US 6,650,149 B1Granted

Latched active fail-safe circuit for protecting a differential receiver

AssigneePericom Semiconductor Corp.

InventorAnthony Yap Wong

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Abstract

A fail-safe circuit for a differential receiver can tolerate noise. A latch is enabled when both differential inputs V+, V− rise above a reference voltage that is close to Vcc. The latch, once enabled, is set by an offset amplifier, signaling the fail-safe condition. The offset amplifier sets the latch when V+ is above or equal to V−. The differential amplifier has a small offset voltage to allow the latch to remain set when V+ and V− are equal in voltage. An output from a differential amplifier receiving V+ and V− can be blocked by a gate when the fail-safe condition is latched. Pullup resistors pull V+, V− to Vcc when an open failure occurs. The latch remains set when common-mode noise occurs on V+, V−, preventing noise from prematurely disabling the fail-safe condition. Such noise coupled into a broken cable is usually common-mode.

Claims19

Dependent claims

A fail-safe differential receiver comprising:

a pair of differential inputs that comprise a first differential input and a second differential input;
a differential amplifier, receiving the first and second differential inputs, for generating a difference output;
a first comparator, receiving the first differential input and a reference voltage, for generating a first compare signal;
a second comparator, receiving the second differential input and a reference voltage, for generating a second compare signal;
a combining gate, receiving the first and second compare signals, for generating an enable signal;
and
a latch, triggered by the enable signal when the first and second compare signals are both active, for outputting a blocking signal;
a blocking gate, receiving the difference output from the differential amplifier and the blocking signal, for driving a safe output to a fixed state when the latch activates the blocking signal, but for passing the difference output to the safe output when the blocking signal is not activated;
wherein the safe output is driven to the fixed state when the latch activates the blocking signal, but the difference output from the differential amplifier is applied to the safe output when the blocking signal is not activated,
whereby the first and second differential inputs are compared to the reference voltage to determine when to block the difference output from the differential amplifier.

7 more elements

A differential receiver comprising:

a first differential input;
a second differential input;
differential amplifier means, responsive to the first and second differential inputs, for generating a difference signal in response to a voltage difference between the first and second differential inputs;
first compare means, coupled to the first differential input, for determining when the first differential input is outside of a voltage limit;
second compare means, coupled to the second differential input, for determining when the second differential input is outside of the voltage limit;
offset compare means, coupled to the first and second differential inputs, for comparing voltages of the first and second differential inputs to generate and activate a data signal when the first differential input is greater or equal in voltage to the second differential input;
latching means, receiving the data signal from the offset compare means, for latching the data signal when the first and second compare means determine that the first and second differential inputs are both outside the voltage limit;
blocking means, coupled to the latching means, for blocking the difference signal from the differential amplifier means when both the first differential input and the second differential input are outside of the voltage limit and the data signal is activated and latched;
whereby the difference signal from the differential amplifier means is blocked when both the first differential input and the second differential input are outside of the voltage limit and the data signal is activated and latched.

6 more elements

A differential receiver with open-cable detection comprising:

a cable input having a first differential line and a second differential line;
a load resistor coupled between the first and second differential lines to generate a voltage difference;
a differential amplifier for amplifying the voltage difference across the load resistor, the differential amplifier receiving the first and second differential lines and generating a difference output;
a first comparator, receiving the first differential line and coupled to a reference node, for comparing a voltage on the first differential line to a reference voltage on the reference node and generating a first compare output;
a second comparator, receiving the second differential line and coupled to the reference node, for comparing a voltage on the second differential line to the reference voltage on the reference node and generating a second compare output;
a third comparator, receiving the first and second differential lines, for comparing a voltage on the first differential line to an offset of a voltage on the second differential line and generating a third compare output;
a storage element, having a state that drives a blocking signal, the state being set when the first compare output, the second compare output, and the third compare output indicate that the first and second differential lines are both above the reference voltage and the first differential line is greater or equal to the second differential line;
blocking logic, receiving the blocking signal, for blocking the difference output and driving a final output with a constant signal when the state in the storage element is set, but otherwise passing the,difference output to the final output.

5 more elements

Dependencies are inferred from the claim text.