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AN453
Application note
TDE1897C, TDE1897R, TDE1898C and TDE1898R
in extreme overload conditions
Introduction
The purpose of this document is to provide the circuit designer with some insight into how
the TDE1897C, TDE1897R, TDE1898C and TDE1898R devices behave in extreme
overload conditions. Although the conditions may range outside the limits of the guaranteed
performances described in the device datasheet, erroneous connections during the
installation phase may occur and momentarily create such conditions. The performed tests
confirm the extreme ruggedness of this device and its ability to overcome the accidental
overload.
The TDE1897C, TDE1897R, TDE1898C and TDE1898R are monolithic intelligent power
switch (IPS) in high-side configuration made in BCD technology (see Figure 1). They can
drive resistive and inductive loads such as lamps, relays, electrovalves and so on. An
internal voltage clamping diode to +VS creates, in inductive loads, a fast demagnetization
path with no external components. Suitable for industrial applications, the device operates in
the 18- to 35 V supply range, delivering output currents up to 500 mA. In typical applications,
it can drive up to 1 or 1.5 H load coils (48 to 60 Ω typical associated resistance).
Figure 1.
December 2008
Block diagram
Rev 3
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Contents
AN453
Contents
1
Overload conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Overload operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Measurements and calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1
Overload conditions
Overload conditions
To investigate how the TDE1897C, TDE1897R, TDE1898C and TDE1898R behave in
extreme inductive overload conditions, which can occur when too big a load is connected to
the device output, tests have been performed in bias conditions that lead the device to
function out of the operatives and rated limits specified in the datasheet.
The test conditions (depicted in Figure 2) are the following: VS = +24 V,
IO = internal limited, Tamb = 25 °C, L = 1.4 H (non saturating), RL = 12 Ω, Vi = 2 V (Vih)(a),
Tj = from Θ Lim-TH to Θ Lim and above(b).
Figure 2.
Inductive load equivalent circuit and demagnetization cycle waveforms
a. The input signal asks for a permanent "on" state.
b. Θ Lim and TH = thresholds of intervention and hysteresis of the internal thermal protection circuit.
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Overload operation
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AN453
Overload operation
Due to the internal limitation (ISC), the output current (IO) is not limited by the load (VS/Rl = 2
A, ISC ≤ 1.5 A) but by the device itself. As soon as the current reaches ISC, the IPS goes out
of the minimum resistance state and increases its voltage drop so that IO = ICS. The silicon
temperature of the DUT increases rapidly up to the thermal protection threshold value
(Θ Lim) and such protection tries to cut-off the output DMOS. The output’s turn-off forces the
demagnetization cycle, which discharges the energy of the inductive load (to VS) through
the device.
Because of the higher energy in the magnetic load and the higher peak power (see Note 1),
the higher-clamped current value (ISC) produces, during demagnetization, more stress
conditions.
During the "on" state, the power (Pdon) on the DUT (see the 225 msec. interval in Figure 3)
is defined by the IO (ISC) and Rl values. The chip temperature rapidly increases and reaches
the upper thermal protection threshold value (Θ Lim).
At that moment the protection is triggered on, inducing a switch-off of the output channel.
Due to the inductive component in the load, you must wait for the associated
demagnetization phase (some 50 msec. after the 225 msec. interval) to see the actual
switch-off.
The DUT then starts to cool down staying in the off state until the chip temperature goes
down to a lower thermal threshold value (Θ Lim-TH). When this lower limit is attained, the
thermal protection circuit withdraws itself and the chip resumes its normal functions and
restarts another cycle. In fact, its input will have been connected permanently to a voltage
level of more than 2 V, meaning a continuous request for conduction. A new overload cycle
begins and a periodic repetition of the following:
●
Load charging
●
Current limitation
●
Over-temperature and demagnetization
●
Cooling down in the off state.
It can be noted that, for given thermal parameters (Zth, thermal protection levels and
hysteresis), differences in Pdon affect only the "TON" and "TOFF" duration and ratio of such
periodic repetitions.
Note:
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1
During the demagnetization phase, the power dissipated inside the IPS chip is: IO(t) * VCL.
IO (t) decays to zero from ISC, VCL is set by the IPS itself to approximately 50 V.
AN453
3
Measurements and calculations
Measurements and calculations
For a typical TDE1897C or TDE1897R sample in Minidip package (see Figure 3) in
"thermal" periodic repetition, the current (self-limited region) is limited to 1.1 A and the
voltage across the DUT is equal to 10.8 V for 225 msecs of "on" time.
The energy dissipated on the DUT in the demagnetization cycle is equal to 1.28 J.
●
The repetition cycle rate is equal to 0.27 Hz (t = 3.7 seconds)
●
Pdon (average) = 1.1 A × 10.8 V × 0.225 sec/3.7s = 0.72 W
●
Pdem (average) = 1.28 J × 0.27 cycles/s = 0.346 W.
Adding the small power dissipated to operate the quiescent current and for IO(t)^2*RON in
the load charging region, the total power P(tot) of 1.1 W is considered a realistic value.
Minidip (on the test-socket) Rthj-amb is approximately 85 °C/W, which leads the average
temperature in the hot region of the chip to 115/120 °C (the chip is not homogeneous in
temperature; higher temperatures are reached, during dissipation, in the area of the output
DMOS).
Figure 3.
TDE1897R or TDE1897C in Minidip Figure 4.
package output voltage (CH2) and
output current (CH1) vs. time in
thermal periodic repetition
TDE1897R or TDE1897C in Minidip
package output current and
temperature in the test point vs.
time
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Conclusion
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AN453
Conclusion
The complex protection system of the TDE1897C, TDE1897R, TDE1898C and TDE1898R
also prove effective in extreme overload conditions. Although the behavior of such devices in
these conditions cannot be guaranteed due to the high temperatures that accelerate the
intrinsic ageing mechanism, the tests performed show that there is a lot of margin beyond
the limits guaranteed in the device datasheet.
These tests also show that it is quite likely that such devices will survive non-permanent
overloads like the ones that can occur in practice during the installation or modification of an
industrial control system.
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5
Revision history
Revision history
Table 1.
Document revision history
Date
Revision
Changes
December
2003
1
Initial release
July 2005
2
– Updated the layout look & feel.
– Changed title
10-Dec-2008
3
– Document reformatted. No content change.
– Obsoleted SIP9 package reference
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AN453
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