NCV1124
http://onsemi.com
4
THEORY OF OPERATION
NORMAL OPERATION
Figure 2 shows one channel of the NCV1124 along with
the necessary external components. Both channels share the
IN
Adj
pin as the negative input to a comparator. A brief
description of the components is as follows:
V
RS
− Ideal sinusoidal, ground referenced, sensor output
− amplitude usually increases with frequency, depending on
loading.
R
RS
− Source impedance of sensor.
R1/R
Adj
− External resistors for current limiting and
biasing.
INP1/IN
Adj
− Internal current sources that determine trip
points via R1/R
Adj
.
COMP1 − Internal comparator with built−in hysteresis
set at 160 mV.
OUT1 − Output 0 V − 5.0 V square wave with the same
frequency as V
RS
.
By inspection, the voltage at the (+) and (−) terminals of
COMP1 with V
RS
= 0V are:
V
+
+ INP1(R1 ) R
RS
)
(1)
V
−
+ IN
Adj
R
Adj
(2)
As V
RS
begins to rise and fall, it will be superimposed on
the DC biased voltage at V
+
.
V
+
+ INP1(R1 ) R
RS
) ) V
RS
(3)
To get comparator COMP1 to trip, the following
condition is needed when crossing in the positive direction,
V
+
u V
−
) V
HYS
(4)
(V
HYS
is the built−in hysteresis set to 160 mV), or when
crossing in the negative direction,
V
+
t V
−
* V
HYS
(5)
Combining equations 2, 3, and 4, we get:
INP1(R1 ) R
RS
) ) V
RS
u IN
Adj
R
Adj
) V
HYS
(6)
therefore,
V
RS(+TRP)
t IN
Adj
R
Adj
* INP1(R1 ) R
RS
) ) V
HYS
(7)
It should be evident that tripping on the negative side is:
V
RS(−TRP)
t IN
Adj
R
Adj
* INP1(R1 ) R
RS
) * V
HYS
(8)
In normal mode,
INP1 + IN
Adj
(9)
We can now re−write equation (7) as:
V
RS(+TR)
u INP1(R
Adj
* R1 * R
RS
) ) V
HYS
(10)
By making
R
Adj
+ R1 ) R
RS
(11)
you can detect signals with as little amplitude as V
HYS
.
A design example is given in the applications section.
OPEN SENSOR PROTECTION
The NCV1124 has a DIAG pin that when pulled high (5.0
V), will increase the IN
Adj
current source by roughly 50%.
Equation (7) shows that a larger V
RS(+TRP)
voltage will be
needed to trip comparator COMP1. However, if no V
RS
signal is present, then we can use equations 1, 2, and 4
(equation 5 does not apply in this mode) to get:
INP1(R1 ) R
RS
) u INP1 K
I
R
Adj
) V
HYS
(12)
Since R
RS
is the only unknown variable we can solve for
R
RS
,
R
RS
+
INP1 K
I
R
Adj
) V
HYS
INP1
* R1
(13)
Equation (13) shows that if the output switches states
when entering the diag mode with V
RS
= 0, the sensor
impedance must be greater than the above calculated value.
This can be very useful in diagnosing intermittent sensor.
INPUT PROTECTION
As shown in Figure 2, an active clamp is provided on each
input to limit the voltage on the input pin and prevent
substrate current injection. The clamp is specified to handle
±12 mA. This puts an upper limit on the amplitude of the
sensor output. For example, if R1 = 20 k, then
V
RS(MAX)
+ 20 k 12 mA + 240 V
Therefore, the V
RS(pk−pk)
voltage can be as high as 480 V.
The NCV1124 will typically run at a frequency up to 1.8
MHz if the input signal does not activate the positive or
negative input clamps. Frequency performance will be
lower when the positive or negative clamps are active.
Typical performance will be up to a frequency of 680 kHz
with the clamps active.
