CS1124 Datasheet, PDF (6/8 Pages) ON Semiconductor – Dual Variable−Reluctance Sensor Interface IC

CS1124
APPLICATION INFORMATION
Referring to Figure 2, the following will be a design
example given these system requirements:
RRS + 1.5 kW (u 12 kW is considered open)
VRS(MAX) + 120 Vpk
VRS(MIN) + 250 mVpk
FVRS + 10 kHz @ VRS(MIN) + 40 Vpk−pk
1. Determine tradeoff between R1 value and power
rating. (use 1/2 watt package)
ǒ Ǔ120 2
Ǹ₂
PD + _R1 t 1ń2 W
Set R1 = 15 k. (The clamp current will then be 120/15 k
= 8.0 mA, which is less than the 12 mA limit.)
2. Determine R_Adj
Set R_Adj as close to R1 + R_RS as possible.
Therefore, R_Adj = 17 k.
3. Determine V_RS(+TRP) using equation (7).
VRS(+TRP) + 11mA 17 k * 11mA(15 k ) 1.5 k) ) 160 m
VRS(+TRP) + 166 mV typical
(easily meets 250 mV minimum)
4. Calculate worst case V_RS(+TRP)
Examination of equation (7) and the spec reveals the worst
case trip voltage will occur when:
V_HYS = 180 mV
IN_Adj = 16 mA
INP1 = 15 mA
R1 = 14.25 k (5% low)
R_Adj = 17.85 k (5% High)
VRS(+)MAX + 16 mA(17.85 k)
* 15mA(14.25 k ) 1.5 k) ) 180 mV
+ 229 mV
which is still less than the 250 mV minimum amplitude of
the input.
5. Calculate C1 for low pass filtering
Since the sensor guarantees 40 V_pk−pk @ 10 kHz, a low
pass filter using R1 and C1 can be used to eliminate high
frequency noise without affecting system performance.
Gain
Reduction
+
0.29 V
20 V
+
0.0145
+
*36.7
dB
Therefore, a cut−off frequency, f_C, of 145 Hz could be
used.
C1
v
1
2pfCR1
v
0.07
mF
Set C1 = 0.047 mF.
6. Calculate the minimum R_RS that will be indicated as
an open circuit. (DIAG = 5.0 V)
Rearranging equation (7) gives
ƪVHYS ) [INP1 KI
* VRS(+TRP)
RRS +
INP1
Æ« ^RAdj]
* R1
But, V_RS = 0 during this test, so it drops out.
Using the following as worst case Low and High:
IN_Adj
R_Adj
V_HYS
INP1
R1
K_I
Worst Case Low (R_RS)
23.6 mA = 15 mA × 1.57
16.15 k
135 mV
16 mA
15.75 k
1.57
Worst Case High (R_RS)
10.7 mA = 7.0 mA × 1.53
17.85 k
185 mV
6.0 mA
14.25 k
1.53
RRS
+
135
mV
)
23.6 mA
16 mA
+ 16.5 k
16.15 k _{* 15.75 k}
Therefore,
RRS(MIN) + 16.5 k (meets 12 k system spec)
and,
RRS(MAX)
+
185
mV
)
10.7 mA
6.0mA
+ 48.4 k
17.85 k _{* 14.25 k}
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