BUILT-IN DELAY CIRCUIT (EXTERNAL DELAY TIME SETTING) VOLTAGE DETECTOR WITH SENSE PIN
S-1004 Series
Rev.2.1_01
2. SENSE pin
2. 1 Error when detection voltage is set externally
By connecting a node that was resistance-divided by the resistor (RA) and the resistor (RB) to the SENSE pin as
seen in Figure 26, the detection voltage can be set externally.
For conventional products without the SENSE pin, RA cannot be too large since the resistance-divided node must
be connected to the VDD pin. This is because a feed-through current will flow through the VDD pin when it goes
from detection to release, and if RA is large, problems such as oscillation or larger error in the hysteresis width may
occur.
In the S-1004 Series, RA and RB are easily made larger since the resistance-divided node can be connected to the
SENSE pin through which no feed-through current flows. However, be careful of error in the current flowing through
the internal resistance (RSENSE) that will occur.
Although RSENSE in the S-1004 Series is large (5 MΩ min.) to make the error small, RA and RB should be selected
such that the error is within the allowable limits.
2. 2 Selection of RA and RB
In Figure 26, the relation between the external setting detection voltage (VDX) and the actual detection voltage
(−VDET) is ideally calculated by the equation below.
RA
RB
VDX = −VDET
×
1 +
··· (1)
(
)
However, in reality there is an error in the current flowing through RSENSE
.
When considering this error, the relation between VDX and −VDET is calculated as follows.
RA
VDX = −VDET
= −VDET
×
×
1 +
(
)
RB || RSENSE
RA
1 +
RB × RSENSE
RB + RSENSE
RA
RB
RA
= −VDET
×
1 +
+
× −VDET
··· (2)
(
)
RSENSE
RA
RSENSE
By using equations (1) and (2), the error is calculated as −VDET
×
.
The error rate is calculated as follows by dividing the error by the right-hand side of equation (1).
RA × RB
RSENSE × (RA + RB)
RA || RB
RSENSE
× 100 [%] =
× 100 [%]
··· (3)
As seen in equation (3), the smaller the resistance values of RA and RB compared to RSENSE, the smaller the error
rate becomes.
Also, the relation between the external setting hysteresis width (VHX) and the hysteresis width (VHYS) is calculated
by equation below. Error due to RSENSE also occurs to the relation in a similar way to the detection voltage.
RA
RB
VHX = VHYS
×
1 +
··· (4)
(
)
RA
VDD
SENSE
RSENSE
OUT
CD
VDX
−VDET
RB
VSS
Figure 26 Detection Voltage External Setting Circuit
Caution If RA and RB are large, the SENSE pin input impedance becomes higher and may cause a
malfunction due to noise. In this case, connect a capacitor between the SENSE pin and the VSS
pin.
18
SII [ SEIKO INSTRUMENTS INC ]
