HEF4541B All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2015. All rights reserved.
Product data sheet Rev. 5 — 15 December 2015 10 of 16
NXP Semiconductors
HEF4541B
Programmable timer
12. Application information
RC oscillator timing component limitations
The oscillator frequency is mainly determined by R
TC
C
TC
, provided R
TC
<< R
S
and
R
S
C
2
<< R
TC
C
TC
. The function of R
S
is to minimize the influence of the forward voltage
across the input protection diodes on the frequency. The stray capacitance C
2
should be
kept as small as possible. In consideration of accuracy, C
TC
must be larger than the
inherent stray capacitance. R
TC
must be larger than the LOCMOS ‘ON’ resistance in
series with it, which typically is 500 at V
DD
= 5 V, 300 at V
DD
= 10 V and 200 at
V
DD
=15V.
The recommended values for these components to maintain agreement with the typical
oscillation formula are: C
TC
100 pF, up to any typical value, 10 k R
TC
1 M.
Typical formula for oscillator frequency: .
Fig 6. External component connection for RC oscillator; R
S
≈ R
TC
&
5
6
&
7&
5
7&
56
&7&
57&
FORFNWR
FRXQWHU
UHVHW
IURPORJLF
f
osc
1
2.3 R
TC
C
TC
----------------------------------------
=
a. C
TC
curve at R
TC
= 56 k; RS = 120 k.b.R
TC
curve at C
TC
= 1 nF; RS = 2 R
TC
.
Fig 7. RC oscillator frequency as a function of R
TC
and C
TC
at V
DD
= 5 to 15 V; T
amb
= 25 C
)
I
RVF
+]
I
RVF
+]
