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ZN449 Datasheet(PDF) 11 Page - List of Unclassifed Manufacturers |
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ZN449 Datasheet(HTML) 11 Page - List of Unclassifed Manufacturers |
11 / 18 page ZN448/9 11 Several suitable circuits are shown in Fig.13. The principle of operation is the same in each case. Whilst the BUSY output is high, capacitor C1 is charged to about 4-4.5V. During a conversion the BUSY output goes low and the upper end of C1 is thus also pulled low. The lower end of C1 therefore applies about -4V to R2, thus providing the tail current for the comparator. The time constant R2. C1 is chosen according to the clock frequency so that droop of the capacitor voltage is not significant during a conversion. The constraint on using this type of circuit is that C1 must be recharged whilst the BUSY output is high. If the BUSY output is high for greater than one converter clock period then the circuit of Fig.13a will suffice. If this is not the case, for example, in the continuous conversion mode, then the circuits of Figs. 13b and 13c are recommended, since these can pump more current into the capacitor. Where several ZN448/9's are used in a system the self- oscillating diode pump circuit Fig.14 is recommeded. Alternatively, if a negative supply is available in the system then this may be utilised. A list of suitable resistor values for different supply voltages is given in Table 1. Fig.14 Diode pump circuit to supply comparator tail current for up to five ZN448/9's 22n 330 5V 470 100n 100n IN914 IN914 -3.5V V – (volts) 3 5 10 12 15 20 25 30 R EXT (kΩ) 47 82 150 180 220 330 390 470 Table 1 |
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