december 2013 docid1745 rev 4 1/7 AN495 application note effective filtering of tde1707 introduction the tde1707 is an industrial ips (intelligent power switch) dedicated for use in proximity detectors. it can deliver up to 0.5 a of current to a ?configurable? load (battery or ground configuration), as indicated in the block diagram below. figure 1. block diagram 5 v 2.6v 4v 5v thermal shutdown 70v isc cmp 6 5 3 7 1 8 2 4 vreg +5v in delay 4ua +vs led driver l.s. out h.s. out gnd ri 1mohm 3ma led driver www.st.com
effective filtering AN495 2/7 docid1745 rev 4 1 effective filtering in the application circuit ( figure 2 ) taken from the tde1707 device datasheet, the use of filter capacitors is recommended on both: ? 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 detec tors); 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 powe r 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 curr ent variations with wide volt age 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 possibl e 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 10nf 1msec/nf 10nf 8 2 4 3 6 5 1 7 tde1707 input vcc ground load 10nf to 1uf high side load 10nf 1msec/nf 10nf 10nf to 1uf 1 2 4 3 6 5 7 8 tde1707 input vcc ground load low side load
docid1745 rev 4 3/7 AN495 effective filtering 7 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 ca pacitor 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 c an, with the same filtering effectiveness of a capacitor on +vs, be 20 times smaller in value. nonetheless, a capacitor on +vs, although no t 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 h are too high to be found in practice. ? 4.7 nf on +vs is barely sufficient with 325 h (with 325 h, 10 nf would be preferable). ? 4.7 nf on vreg is sufficie nt 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.
effective filtering AN495 4/7 docid1745 rev 4 figure 3. test circuit 6 v.reg out 4.7nf 5 3 1 7 8 2 4 in delay 50ohm delay cap led out vs low side out high side out tde1707 ground 325uh 4.7nf 24v load 75ohm
docid1745 rev 4 5/7 AN495 effective filtering 7 figure 4. voltage on vs and vreg without filter capacitor figure 5. voltage on vs and vreg with filter capacitor ch1 24v ch2 5v on off ch1 20v/div t = 20sec/div ch2 2v/div ch1 24v ch2 5v ch1 20v/div t = 20sec/div ch2 2v/div on off
revision history AN495 6/7 docid1745 rev 4 2 revision history table 1. revision history date revision changes nov-2003 1 first issue 14-sep-2005 2 new template, no content change 14-dec-2006 3 the document has been reformatted 17-dec-2013 4 minor formatting and text changes
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