMIT (NOM) =
VCB(NOM)
RDSON(NOM)
(3)
The minimum load limit current is given by:
ILIMIT (MIN) =
VCB(MIN)
RDSON(MAX)
(4)
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The maximum load limit current is given by:
ILIMIT (MAX) =
Example Current Limit Calculation
VCB(MAX)
RDSON(MIN)
(5)
Most MOSFET data sheets have an RDSON specification
with typical and maximum values but no minimum value.
Assuming a normal distribution with typical as mean, the
minimum value can be estimated as
RDSON(MIN) = 2 • RDSON(NOM) − RDSON(MAX)
(6)
The LTC4213 gives higher gate drive than the manufacturer specified gate drive for RDSON. This gives a slightly
lower RDSON than specified. Operating temperature also
modulates the RDSON value.
An Si4410DY is used for current detection in a 5V supply
system with the LTC4213 VCB at 25mV (ISEL pin grounded).
The RDSON distribution for the Si4410DY is
Typical RDSON = 0.015Ω = 100%
Maximum RDSON = 0.02Ω = 133.3%
Estimated MIN RDSON = 2 • 15 – 20 = 0.010Ω = 66.7%
The RDSON variation due to gate drive is
RDSON @ 4.5VGS = 0.015Ω = 100% (spec. TYP)
RDSON @ 4.8VGS = 0.014Ω = 93% (MIN ∆VGSMAX)
RDSON @ 7VGS = 0.0123Ω = 82% (NOM ∆VGSMAX)
RDSON @ 8VGS = 0.012Ω = 80% (MAX ∆VGSMAX)
CIRCUIT BREAKER TRIPS
GATE AND READY PINS PULL LOW
SHORT CIRCUIT
A
B
>VCB
VCB
∆VSENSE
CB TRIPS
VGATE
VREADY
tFAULT
4213 F02
Figure 2. Short Circuit Fault Timing Diagram
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CIRCUIT BREAKER TRIPS
GATE AND READY PINS PULL LOW
SHORT CIRCUIT
NOT RESET
RESET REINITIALIZE
RESTART
VCC > 2.07V
1
2
3
4
5
67
8
VON
0.8V
0.76V
0.4V
0V
>VCB
VCB
∆VSENSE
tFAULT
CB TRIPS
VGATE
VREADY
VON < 0.4V
DURATION > tRESET
NORMAL MODE
FAULT LATCHED OFF
tDEBOUNCE
STARTUP CYCLE
4213 F03
Figure 3. Resetting Fault Timing Diagram
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Table 1. Nominal Operating ∆VGSMAX for Typical Bias
Supply Voltage
Operating temperature of 0° to 70°C.
RDSON @ 25°C = 100%
VCC (V)
∆VGSMAX (V)
RDSON @ 0°C = 90%
2.3
4.3
RDSON @ 70°C = 120%
2.5
5.0
2.7
5.6
3.0
6.5
3.3
7.0
5.0
7.0
6.0
7.0
MOSFET resistance variation:
RDSON(NOM) = 15m • 0.82 = 12.3mΩ
RDSON(MAX) = 15m • 1.333 • 0.93 • 1.2 = 15m • 1.488
= 22.3mΩ
RDSON(MIN) = 15m • 0.667 • 0.80 • 0.90 = 15m • 0.480
= 7.2mΩ
VCB variation:
NOM VCB = 25mV = 100%
MIN VCB = 22.5mV = 90%
MAX VCB = 27.5mV = 110%
The current limits are:
ILIMIT(NOM) = 25mV/12.3mΩ = 2.03A
ILIMIT(MIN) = 22.5mV/22.3mΩ = 1.01A
ILIMIT(MAX) = 27.5mV/7.2mΩ = 3.82A
For proper operation, the minimum current limit must
exceed the circuit maximum operating load current with
margin. So this system is suitable for operating load
current up to 1A. From this calculation, we can start with
the general rule for MOSFET RDSON by assuming maximum operating load current is roughly half of the
ILIMIT(NOM). Equation 7 shows the rule of thumb.
IOPMAX =
VCB(NOM)
2 • RDSON(NOM)
(7)
Note that the RDSON(NOM) is at the LTC4213 nominal
operating ∆VGSMAX rather than at typical vendor spec.
Table 1 gives the nominal operating ∆VGSMAX at the
various operating VCC. From this table users can refer to
the MOSFET’s data sheet to obtain the RDSON(NOM) value.
Load Supply Power-Up after Circuit Breaker Armed
Figure 4 shows a normal power-up sequence for the
circuit in Figure 1 where the VIN load supply power-up after
circuit breaker is armed. VCC is first powered up by an
auxiliary bias supply. VCC rises above 2.07V at time
point 1. VON exceeds 0.8V at time point 2. After a 60µs
debounce delay, the GATE pin starts ramping up at time
point 3. The external MOSFET starts conducting at time
point 4. At time point 5, VGATE exceed ∆VGSARM and the
circuit breaker is armed. After 50µs (tREADY delay), READY
pulls high by an external resistor at time point 6. READY
signals the VIN load supply module to start its ramp. The
load supply begins soft-start ramp at time point 7. The load
supply ramp rate must be slow to prevent circuit breaker
tripping as in equation (8).
∆VIN IOPMAX − ILOAD
<
∆t
C LOAD
(8)
Where IOPMAX is the maximum operating current defined
by equation 7.
For illustration, VCB = 25mV and RDSON = 3.5mΩ at the
nominal operating ∆VGSMAX. The maximum operating
current is 3.5A (refer to equation 7). Assuming the load
can draw a current of 2A at power-up, there is a margin of
1.5A available for CLOAD of 100µF and VIN ramp rate should
be VCB.
At time point 7, the GATE voltage peaks. 50µs after time
point 6, READY goes HIGH.
Startup Problems
There is no current limit monitoring during output charging for the figure 5 power-up sequence where the load
supply is powered up before VCC. This is because the GATE
voltage is below ∆VGSARM and the MOSFET may not reach
the specified RDSON. The VIN load supply should have
sufficient capability to handle the inrush as the output
charges up. For proper startup, the final load at time
CIRCUIT BREAKER ARMS
VSENSEP – VSENSEN = VCB
VGATE MAXES OUT
READY SIGNALS
0
1
2
3
4
5
6
7
8
VCC > 2.07V
VON > 0.8V
VCC, VON
∆VGSMAX + VSENSEN
∆VGSARM + VSENSEN
Vth
VGATE
VSENSEP
VSENSEN
VREADY
tREADY
RESET MODE
tDEBOUNCE
STARTUP CYCLE
NORMAL CYCLE
4213 F05
Figure 5. Load Supply Power-Up Before VCC
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point 6 should be within the circuit breaker limits. Otherwise, the system fails to start and the circuit breaker trips
immediately after arming. In most applications additional
external gate capacitance is not required unless CLOAD is
large and startup becomes problematic. If an external gate
capacitor is employed, its capacitance value should not be
excessive unless it is used with a series resistor. This is
because a big gate capacitor without resistor slows down
the GATE turn off during a fault. An alternative method
would be a stepped ISEL pin to allow a higher current limit
during startup.
In the event of output short circuit or a severe overload, the
load supply can collapse during GATE ramp up due to load
supply current limit. The chosen MOSFET must withstand
this possible brief short circuit condition before time
point 6 where the circuit breaker is allowed to trip. Bench
short circuit evaluation is a practical verification of a
reliable design. To have current limit while powering a
MOSFET into short circuit conditions, it is preferred that
the load supply sequences to turn on after the circuit
breaker is armed as described in an earlier section.
Power-Off Cycle
The system can be powered off by toggling the ON pin low.
When ON is brought below 0.76V for 5µs, the GATE and
READY pins are pulled low. The system resets when ON is
brought below 0.4V for 80µs.
MOSFET Selection
The LTC4213 is designed to be used with logic (5V) and
sub-logic (3V) MOSFETs for VCC potentials above 2.97V
with ∆VGSMAX exceeding 4.5V. For a VCC supply range
between 2.3V and 2.97V, sub-logic MOSFETs should be
used as the minimum ∆VGSMAX is less than 4.5V.
The selected MOSFET VGS absolute maximum rating should
meet the LTC4213 maximum ∆VGSMAX of 8V.
Other MOSFET criteria such as VBDSS, IDMAX, and RDSON
should be reviewed. Spikes and ringing above maximum
operating voltage should be considered when choosing
VBDSS. IDMAX should be greater than the current limit. The
maximum operating load current is determined by the
RDSON value. See the section on “Calculating Current
Limit” for details.
Supply Requirements
The LTC4213 can be powered from a single supply or dual
supply system. The load supply is connected to the
SENSEP pin and the drain of the external MOSFET. In the
single supply case, the VCC pin is connected to the load
supply, preferably with an RC filter. With dual supplies,
VCC is connected to an auxiliary bias supply VAUX where
VAUX voltage should be greater or equal to the load supply
voltage. The load supply voltage must be capable of
sourcing more current than the circuit breaker limit. If the
load supply current limit is below the circuit breaker trip
current, the LTC4213 may not react when the output
overloads. Furthermore, output overloads may trigger
UVLO if the load supply has foldback current limit in a
single supply system.
VIN Transient and Overvoltage Protection
Input transient spikes are commonly observed whenever
the LTC4213 responds to overload. These spikes can be
large in amplitude, especially given that large decoupling
capacitors are absent in hot swap environments. These
short spikes can be clipped with a transient suppressor of
adequate voltage and power rating. In addition, the LTC4213
can detect a prolonged overvoltage condition. When
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SENSEP exceeds VCC + 0.7V for more than 65µs, the
LTC4213’s internal overvoltage protection circuit activates and the GATE pin pulls down and turns off the
external MOSFET.
Typical Single Supply Hot Swap™ Application
A typical single supply Hot Swap application is shown in
Figure 7. The RESET signal at the backplane is held low
initially. When the PCB long edge makes contact the ON
pin is held low (
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