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M51957A

VOLTAGE DETECTING/ SYSTEM RESETTING IC SERIES

Renesas Electronics 

M51957A,B/M51958A,B

Setting of Delay Capacity

Please use capacitor Cd for the delay within the range of 10 µF or less.

When a value that is bigger than this is set, the problem such as following (1), (2), and (3) becomes remarkable.

t

VCC

tpd

Output

tPHL

Figure 9 Time Chart at Momentary Voltage-Decrease

(1) The difference at delay time becomes remarkable.

A long delay setting of tens of seconds is fundamentally possible. However, when set delay time is lengthened, the

range of the difference relatively grows, too. When a set value is assumed to be ‘tpd’, the difference occurs in the

range from 0.47 × tpd to 2.05 × tpd. For instance, 34 seconds can be calculated at 100 µF. However, it is likely to

vary within the ranges of 16-70 seconds.

(2) Difficulty to react to a momentary voltage decrease.

For example, the reaction time tPHL is 10 µs when delay capacitor Cd = 0.1 µF.

The momentary voltage-decrease that is longer than such tPHL are occurs, the detection becomes possible. When the

delay capacitance is enlarged, tPHL also becomes long. For instance, it becomes about 100 to 200 µs in case of

circuit constant C1 = 100 µF.

(Characteristic graph 1 is used and extrapolation in case of Cd = 100 µF.)

Therefore, it doesn't react to momentary voltage-decrease that is shorter than this.

(3) Original delay time is not obtained.

When the momentary voltage-decrease time ‘t’ is equivalent to tPHL, the discharge becomes insufficient and the

charge starts at that state. This phenomenon occurs at large capacitance. And, original delay time tpd is not

obtained.

Please refer to characteristic graph 2. (Delay time versus input pulse width)

1000

200

100

Characteristic Graph 1

Reaction Time vs. Delay Capacitance

(Example data)

10

Characteristic Graph 2

Delay Time vs. Momentary Voltage Decrease Pulse Width

(Example data)

10000

1000

100

Delay Capacitance

0.01µF

0.033µF

0.1µF

0.33µF

1µF

2.2µF

3.3µF

10

1

0.01

0.1

1

10

100

Delay Capacitance Cd (µF)

1

1

10

100

1000

10000

Pulse Width (µs)

Figure 10 Characteristic Graph

REJ03D0778-0300 Rev.3.00 Sep 18, 2007

Page 10 of 13

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