ST AN453 APPLICATION NOTE

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 +V 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 typical associated resistance).
creates, in inductive loads, a fast demagnetization
S

Figure 1. Block diagram

December 2008 Rev 3 1/8
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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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AN453 Overload conditions

1 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: V I
= internal limited, T
O
T
= from Θ Lim-TH to Θ Lim and above
j
= 25 °C, L = 1.4 H (non saturating), RL = 12 Ω, Vi = 2 V (Vih)
amb
(b)
.
= +24 V,
S
(a)

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 AN453

2 Overload operation

Due to the internal limitation (ISC), the output current (IO) is not limited by the load (VS/Rl = 2 A, I
1.5 A) but by the device itself. As soon as the current reaches ISC, the IPS goes out
SC
of the minimum resistance state and increases its voltage drop so that I 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 V the device.
Because of the higher energy in the magnetic load and the higher peak power (see Note 1), the higher-clamped current value (I
) produces, during demagnetization, more stress
SC
conditions.
= ICS. The silicon
O
) through
S
During the "on" state, the power (P is defined by the I
(ISC) and Rl values. The chip temperature rapidly increases and reaches
O
) on the DUT (see the 225 msec. interval in Figure 3)
don
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-T
). When this lower limit is attained, the
H
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 (Z hysteresis), differences in P
affect only the "TON" and "T
don
, thermal protection levels and
th
" duration and ratio of such
OFF
periodic repetitions.
Note: 1 During the demagnetization phase, the power dissipated inside the IPS chip is: I
I
(t) decays to zero from ISC, VCL is set by the IPS itself to approximately 50 V.
O
(t) * VCL.
O
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AN453 Measurements and calculations

3 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)
P
P
Adding the small power dissipated to operate the quiescent current and for I the load charging region, the total power P(tot) of 1.1 W is considered a realistic value.
(average) = 1.1 A × 10.8 V × 0.225 sec/3.7s = 0.72 W
don
(average) = 1.28 J × 0.27 cycles/s = 0.346 W.
dem
(t)^2*RON in
O
Minidip (on the test-socket) R
is approximately 85 °C/W, which leads the average
thj-amb
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
package output voltage (CH2) and output current (CH1) vs. time in thermal periodic repetition
Figure 4. TDE1897R or TDE1897C in Minidip
package output current and temperature in the test point vs. time
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Conclusion AN453

4 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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AN453 Revision history

5 Revision history

Table 1. Document revision history

Date Revision Changes
December
2003
July 2005 2
10-Dec-2008 3
1 Initial release
– Updated the layout look & feel. – Changed title
– Document reformatted. No content change. – Obsoleted SIP9 package reference
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AN453
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