AN495 STMICROELECTRONICS | Alldatasheet
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Technical content
configuration), as indicated in the block diagram below. Figure 1. Block diagram
1 Effective filtering
- the power supply (+Vs pin 7).
- the +5 V regulated voltage (Vreg pin 6). – To understand the function of the Vreg capacitor, it is necessary to consider that the Vreg pin makes available a regulated voltage that can be used to supply external circuits (typically proximity detectors); but the TDE1707 itself always uses the Vreg to supply most of its internal circuits (see Figure 1: Block diagram). Internally to the TDE1707, Vreg supplies all the functional blocks, except the output power transistor (and the Driver-Led, when the Led is connected as in “load to GND configuration”). – In a real application, the TDE1707 and as sociated circuitry can be significantly farther away from the power supply and the long connection wires will act as a serial inductor. At power-on, or at turn-on of the power transistor, or in coincidence with load variations, this inductance will react to the current variations with wide voltage variations. In proximity sensor applications, for space reasons, the size of the filtering capacitors must be reduced as much as possible. The voltage oscillation can induce two kind of problems in the circuit: 1. Overvoltage on the +Vs pin, connected to the +24 V bus, which can exceed the TDE1707 rated limit. 2. Disturbances inside the circuit (TDE1707), because the noise immunity level is exceeded with disruption of the input/output function. Filtering the overvoltage on the +Vs is possible only by an external capacitance between +Vs (pin7) and Ground (pin4) to limit voltage spikes at the device’s rated values.
Figure 2. Application circuit
Immunity problems in the circuit are essentially related to the +5 V Vreg filtering, because the noise on this voltage can induce errors in the low-voltage circuits. The best solution to avoid immunity problems is to put the filter capacitor between Vreg and Ground even if the Vreg does not supply any external circuits. Compared to the +Vs capacitor filtering effect, which also improves the immunity, the Vreg capacitor offers two advantages: 1. The capacitive value can be significantly lo wer, for the same filtering effect, because the Vreg filter does not have to sustain the load current variations. 2. The nominal voltage of the capacitor is lower (6 V instead of 25-50 V), so that the physical size of the capacitor is also smaller. The higher effectiveness of filtering Vreg instead of +Vs is always evident. On average, in a typical application, a capacitor put on Vreg can, with the same filtering effectiveness of a capacitor on +Vs, be 20 times smaller in value. Nonetheless, a capacitor on +Vs, although not of high value, is recommended. Its value should be adapted in accordance with the equivalent inductance of the supply connections and of the value of the load. For instance, the values used in the test circuit (Figure 3):
- 325 μH are too high to be found in practice.
- 4.7 nF on +Vs is barely sufficient with 325 μH (with 325 μH, 10 nF would be preferable).
- 4.7 nF on Vreg is sufficient in most practical cases.
- Using the test circuit shown in Figure 3, with the input trigger of the TDE1707 in pulsed mode, the effect of the capacitor on Vreg (pin 6) can be noted in Figure 4 and Figure 5: – Figure 4 shows the voltage on +Vs (Ch1) and Vreg (Ch2) at turn-on and turn-off edges, without the filter capacitor on pin 6. – Figure 5 shows the same signals but with 4.7 nF capacitor connected between Vreg (pin 6) and Ground. The improvement of the immunity level and the low values of the capacitors makes the suggested filtering solution the best for proximity detector applications.
Figure 3. Test circuit
2 Revision history
Table 1. Revision history