AN2713 datasheet(17/40 Pages) STMICROELECTRONICS

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AN2713

Other application circuits

Doc ID 14431 Rev 1

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Other application circuits

The following paragraphs present two particular application solutions: the first can be used

to reduce the 18 V to 13 V transition time, while the second is designed to improve lightning

surge protection.

4.1

18 V to 13 V fast transition with high bus capacitance

In cases of very high bus capacitance (Cbus>10 µF) and very low output current, an

external circuit (with a 4.7 V Zener diode and a 100

Ω series resistor) can be added to

reduce 18 V to 13 V transition fall time, as shown in Figure 6.

Figure 6.

External circuit to reduce 18 V to 13 V fall time transition

The TTX bit (or pin) must be set high only during the transition from 18 V to 13 V. The TTX

function activates only the push-pull circuit, but not the tone output. The 22 kHz tone is

activated only when the TEN bit (or DSQIN pin) is also set high and injected into the LNB

bus through the 10 µF capacitor.

When the TTX function is activated, the VoTx voltage is internally set at 5 V (typ). below the

Vout (for example, if Vout=18 V then VoTx=18-5=13 V) and, at the same time, the P-channel

is enabled to sink current (note: the P-channel is internally current-limited).

With a 4.7 V Zener diode, when TTX=high, the current through the Zener is: Iz=(5-

4.7)/100=3 mA. If TTX=low, the current through the Zener is negligible.

If there is a high output capacitance present, during the transition from 18 V to 13 V (with

TTX=high), the voltage drop (Vout-VoTx) is increased because the VoTx goes quickly to low

level (at 13 V-5 V=8 V) and, consequently, the Zener current is also increased until the

output capacitance is discharged to 13 V. For example, with 100 µF on the output, the 18 V

to 13 V fall time is about 25 ms.

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