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LTC4216CDE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LTC4216
16
4216fa
For more information www.linear.com/LTC4216
applicaTions inForMaTion
Figure 9. Normal Power-Up Sequence (with Analog Current Limiting)
SENSEP
ON
TIMER
SS
GATE
V
OUT
I
LOAD
FILTER
RESET
V
CC
POWER GOOD
V
FB
> 0.6V
LOAD CURRENT REGULATING
AT 40mV/R
SENSE
POWER BAD
V
FB
< 0.6V
(V
GATE
– V
OUT
) > V
GS(TH)
(V
SENSEP
– V
SENSEN
) > 25mV
FILTER RAMPS UP WHEN (V
SENSEP
– V
SENSEN
) > 25mV OUTPUT IN ANALOG CURRENT LIMIT,
(V
SENSEP
– V
SENSEN
) = 40mV
(V
SENSEP
– V
SENSEN
) < 25mV
TRACKS SS RAMP
IN REGULATION
20µA
60µA
2.4µA
V
TMR(TH)
2µA
V
TMR(TH)
2µA
10µA
10µA
1µA
CHECK FOR GATE,
FILTER, TIMER,
SS < 0.2V
CHECK FOR GATE, FILTER, TIMER, SS < 0.2V AND FAULT HIGH
PLUG-IN CYCLE
FIRST TIMING CYCLE
POWER-GOOD DELAY
SECOND TIMING CYCLE
4216 F09
1 2 3 4 5 6 7 8 9 10 11 12 16151413
0.72V
0.4V
0.8V
ELECTRONIC CIRCUIT BREAKER ARMED
RESET
GOES HIGH
IN RESET
MODE
(ON < 0.4V)
ON GOES LOW
(ON < 0.72V)
OUTPUT NO LONGER
IN CURRENT LIMIT
RESET PULLED LOW
DUE TO POWER BAD
2ND TIMING CYCLE CANNOT START WITH
OUTPUT IN ANALOG CURRENT LIMIT
V
FILT(TH)
LTC4216
17
4216fa
For more information www.linear.com/LTC4216
applicaTions inForMaTion
Power-Up into an Output-Short
Figure 10 shows the timing diagram in the case when the
output is a dead short during power-up. As GATE ramps
up at time point 6, the MOSFET current increases due to
the output short causing the voltage drop across the sense
resistor to rise above 25mV. FILTER sources 60µA, charg
-
ing the external capacitor. At time point 7, GATE regulates
to limit the output current to 40mV/R
SENSE
. If the output
continues to be in analog current limit when the FILTER
pin voltage reaches its threshold (1.253V) at time point
8, the circuit breaker trips and GATE is pulled low. The
device latches-off and FAULT is pulled low, indicating a
fault condition. The FILTER capacitor discharges through
a 2.4µA pull-down until the device resets.
Resetting the Electronic Circuit Breaker
When the LTC4216’s electronic circuit breaker is tripped
during a fault condition, FAULT is asserted low and the
RESET, SS and GATE pins are all pulled to ground. This is
shown in the timing diagram of Figure 11. The LTC4216
remains latched-off until the external fault is cleared. To
clear the internal fault latch and restart the device, pull
the ON
pin low (< 0.4V) at time point 4 for
at least 100µs,
after which the FAULT will go high at time point 5. Tog-
gling the
ON pin from low to high (> 0.8V) initiates a new
start-up cycle.
Sense Resistor Considerations
The
circuit breaker trip threshold of 25mV and the value
of the sense resistor, R
SENSE
, connected between the
SENSEP and SENSEN pins, determine the trip current level
as given by Equation (10). If the fault current level exceeds
the analog current limit, the current is limited to a value
given by Equation (11). Should the overload condition exist
for more than one fault filter delay as given by Equation
(2), the circuit breaker trips and the device is latched-off.
I
V
R
mV
R
TRIP CB
CB TH
SENSE SENSE
()
()
=
∆
=
25
(10)
I
V
R
mV
R
ACL
ACLTH
SENSE SENSE
=
∆
=
()
40
(11)
For a new circuit design, the sense resistor value is first
calculated from the maximum operating load current under
normal conditions and the minimum circuit breaker trip
threshold. This is given by:
R
V
I
mV
I
SENSE
CB TH MIN
LOAD MAX LOAD MAX
=
∆
=
(, )
()
()
.21 5
(12)
Figure 10. Power-Up into an Output-Short and
Circuit Breaker Trips
Figure 11. Mild Overcurrent Circuit Breaker Trips Followed by
Device Reset
TIMER
2.4µA
2µA
40mV
25mV
10µA
10µA
1µA
1 2 3 4 5 6 7 8
4216 F10
0.8V
TRACKS SS RAMP
FPD
60µA
SENSEP-SENSEN
FILTER
SS
GATE
V
OUT
ON
FAULT
RESET
V
FILT(TH)
V
TMR(TH)
V
GATE
– V
OUT
< V
GS(TH)
FAULT
FILTER
DELAY
GATE
REGULATING
2.4µA
<40mV
25mV
4216 F11
FPD
60µA
t
RST(ONLO)
V
FILT(TH)
SENSEP-SENSEN
FILTER
FAULT
RESET
SS
GATE
V
OUT
1
ON
0.4V
POWER BAD
V
FB
< 0.6V
2 3 4 5
MILD
OVERCURRENT
FAULT LATCH
RESET
CIRCUIT BREAKER TRIPS
AND LATCHED-OFF
RESET PULLED LOW
DUE TO POWER BAD
LTC4216
18
4216fa
For more information www.linear.com/LTC4216
applicaTions inForMaTion
For example, if I
LOAD(MAX)
= 5A, R
SENSE
= 4.3mΩ. The
nearest standard value is 4mΩ.
For proper circuit breaker operation, kelvin-sense PCB
connections between the sense resistor and the LTC4216’s
SENSEP and SENSEN pins are strongly recommended.
Figure 12 illustrates the correct way of making connections
between the LTC4216 and the sense resistor. PCB layout
should be balanced and symmetrical to minimize wiring
errors. In addition, the PCB layout for the sense resistor
should include good thermal management techniques for
optimal sense resistor power dissipation.
The power rating of the sense resistor should accom
-
modate the fault current level during analog current limit
so that the component is not damaged before the circuit
breaker trips.
Circuit Breaker Trip Current Calculation
For a selected R
SENSE
value, the typical load current that
trips the circuit breaker is given by:
I
V
R
mV
R
TRIP TYP
CB TH TYP
SENSE TYP SENSE TYP
()
(, )
()
()
=
∆
=
25
(13)
The minimum load current that trips the circuit breaker
is given by:
I
V
R
mV
R
TRIP MIN
CB TH MIN
SENSE MAX SENSE MAX
()
(, )
()
()
.
=
∆
=
21 5
(14)
Figure 12. Making PCB Connections to the Sense Resistor
where
RR
R
SENSE MAX SENSE TYP
TOL
() ()
•
=+
1
100
The maximum load current that trips the circuit breaker
is given by:
I
V
R
mV
R
TRIP MAX
CB TH MAX
SENSE MIN SENSE MIN
()
(, )
()
()
.
=
∆
=
28 5
where (15)
RR
R
SENSE MIN SENSE TYP
TOL
() ()
•–=
1
100
For example, if a sense resistor of 4mΩ ± 1% R
TOL
is used
for current sensing, the typical trip current, I
TRIP(TYP
) =
6.25A. From Equations (14) and (15), I
TRIP(MIN)
= 5.3A
and I
TRIP(MAX)
= 7.2A respectively.
For proper operation and to avoid tripping the circuit breaker
unnecessarily, the minimum trip current, I
TRIP(MIN)
, must
exceed the maximum operating load current of the circuit
connected to the output of the MOSFET.
MOSFET Selection
The external MOSFET switch must have adequate safe
operating area (SOA) to handle short-circuit conditions
before the circuit breaker trips. These considerations
take precedence over continuous drain current ratings. A
MOSFET with adequate SOA for a given application can
always handle the required drain current, but the opposite
may not be true. Consult the manufacturer’s MOSFET
data sheet for safe operating area and effective transient
thermal impedance curves.
MOSFET selection is a 3-step process by assuming the
absence of a soft-start capacitor. First, R
SENSE
is chosen
and then the time required to charge the load capacitance
is determined. This timing, along with the maximum short-
circuit current and maximum load supply voltage, defines
an
operating point
that is checked against the MOSFET’s
SOA curve.
In addition, consider three other key parameters:
TO
SENSEP
W
TO
SENSEN
TRACK WIDTH W:
0.03˝ PER AMPERE
ON 1OZ COPPER
CURRENT FLOW
TO LOAD
SENSE RESISTOR
CURRENT FLOW
TO LOAD
4216 F12
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26

LTC4216CDE#PBF

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Manufacturer:
Analog Devices Inc.
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