DEMO MANUAL DC1935A
LT4363-1/LT4363-2
High Voltage Surge Stopper
with Current Limit
DESCRIPTION
Demonstration Circuit 1935A showcases the LT
®
4363 surge
stopper in a 12V, 2A application. Inputs of up to 80VDC
and 1ms transients of up to 250V are limited to 16.3V
at the output. The MOSFET is protected against output
overloads by current limiting. Sustained overvoltage or
overcurrent conditions cause the LT4363 to turn off after a
timer delay. The LT4363-1 latches off and is reset by pulling
SHDN
low for at least 100μs. The LT4363-2 automatically
retries after a cool down delay; retry is inhibited if OV is
higher than 1.275V.
By removing 0Ω jumpers R1C and R4C, the operation may
be modified for 28V systems with an output regulation
point of 34.6V in overvoltage conditions.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
PERFORMANCE SUMMARY
PARAMETER
Input Supply
Operating
DC Survival
T
A
= 25°C, R1C = R4C = 0Ω (As Shipped)
MIN
5.7
80
250
15.7
5.2
14.7
2.0
16.3
5.6
15.5
2.5
TYP
28
17.0
5.7
16.2
3.1
MAX
31.6
TYP
12
MAX
14.7
UNITS
V
V
V
V
V
V
A
UNITS
V
V
V
34.6
11.9
33.3
2.6
35.9
12.4
34.9
3.1
V
V
V
A
CONDITIONS
1ms Transient Survival
Output Regulation Voltage
Undervoltage Threshold
Retry Inhibit Threshold
Current Limit
T
A
= 25°C, R1C = R4C = Open (Optional Field Change)
PARAMETER
Input Supply
CONDITIONS
Operating
DC Survival
1ms Transient Survival
Output Regulation Voltage
Undervoltage Threshold
Retry Inhibit Threshold
Current Limit
MIN
12.4
80
250
33.2
11.2
31.6
2.0
dc1935af
1
DEMO MANUAL DC1935A
QUICK START PROCEDURE
Danger!
High voltage testing should be performed by
qualified personnel only. As a safety precaution at least
two people should be present during high voltage testing.
BOARD LAYOUT
DC1935A is designed to withstand 250V peak input voltage
for up to 1ms. This high voltage is stood off by CSNAB,
R4A, R7, R10, Q1, Q2 and Q3. Maximum input voltage is
limited by Q1’s 250V BV
DSS
rating. The permissible time at
250V is limited by R7, which dissipates slightly less than
40W and is capable of doing so for at least 1ms.
The minimum spacing is limited by 1206 pad spacing
where the gap between solder pads is 2mm, or just under
80mils. Thus, the spacing between the input plane and all
other board traces is maintained at a minimum of 2mm.
As a point of reference, a 2mm needle gap in air breaks
down well above 1kV.
250V CAPABILITY
The LT4363 is rated to survive up to 100V. If input spikes
did not exceed 100V, the V
CC
pin could be connected directly
to the input. Nevertheless, in some systems input voltage
spikes may reach levels much higher than 100V. Opera-
tion under these conditions is made possible by limiting
the voltage at the V
CC
pin with an external filter or clamp.
The simplest clamp is a Zener diode or transient voltage
suppressor (D1) and a 1kΩ series limiting resistance (R7).
This arrangement is adequate for millisecond excursions
of up to 250V. The voltage is limited by the breakdown
of Q1 and the peak voltage rating of R7, and the time is
limited by the pulse power capability of R7, D1, and Q1.
The OV and UV pins are limited to 100V absolute maximum.
Practical divider ratios attenuate peak inputs of 250V to
safe levels at the OV and UV pins. For the resistors used in
DC1935A, the peak voltage at the UV pin is less than 60V
when the input spikes to 250V. If there is an input voltage/
divider ratio combination that leads to greater than 100V at
OV or UV, the top of the divider may be moved to the V
CC
clamp for protection, with a small penalty in accuracy, or
a Zener clamp may be added to the affected pin.
While the input of the DC1935A can handle 250V, several
points should be brought to mind. First, the dissipation
of several components rises to significant levels when
250V is applied. R4 dissipation rises to near 500mW, D1
dissipates over 10W and R7 dissipates nearly 40W. These
components dissipate high, destructive power even though
the LT4363 regulates and then shuts down the output,
saving the load and Q1 from destruction. The maximum
safe DC input is 80V, limited by the dissipation of R7 and
D1. Always remember that 250V should be applied only
as a spike or transient, not exceeding 1ms duration.
Danger!
High voltage testing should be performed by
qualified personnel only. As a safety precaution at least
two people should be present during high voltage testing.
DC1935A-A AND DC1935A–B STUFFING OPTIONS
The DC1935A-A is fitted with the LT4363-1 which latches
off after a timer delay in the presence of overvoltage or
overcurrent conditions. Once latched off the LT4363-1 ver-
sion’s timer executes a cool-down cycle, after which the
output may be restarted by pulsing the
SHDN
pin low for
at least 100μs, or by briefly disconnecting the input supply.
The DC1935A-B is fitted with the LT4363-2 which automati-
cally retries after its cool-down cycle. Retry is inhibited by
the OV pin, if the input is greater than 15.5V.
OPERATION
The shutdown pin,
SHDN,
is floating so that when power
is applied to the input, the LT4363 automatically turns on.
The LT4363 protects the load from destruction by regulating
the output voltage to a safe level during intervals of input
overvoltage. DC1935A is designed to regulate the output
at 16.3V. If the input voltage is less than 16.3V, power
passes through directly to the output. The output voltage
is sensed by the R1/R2 divider and the FB pin. The GATE
pin controls Q1 to regulate the output voltage in the event
the input rises above 16.3V.
Overcurrent is sensed by R
SNS
and the SNS and OUT pins
of the LT4363. If the load current reaches 50mV/20mΩ,
the GATE pin will control Q1 to regulate the output cur-
rent at 2.5A.
dc1935af
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DEMO MANUAL DC1935A
QUICK START PROCEDURE
In both overcurrent and overvoltage conditions, current is
sourced by the TMR pin into the timer capacitor, CTMR. It
charges and upon reaching 1.375V, causes the LT4363 to
turn off the MOSFET. As previously mentioned the LT4363-1
version latches off, while the LT4363-2 version automati-
cally tries to restart the load after a cool-down interval.
The timer interval before the MOSFET turns off is variable,
depending on the type and severity of the fault, and ranges
from 150μs to 10ms. The cool-down time is 158ms.
For the LT4363-2 version, automatic retry is inhibited by
the OV pin if the input remains higher than 15.5V, as set
by the R4-R5-R6 divider. For the LT4363-1 version, the OV
pin becomes a ground pin (GND, Pin 7) and it is shorted
to ground by a 0Ω jumper at R6. The LT4363-1 version
simply latches off in response to a fault. Restart by pulling
SHDN
low for at least 100μs, or by briefly disconnecting
the input supply.
The combined tolerances of the LT4363 and external re-
sistor dividers are approximately 4% for output voltage
regulation (FB pin), 5% for undervoltage (UV pin) and
retry inhibit threshold (OV pin), and 15% for current limit.
A performance summary is shown in a table on the
schematic diagram and is silk-screened on the back of
the demo board for easy reference.
BOARD MODIFICATIONS
R1-2 and R4-6 may be changed to customize the UV and
OV thresholds, as well as the output regulation voltage.
If additional
SHDN
pin pull-up is desired, R9 may be
populated. The board layout is designed for four other
modifications.
Modification 1: 28V Operation
As shipped, DC1935A is designed for 12V operation. It
may be modified for 28V operation by removing the 0Ω
jumpers R1C and R4C. The resulting voltage ranges and
thresholds are shown in a table on the schematic diagram
and on the silkscreen.
Modification 2: Higher DC Input Capability
Provision is made for buffering the D1 shunt clamp, permit-
ting the use of a much higher value for R7, which in turn
reduces its dissipation and allows for longer overvoltage
times or higher DC inputs. To modify, remove 0Ω jumper
R8, install a PZTA42 for Q2 and increase R7 to 150kΩ.
This arrangement can tolerate 250VDC, limited by board
spacing considerations, dissipation in R7 (a 1206 resistor
can dissipate 250mW), Q3’s dissipation (about 750mW
at 250V) and Q1’s 250V BV
DSS
rating.
Bear in mind that the circuit board’s 2mm spacing is
adequate for only 100V long-term continuous bias. Also
note that the V
BE
of Q2 adds to the LT4363’s inherent 4V
minimum operating voltage, as does the drop in R7.
Modification 3: Higher Current
For higher current applications, DC1935A may be grafted
onto a separate assembly comprising a MOSFET, sense
resistor and output capacitor (see Figure 1). Through-
holes are included to facilitate this modification. Here is
the procedure:
1. Remove Q1, R
SNS
and C
L
.
2. Connect short lengths of hook-up wire between
DC1935A and the MOSFET assembly. It is imperative
that a 10Ω resistor be installed at the MOSFET gate
lead to replicate the function of R3.
3. Connect the input supply to the MOSFET drain, and
connect the load to the output side of the sense resistor.
4. If parasitic oscillations are observed at start-up, or
during voltage or current limiting, add a 100Ω/10nF
snubber to the drain of the MOSFET.
+
Q1
INPUT
SUPPLY
R
SNUB
*
100Ω
C
SNUB
*
10nF
TP1
R
SNS
C
L
≥22μF
10Ω
LOAD
TP4
TP2
TP3
DC1935A
Q1, R
SNS
, C
L
REMOVED TP5
DC1935A F01
*MAY BE NECESSARY TO SUPPRESS
PARASITIC OSCILLATIONS IN MOSFET
Figure 1. For High Current Applications Assemble Power Path
Off-Board and Attach DC1935A Using Through-Hole Test Points
dc1935af
3
DEMO MANUAL DC1935A
QUICK START PROCEDURE
Modification 4: Higher Operating Voltage
Figure 2 shows a combination of modifications 2 and 3 for
high voltage operation. The power path and the V
CC
pre-
regulator are moved off-board to eliminate spacing issues.
+
Q1
INPUT
SUPPLY
R7
Q2
R
SNUB
*
100Ω
C
SNUB
*
10nF
R
SNS
C
L
≥22μF LOAD
10Ω
FLT
and ENOUT are open-collector outputs. If used, pull
up to the output or an output-derived external logic sup-
ply. There are no pull-ups included on DC1935A.
FLT
and
ENOUT have 100V ratings and can sink 100μA to less
than 800mV.
TMR is brought out to a turret for the purpose of monitor-
ing the waveform there. It may also be used to connect
external timer capacitors.
QUICK START
Connect a 12V supply to input, and connect a load to
output as shown in Figure 3. The circuit will turn on au-
tomatically when power is applied, and green LED D3 will
show that the output is up. LED D4 indicates that input
power is present. If the input voltage is increased above
16.3V, the output will shut off and D3 will extinguish. The
output will remain off until the input is brought below
15.5V. Similarly, if the input voltage is decreased to less
than 5.6V the output will shut off and D3 will extinguish.
It will restart when the input rises above 5.7V.
TP6 TP7 TP1
TP2 TP3
DC1935A
R7, Q2, Q1, R
SNS
, C
L
REMOVED
TP4 TP5
DC1935A F02
*MAY BE NECESSARY TO SUPPRESS
PARASITIC OSCILLATIONS IN MOSFET
Figure 2. For Higher Input Voltage Operation, Assemble Power
Path and V
CC
Pre-Regulator (Q2 and R7) Off-Board and Attach
DC1935A Using Through-Hole Test Points
TEST POINTS
A summary of the test points and their functions is shown
in Table 1.
Table 1. Test Point Connections
TEST POINT
1
2
3
4
5
6
7
EXTERNAL
CONNECTION
Drain/Input
Gate
Sense
+
Sense
–
Ground
Base Q2, R7
Emitter Q2
INPUT
OUTPUT
DC1935A
0A TO 2A
LOAD
LT4363 PIN
—
GATE
SNS
OUT
GND
—
V
CC
12V NOMINAL
(0V TO 80V)
≥3A
+
–
D4
GND
D3
GND
DC1935A F03
Figure 3. Basic Test Setup
SMALL TURRETS
No connection to any of the small turrets is necessary to
make the board operate—the LT4363 defaults to the ON
state. If the
SHDN
turret is left open, the board will turn
on when power is applied. Short this turret to ground to
turn off the LT4363.
To test the dynamic behavior, apply a transient to the input
as shown in Figure 4. Use a storage oscilloscope to moni-
tor both the input and the output, and use the NSG5500
trigger output to trigger the oscilloscope sweep.
dc1935af
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DEMO MANUAL DC1935A
QUICK START PROCEDURE
INPUT
SCHAFFNER
NSG 5500
OR
EQUIVALENT
TRIGGER
OUTPUT
OUTPUT
A
DC1935A
0A TO 2A
LOAD
GND
STORAGE
B OSCILLOSCOPE
TRIGGER
INPUT
DC1935A F04
+
–
GND
Figure 4. Testing Dynamic Behavior
1k
10W
0V TO 100V
≥200mA
SWITCH
OR RELAY
NC NO
COM
R
GEN
*
470mΩ
2W
MUR415
INPUT
OUTPUT
+
1k
10W
+
C
GEN
*
6800μF
100V
+
12V
≥3A
GND
DC1935A
0A TO 2A
LOAD
GND
DC1935A F05
*RGA TYPE MPC70 METAL PLATE
CEMENT RESISTOR OR EQUIVALENT
Figure 5. Simple Transient Generator
Another method of developing transients is to discharge a
capacitor into the input of DC1935A, as shown in Figure 5.
Monitor the behavior with a storage oscilloscope. Allow
30 seconds for complete recharging of the 6800μF capaci-
tor. C
GEN
may be varied to achieve any desired transient
duration. Note that when DC1935A turns off, the input
waveform will stall since the high current discharge path is
disconnected. Either a SPDT switch or a relay can be used
to transfer C
GEN
from the charging supply to DC1935A. A
so-called “3-way” light switch is a SPDT configuration and
will survive many switching cycles in this circuit.
HIGH VOLTAGE WARNING
DC1935A is designed to handle up to 250V input. When
testing the circuit behavior at these levels, beware of lethal
voltages produced by transient generators and of charge
stored on capacitors such as C
GEN
in Figure 5. Further,
circuit components may explode or ignite as a result of
unforeseen operating conditions or component failure.
Take all necessary steps to shield and protect personnel
and property from these hazards.
Danger!
High voltage testing should be performed by
qualified personnel only. As a safety precaution at least
two people should be present during high voltage testing.
dc1935af
5
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