
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
(k
)
47
82
150
180
220
330
390
470
Table 1