lm9011 National Semiconductor Corporation, lm9011 Datasheet - Page 11

• Manufacturer Part Number: lm9011
• Description: Electronic Ignition Interface
• Manufacturer: National Semiconductor Corporation

Timing Diagrams
Circuit Description
VR SENSOR INTERFACE
The differential inputs, VR_HI and VR_LO are low imped-
ance inputs with a DC voltage bias of one half of Vcc, Both
inputs require equal value series resistance on their respec-
tive pins to convert the VR sensor voltage to a differential
input current. The differential input current range is typically
2.5µA peak-to-peak to 2.5mA peak-to-peak. Each input has
active current limiting that will clamp the current at typically
+/-5mA. This is intended for short circuit protection and not
for input signal limiting.
FIGURE 9. VR Interface Timing Diagram
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Differential voltages of 500mV peak-to-peak to 500V peak-
to-peak can be processed with the specified 100KΩ series
resistor on each input. Numerous variables will determine
the output voltage signal from a VR sensor across a fre-
quency range. The input resistors can be scaled from typi-
cally 50KΩ to 200KΩ to keep the differential input current
with-in the recommended range for a given VR Sensor out-
put voltage. Bypass capacitors can be added to form a low
pass filter to limit the differential input signal at the higher
frequencies.
The VR Sensor interface utilizes a dynamic hysteresis which
will increase the hysteresis level as the input signal from the
VR Sensor increases. The circuit requires two external com-
ponents to fully implement the hysteresis function: a capaci-
tor on VR_FC to filter and store the peak detector signals;
and a 150KΩ resistor on VR_BIAS to set a reference current
for the hysteresis circuit. The typical value range for the peak
detector storage capacitor is 0.1µF to 0.47µF.
The peak detector has an internal 3KΩ (typical) current
limiting resistor to Vcc for charging the storage capacitor. An
external resistor in parallel with the peak detector storage
capacitor is used to set the RC discharge rate of the peak
detector capacitor.
For input levels greater than typically 10µA peak-to-peak the
voltage on the peak detector output pin VR_FC is used to
actively derive the hysteresis level. The active hysteresis will
typically be 30% of the peak input signal. As the input level
falls below typically 10µA peak-to-peak the hysteresis level
will begin to rise as the static hysteresis level takes effect.
The static hysteresis level is set by the current out of the
VR_BIAS pin and is a constant level of typically 1µA peak
with a VR_BIAS resistor of 150KΩ. This static hysteresis
level acts as the minimum detect threshold as there will be
no output if the input signal is not greater than the static
hysteresis level.
The VR_BIAS resistor can be scaled from typically 50KΩ to
500KΩ, but the practical range is typically 75KΩ to 300KΩ.
Increasing the resistance (i.e. reducing the current) will lower
the minimum hysteresis level. Conversely, reducing the re-
sistance will raise the minimum hysteresis level. Since the
VR_BIAS current is modified by the same square root circuit
used for the input signal, the relationship between the VR-
_BIAS resistor value and the minimum detect level is not
FIGURE 10. VR Sensor Interface Block Diagram
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