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VN21
6/13
Figure 7. Switching Time Waveforms
FUNCTIONAL DESCRIPTION
The device has a diagnostic output which
indicates open load conditions in off state as well
as in on state, output shorted to V
CC
and
overtemperature. The truth table shows input,
diagnostic and output voltage level in normal
operation and in fault conditions. The output
signals are processed by internal logic. The open
load diagnostic output has a 5 ms filtering. The
filter gives a continuous signal for the fault
condition after an initial delay of about 5 ms. This
means that a disconnection during normal
operation, with a duration of less than 5 ms does
not affect the status output. Equally, any re-
connection of less than 5 ms during a
disconnection duration does not affect the status
output. No delay occur for the status to go low in
case of overtemperature conditions. From the
falling edge of the input signal the status output
initially low in fault condition (over temperature or
open load) will go back with a delay (t
povl
) in case
of overtemperature condition and a delay (t
pol
) in
case of open load. These feature fully comply with
International Standard Office (I.S.O.) requirement
for automotive High Side Driver.
To protect the device against short circuit and over
current conditions, the thermal protection turns the
integrated Power MOS off at a minimum junction
temperature of 140 °C. When the temperature
returns to 125 °C the switch is automatically turned
on again. In short circuit the protection reacts with
virtually no delay, the sensor being located in the
region of the die where the heat is generated.
Driving inductive loads, an internal function of the
device ensures the fast demagnetization with a
typical voltage (V
demag
) of -18V.
This function allows to greatly reduce the power
dissipation according to the formula:
P
dem
= 0.5 • L
load
• (I
load
)
2
• [(V
CC
+V
demag
)/
V
demag
] • f
where f = switching frequency and
V
demag
= demagnetization voltage
Based on this formula it is possible to know the
value of inductance and/or current to avoid a
thermal shut-down. The maximum inductance
which causes the chip temperature to reach the
shut down temperature in a specific thermal
environment, is infact a function of the load current
for a fixed V
CC
, V
demag
and f.
PROTECTING THE DEVICE AGAIST LOAD
DUMP - TEST PULSE 5
The device is able to withstand the test pulse No.
5 at level II (V
s
= 46.5V) according to the ISO T/R
7637/1 without any external component. This
means that all functions of the device are
performed as designed after exposure to
disturbance at level II. The VN21 is able to
withstand the test pulse No.5 at level III adding an
external resistor of 150 ohm between pin 1 and
ground plus a filter capacitor of 1000 µF between
pin 3 and ground (if R
LOAD
≤ 20 Ω).
PROTECTING THE DEVICE AGAINST
REVERSE BATTERY
The simplest way to protect the device against a
continuous reverse battery voltage (-26V) is to
insert a Schottky diode between pin 1(GND) and
ground, as shown in the typical application circuit
(Figure 10).
The consequences of the voltage drop across this
diode are as follows:
– If the input is pulled to power GND, a negative
voltage of -V
f
is seen by the device. (V
IL
, V
IH
thresholds and V
STAT
are increased by V
f
with
respect to power GND).
– The undervoltage shutdown level is increased
by V
f
.
If there is no need for the control unit to handle
external analog signals referred to the power
GND, the best approach is to connect the
reference potential of the control unit to node [1]
(see application circuit in Figure 11), which
becomes the common signal GND for the whole
control board avoiding shift of V
IH
, V
IL
and V
STAT
.
This solution allows the use of a standard diode.
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