NIS5020, NIS5021, NIS5820
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10
APPLICATIONS INFORMATION
Paralleling eFuses
If the output current capability required by an application
is higher than the current which can be carried by a single
eFuse, it is possible to parallel eFuses to achieve a higher
current throughput. Up to four eFuses can be paralleled to
achieve a higher current. All of the eFuses will have a
common thermal shutdown. Refer to Figure 5 for the
schematic connection of parallel eFuses. The VCC pins of
every eFuse must be shorted together. The Source pins of
each eFuse must be shorted together. Each eFuse should be
configured either in Kelvin or Direct mode and have its
individual current limiting resistor Rlim connected between
ILIMIT and Source pins. The Enable pins of all the eFuses
must be shorted together for common shutdown
functionality and connected to an open−drain or open
collector device in case it is desired to turn off all the eFuses
at the same time. The dv/dt pins of eFuses must NOT be
shorted together; they should be either left floating for a
standard output ramp−up time or have individual dvdt
capacitor to ground.
Every eFuse will carry equal amount of current during
normal operation and overcurrent events. If any of the
eFuses goes to thermal shutdown first, it will pull down the
Enable pin and make the other eFuses to shut down as well.
Basic Operation
This device is a self−protected, resettable, electronic fuse.
It contains circuits to monitor the input voltage, output
voltage, output current and die temperature.
On application of the input voltage, the device will apply
the input voltage to the load based on the restrictions of the
controlling circuits. The output voltage, which is controlled
by an internal dV/dt circuit, will slew from 0 V to the rated
output voltage in 1 ms. The device will remain on as long as
the temperature does not exceed the 175°C limit that is
programmed into the chip.
The internal current limit circuit does not shut down the
part but will reduce the conductivity of the FET to maintain
a constant current at the internally set current limit level. The
input overvoltage clamp also does not shutdown the part, but
will limit the output voltage in the event that the input
exceeds the Vclamp level.
An internal charge pump provides bias for the gate voltage
of the internal n−channel power FET and also for the current
limit circuit. The remainder of the control circuitry operates
between the input voltage (VCC) and ground.
Current Limit
The current limit circuit uses a SENSEFET along with a
reference and amplifier to control the peak current in the
device. The SENSEFET allows for a small fraction of the
load current to be measured, which has the advantage of
reducing the losses in the sense resistor.
The current limit circuit has two limiting values, one for
short circuit events which are defined as the mode of
operation in which the gate is high and the FET is fully
enhanced. The overload mode of operation occurs when the
device is actively limiting the current and the gate is at an
intermediate level. For a more detailed description of this
circuit please refer to application note AND9441.
Connection of RLIMIT current limit setting resistor can be
made as shown in Figure 2 (Kelvin connection), or Figure 4
(Direct connection). Both connections result in a similar
current limit thresholds and behavior. It is important to make
sure that layout trace connecting RLIMIT resistor to pins 4
and 6 is as short as possible. The shortest possible distance
on a PCB must be used to connect pin 6 to RLIM resistor
before pin 6 is connected to a common load node.
Overvoltage Clamp
The overvoltage clamp consists of an amplifier and
reference. It monitors the output voltage and if the input
voltage exceeds 14 V, the gate drive of the main FET is
reduced to limit the output. This is intended to allow
operation through transients while protecting the load. If an
overvoltage condition exists for many seconds, the device
may overheat due to the voltage drop across the FET
combined with the load current. In this event, the thermal
protection circuit would shut down the device. Refer to
Figure 12 for typical overvoltage clamp behavior
Undervoltage Lockout
The undervoltage lockout circuit uses a comparator with
hysteresis to monitor the input voltage. If the input voltage
drops below the specified level, the output switch will be
switched to a high impedance state.
Slew Rate Control
The dV/dt circuit brings the output voltage up under a
linear, controlled rate regardless of the load impedance
characteristics. An internal ramp generator creates a linear
ramp, and a control circuit forces the output voltage to
follow that ramp, scaled by a factor.
The default ramp time is approximately 1 ms. This pin
includes an internal current source of approximately 1 mA.
Since the current level is very low, it is important to use a
ceramic cap or other low leakage capacitor. Aluminum
electrolytic capacitors are not recommended for this circuit.
The ramp time from 10% to 90% of the nominal output
voltage can be determined by the following equation:
Cext +ǒt
0.5E06Ǔ*1.4 nF
Where: C is in Farads,
t is in Seconds
Anytime that the unit shuts down due to a fault, enable shut−
down, or recycling of input power, the timing capacitor will be
discharged and the output voltage will ramp from 0 at turn on.
Refer to Figures 9 and 11 for slew rate control and typical Slew
Rate behavior.