NIS5101
http://onsemi.com
11
An overload condition is one in which the FET is fully
enhanced and operating at it’s minimum R
DSon
. A short
circuit condition occurs when either the load has shorted or
upon turn on, as the load capacitor to the hot swap device
initially looks like a short circuit.
A single resistor will determine both the short circuit and
overload current. For example, a 110 W resistor would
result in a 1 A current limit when charging the capacitance
at turn on, but once the FET is fully enhanced, it would
allow the load to operate at a current up to 2.5 A. Once the
2.5 A limit is reached, any further reduction in load
impedance will result in a short circuit condition and the
current will be reduced to 1 amp.
As with all SMART HotPlug devices, the current limit
will never shut down the limiter. Only the thermal limit will
stop the flow of current to the load. Once the current is
stopped due to the thermal limit, it will remain off until
input power is recycled for the latching version, or it will
continuously retry to start again if it is the auto−retry
version.
The I
Limit
graph shown in Figure 2 was generated from
the data of the I
Limit
characterization, the formulas for each
of the curves and temperatures are shown below:
R
ILimit
(W) = (56.55 / y)
1.20
; for T
J
= 25C
R
ILimit
(W) = (52.91 / y)
1.22
; for T
J
= 120°C
R
ILimit
(W) = (44.80 / y)
1.33
; for T
J
= −40°C
where “y” is the desired ILimit value.
Main/Mirror MOSFET Current Ratio. The ratio varies
with current and sense resistance. The key parameter that
it is important to know is that the current sense reference
voltage of the device is 50 mV. Knowing this information,
it is possible to use Figure 2 on the datasheet for the current
limit to calculate the ratio for any condition.
For ”normal” operating condition, the overload curve
would apply. If a 100 W for the I
Limit
resistor is used, the
sense current would be 50 mV/ 100 W at the current limit
level, which results in 500 mA. The drain current is 2.7 A
under this condition, so the ratio is 5400:1.
Same analysis can be made for “short circuit” conditions,
the only difference is that the short circuit curve of
Figure 2 is used to do the ratio calculations instead.
There is a 5 W resistor in series with the sense cells. This
has a tolerance of about 10% and should be taken into
account when making the above calculations.
Turn−on Surge: During the turn−on event, there is a large
amount of energy dissipated due to the linear operation of
the power device. The energy rating is the amount of energy
that the device can absorb before the thermal limit circuit
will shut the unit down. This is very important specially for
the latch off device as it determines the maximum load
capacitance that the device can charge before the thermal
limit shuts the device down. The calculation of this is not
very simple as it depends on several factors such as the
input voltage (V
in
), load capacitance (C
L
), current limit
settings (I
Limit
) and device’s thermal transient response,
therefore, it is recommended to do lab evaluations for these
purposes. Figure 19 shows the device’s thermal transient
response for minimum pad.
0.01
0.1
1
10
100
0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000
Figure 19. Thermal Transient Response
TIME (seconds)
THETA J(t) (°C/W)
