Output slew rate control in low voltage differential signal (LVDS) driver
View Patent ↗A differential signal driver includes a pre-driver configured to generate a constant charging current and a constant discharging current. A first capacitor of the pre-driver is charged with the charging current when a differential input signal has a first state, and discharged with the discharging current when the differential input signal has a second state, thereby developing a first output control voltage on the first capacitor. A second capacitor of the pre-driver is discharged with the discharging charging current when the differential input signal has the first state, and charged with the charging current when the differential input signal has the second state, thereby developing a second output control voltage on the second capacitor. An output driver circuit generates a differential output signal in response to the first and second output control voltages. The slew rate of the differential output signal is controlled by the charging and discharging currents.
1. A differential signal driver comprising:
a pre-driver circuit configured to receive a first differential input signal and a second differential input signal, and in response, provide a first output control voltage and a second output control voltage; and
an output driver circuit configured to provide a differential output signal in response to the first and second output control voltages,
wherein the pre-driver circuit comprises:
a current source configured to provide a charging current;
a current sink configured to provide a discharging current;
a first capacitor;
a second capacitor;
a first set of switching transistors configured to selectively couple the first capacitor to either the current source or the current sink in response to the first differential input signal, thereby developing the first output control voltage on the first capacitor;
a second set of switching transistors configured to selectively couple the second capacitor to either the current source or the current sink in response to the second differential input signal, whereby the second output control voltage is developed on the second capacitor.
2. The differential signal driver of claim 1 , wherein the output driver circuit comprises:
an output current source configured to provide an output current;
a load resistor; and
a plurality of switches configured to steer the output current through the load resistor in response to the first and second output control voltages.
3. The differential signal driver of claim 2 , wherein the plurality of switches comprise four transistors having the same conductivity type.
4. The differential signal driver of claim 3 , wherein the pre-driver circuit further comprises a voltage regulator configured to provide a constant voltage to the current source.
5. The differential signal driver of claim 1 , wherein the pre-driver circuit further comprises a bias control circuit configured to provide a first set of bias voltages to the current source, thereby causing the current source to provide the charging current, and a second set of bias voltages to the current sink, thereby causing the current sink to provide the discharging current.
6. The differential signal driver of claim 5 , wherein the pre-driver circuit further comprises a voltage regulator configured to provide a constant voltage to the bias control circuit.
7. The differential signal driver of claim 1 , wherein the charging current is equal to the discharging current.
8. The differential signal driver of claim 7 , wherein the current source sources the charging current from a first supply voltage and the current sink sinks the discharging current to a second supply voltage, wherein a cross point of the first and second output control voltages occurs at a voltage equal to one half the sum of the first supply voltage and the second supply voltage.
9. The differential signal driver of claim 1 , wherein the charging current is greater than the discharging current.
10. The differential signal driver of claim 9 , wherein the current source sources the charging current from a first supply voltage and the current sink sinks the discharging current to a second supply voltage, wherein a cross point of the first and second output control voltages occurs at a voltage greater than one half the sum of the first supply voltage and the second supply voltage.
11. The differential signal driver of claim 1 , wherein the current source comprises two p-channel transistors connected in a cascode configuration, and the current sink comprises two n-channel transistors connected in a cascode configuration.
12. The differential signal driver of claim 1 , wherein the first set of switching transistors comprise:
a first switching transistor configured to couple the current source to the first capacitor when the first differential input signal has a first logic state; and
a second switching transistor configured to couple the current sink to the first capacitor when the first differential input signal has a second logic state; and
wherein the second set of switching transistors comprise:
a third switching transistor configured to couple the current source to the second capacitor when the second differential input signal has a first logic state, and;
a fourth switching transistor configured to couple the current sink to the second capacitor when the second differential input signal has a second logic state.
13. The differential signal driver of claim 1 , further comprising a voltage supply terminal coupled to the first capacitor and the second capacitor, whereby the voltage supply terminal applies a common supply voltage to the first capacitor and the second capacitor, and wherein the current sink provides the discharging current to the voltage supply terminal.
14. A differential signal driver comprising:
a pre-driver circuit configured to receive a first differential input signal and a second differential input signal, and in response, provide a first output control voltage and a second output control voltage; and
an output driver circuit configured to provide a differential output signal in response to the first and second output control voltages,
wherein the pre-driver circuit comprises:
a current source configured to provide a charging current;
a current sink configured to provide a discharging current;
a first capacitor;
a second capacitor;
a first set of switching transistors configured to selectively couple the first capacitor to either the current source or the current sink in response to the first differential input signal, thereby developing the first output control voltage on the first capacitor;
a second set of switching transistors configured to selectively couple the second capacitor to either the current source or the current sink in response to the second differential input signal, whereby the second output control voltage is developed on the second capacitor; and
a bias control circuit configured to provide a first set of bias voltages to the current source, thereby causing the current source to provide the charging current, and a second set of bias voltages to the current sink, thereby causing the current sink to provide the discharging current, wherein the bias control circuit includes a reference current generator configured to generate a reference current, current mirror circuitry configured to generate first and second sets of bias voltages in response to the reference current.
15. A method of operating a differential signal driver comprising:
generating a constant charging current;
generating a constant discharging current;
charging a first capacitor with the constant charging current when a differential input signal has a first state, and discharging the first capacitor with the constant discharging current when the differential input signal has a second state, thereby developing a first output control voltage on the first capacitor;
charging a second capacitor with the constant charging current when the differential input signal has the second state, and discharging the second capacitor with the constant discharging current when the differential input signal has the first state, thereby developing a second output control voltage on the second capacitor; and
driving a differential output signal in response to the first and second output control voltages.
16. The method of claim 15 , further comprising selecting the constant charging current and the constant discharging to specify a slew rate of the first and second output control voltages.
17. The method of claim 15 , further comprising selecting the constant charging current and the constant discharging to specify a slew rate of the differential output signal.
18. The method of claim 15 , further comprising selecting the constant charging current and the constant discharging current to specify a cross point of the first output control voltage and the second output control voltage.