HEF4541BP データシートの表示（PDF） - NXP Semiconductors.

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HEF4541BP

Programmable timer

NXP Semiconductors. 

NXP Semiconductors

HEF4541B

Programmable timer

12. Application information

RC oscillator timing component limitations

The oscillator frequency is mainly determined by RTCCTC, provided RTC << RS and

RSC2 << RTCCTC. The function of RS is to minimize the influence of the forward voltage

across the input protection diodes on the frequency. The stray capacitance C2 should be

kept as small as possible. In consideration of accuracy, CTC must be larger than the

inherent stray capacitance. RTC must be larger than the LOCMOS ‘ON’ resistance in

series with it, which typically is 500 Ω at VDD = 5 V, 300 Ω at VDD = 10 V and 200 Ω at

VDD = 15 V.

The recommended values for these components to maintain agreement with the typical

oscillation formula are: CTC ≥ 100 pF, up to any typical value, 10 kΩ ≤ RTC ≤ 1 MΩ.

reset

from logic

clock to

counter

RS

3

C2

RS

CTC

2

CTC

RTC

1

RTC

001aai584

Fig 6.

Typical formula for oscillator frequency: fosc = 2----.-3----×-----R----T-1--C-----×-----C----T---C- .

External component connection for RC oscillator; RS ≈ RTC

105

fosc

(Hz)

104

001aai586

105

fosc

(Hz)

104

001aai585

103

103

102

102

10

10-4

10-3

10-2

10-1

CTC (μF)

a. CTC curve at RTC = 56 kΩ; RS = 120 kΩ.

10

103

104

105

106

RTC (Ω)

b. RTC curve at CTC = 1 nF; RS = 2 RTC.

Fig 7. RC oscillator frequency as a function of RTC and CTC at VDD = 5 to 15 V; Tamb = 25 °C

HEF4541B

Product data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 4 — 25 June 2012

© NXP B.V. 2012. All rights reserved.

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