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

47

82

150

180

220

330

390

470

Table 1