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 in-

put 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.

Differential voltages of 500mV peak-to-peak to 500V peak-

to-peak can be processed with the specified 100K

Ω 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 50K

Ω 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 fre-

quencies.

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 150K

Ω 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 3K

Ω (typical) current lim-

iting resistor to Vcc for charging the storage capacitor. An ex-

ternal resistor in parallel with the peak detector storage ca-

pacitor 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 ac-

tively 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 150K

Ω. This static hysteresis level

acts as the minimum detect threshold as there will be no out-

put if the input signal is not greater than the static hysteresis

level.

The VR_BIAS resistor can be scaled from typically 50K

Ω to

500K

Ω, 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 lin-

ear. For VR_BIAS values greater than 500K

Ω, the minimum

detect level is typically determined more by the internal de-

vice offsets, and thermal effects.

Voltage Comparators

The circuit includes four general purpose voltage compara-

tors that use an internal reference voltage to set their voltage

thresholds. Three of the comparators have their non-

inverting inputs tied to the internal reference voltage, and

their inverting-inputs are brought out. The remaining one

comparator has its inverting input tied to the internal voltage

reference, and its non-inverting input is brought out. All four

comparators include hysteresis to improve noise immunity.

The comparator outputs are internally pulled up to V

un-used comparator should have its input connected to de-

vice ground.

DS101264-21

FIGURE 10. VR Sensor Interface Block Diagram

DS101264-22

FIGURE 11. Voltage Comparator Block Diagram

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